Your questions answered: Design considerations for generator distribution switchboards

NFPA 70: National Electrical Code Article 700 specifies the requirements to install emergency power systems such as lighting when power from the normal source is interrupted. But while properly configuring the distribution switchboards of the generators in emergency systems is crucial, it is often a source of anxiety and confusion for installers. Electrical contractors and consultants need to understand all the applicable codes so they can arrive at a solution that complies with NEC 2020 requirements.

This session discusses some of the traditional areas of confusion surrounding generator distribution switchboard design. For instance, the presenters will go into how to properly segregate 701 and 702 loads. 701 loads consist of “legally required standby” loads which include elevators, ventilation, and fire-detection equipment. 702 loads are considered “optional standby,” covering areas such as heating, refrigeration, and data processing. One of the questions that often comes up is whether wiring for optional standby loads can share the raceway, box, and cabinets with legally required wiring. The reason for confusion is that more than one interpretation of the code is possible, requiring the owner and AHJ to decide on a preference.

The presenters ended the webcast on “Design considerations for generator distribution switchboards” with several questions left unanswered. Read the responses here.

Expert presenters:

• Gary Gluck, Western Region Business Development Manager, Siemens Low Voltage Distribution Products, Siemens
• James Alvers, Business Development Manager, Siemens Low Voltage Distribution Products, Siemens

According to the example I heard in this presentation, an 800A standby switchboard would need to be rated no more than 8 kAIC. Won’t we think also about the normal feed to this switchboard? If the normal source rated 100 kAIC (as an example) a few feet away from the standby switchboard, shouldn’t the standby switchboard be rated 100 kAIC to match the worst-case scenario feed.

Gary Gluck: Emergency switchboards are typically isolated form the normal supply, so the short circuit current from the normal supply need not be considered.

Any thoughts about protection/mitigation controls for basement flooding (when the board is located on sub-ground floors)?

James Alvers: These switchboards should be on pads. The switchboard should also include GFP. We’ve seen sump pumps used to empty water with pumps on a dedicated circuit from outside the room.

Are the elevators not supposed to work during a fire emergency, not sure why elevator feeder is included here?

Gary Gluck: This is why there is a debate between those that classify it as a NFPA 70: National Electrical Code Article 700 load versus those that have them in 701.

Are we not concerned with the normal source fault current if the switchboard is connected to such source that may have high fault current?

Gary Gluck: If the generator switchboard is connected to the normal source, we would need to consider the fault current. We typically don’t see that, however. The two systems are normally isolated and only meet at the transfer switch.

As NEC requires a front clear working space, it follows that doors that open 90 degrees and draw-out breakers cannot encroach on this space. Therefore, front working space has to be increased. Agree?

James Alvers: Agreed. The presentation only summarized working space requirements. Refer to Article 110 for complete requirements.

Based on what you see across the U.S., typically you are saying it is allowed by the local AHS’s to combine legally required and optional standby loads in the same vertical section?

Gary Gluck: We are just taking what’s in the NEC and relaying that information to you. We are not spokesmen for the local authority having jurisdiction. They are free to interpret the code as they see fit.

Bells and whistles for switchboard: load monitoring for each feeder to sub panels?

James Alvers: Load monitoring is possible and often desirable in any breaker in an emergency switchboard.

Can 701 and 702 share ATS?

Gary Gluck: This may be permissible, however, please refer to the AHJ for interpretation.

Can a single feed come from the generator to the switchboard which would inherently combine the various loads into a single raceway?

James Alvers: Typically, these switchboards have one generator with a single feed to the switchboard. So, 700, 701 and 702 loads are combined from the generator to the switchboard.

Can fire pump grouped with life safety in health care?

Gary Gluck: Yes, per the reference to article 700 in 517.

Can switchboards be configured to shed loads automatically when generators (smaller than the connected load) are used?

James Alvers: Yes, they can. However, the generator must be sized and configured to prioritize and support all the 700 loads including motor restarts. Other loads may be shed.

Can the GEN switchboard MCB be placed in the same vertical section with the Fire Pump and emergency feeder circuit breakers?

Gary Gluck: Physically yes but our message was that it was probably best to put it in its own section. There is nothing in the code that condemns nor sanctions the decision to do this

Can the GFP interfere with selective coordination? Main trips on ground fault before short time trip downstream?

Gary Gluck: If you are talking about the emergency side, it shouldn’t if you are using ground fault alarm only. If you are using ground fault alarm and trip, then it needs to be considered.

Can you discuss what type of buildings require Emergency systems (specifically emergency lights)?

Gary Gluck: This is not our area of expertise, but it is our understanding that buildings that are at least 75 feet high or can accommodate 1000 people or more would require an emergency system. Typically, this is defined in IBC 2018 and NFPA 5000 2021. The AHJ would be involved as well.

James Alvers: Typically, this is defined in IBC 2018 and NFPA 5000 2021. The AHJ would be involved as well.

Can you elaborate on conditions when a separate electrical room is required for emergency distribution equipment, automatic transfer switches, emergency switchboards, panels?

James Alvers: Refer to NFPA 110: Standard for Emergency and Standby Power Systems and your AHJ. Generally, the emergency generator switchboard should be in a different room so that a fault or fire in the normal equipment will not affect the emergency equipment. They can’t be in the same room unless separated by a fire-rated wall.

Can you go over again the requirement for need of a fused disconnect for the ATS of the elevator as mentioned due to fault current?

Gary Gluck: Refer to published withstand rating data from ATS manufacturers. For low rated switches, one would find that the only way to substantially enhance withstand ratings is with a fuse.

Can you please clarify why arc flash mitigation is not required at the EMDB Switchboard if it rated for less than 1200 amps? Are we assuming that incident energy has already been determined not be an issue?

Gary Gluck: We are basing this statement on the criteria set up in 240.87 of the NEC

Could you provide the NEC reference that requires a dedicated Life Safety section?

Gary Gluck: Please download the white paper that was advertised with this presentation. It’s covered in that paper.

Difference between unit-mounted and group-mounted?

Gary Gluck: When you think group mounted think about how breakers mount in a panelboard. Unit mount is a single mounted breaker where the line side of the breaker is fed directly from either lugs or the cross bus

Do elevators need a separate 701 ATS? Or can they be grouped with the 700 ATS?

Gary Gluck: Elevators can be grouped with whichever load you have it called out for. So, if you have it as legally required, it can be added to other 701 loads.

Do switchboards have arc flash quenching devices to protection the equipment and personnel?

James Alvers: Arc quenching technologies are not normally found in UL891 switchboards.

Do we need always load bank for generators?

James Alvers: Generators require load for proper testing and exercising. The easiest way is usually to use load banks, though in certain applications building loads can be used.

Does the ATS size in the last example depend upon the locked rotor amps of the fire pump?

Gary Gluck: It does not. The ATS is not an overcurrent device and is not subject to the same rules as a breaker. It’s no different from, say, the cabling

Don’t we have to add a section in the EM switchboard for mobile type generator as part of Article 700?

James Alvers: If there is just one generator or service almost always yes. This allows compliance with 700.3(F).

Elevators for high-rise Buildings: 700 or 701?

Gary Gluck: That is dependent on what the elevator is going to be used for. If first responders could reasonably be expected to be using the elevator during an emergency, I would maintain that 701 is the applicable standard

Feeders will be sized based on the trip size of the breaker or frame size of the breaker?

Gary Gluck: Trip-rating of the breaker.

Does that load bank circuit breaker at the generator mentioned require vertical separation from the circuit breaker feeding the generator distribution board?

James Alvers: This is best addressed with the AHJ, as it is not covered in the code. We generally we do not see separate sections as the generator is allowed multiple feeds. Since the load bank is only used during maintenance, the load bank breaker should be mechanically interlocked with the generator main or the generator should be disconnected from the generator switchboard. This ensures the generator is offline while being exercised for maintenance.

For generator distribution switchboard wouldn’t the normal power source available fault current drive the AIC Rating?

Gary Gluck: Not unless they are connected-which they typically aren’t for the bracing of the switchboards in a closed transition transfer scheme do you consider both sources of fault current thus brace the board for the contribution from both sources

James Alvers: Yes, both sources must be considered.

For the switchboard when should we consider individually mounted feeder/branch circuit breakers versus group mounted feeder/branch circuit breakers?

Gary Gluck: The decision to group mount or individually mount is dependent on frame size, cable size and switchboard size.

Under NEC 517 do optional loads (misc. owner loads) that need to be on the generator will they get lumped into the “equipment” group?

Gary Gluck: My suggestion would be to have optional loads in their own section separate from the “Essential Electrical System”

What is the difference between a switchboard and switchgear?

Gary Gluck: Switchboard is built to UL891 standards; Switchgear is built to UL 1558 standards.

Gary mentioned that breaker selection can change depending on 700, 701 or 702. Can you give an example of some of the differences?

Gary Gluck: One example would be selective coordination. A poorly designed 700/701 system compounded by an area where the threshold for selective coordination is 0.01 seconds could result in oversized overcurrent devices in order to satisfy the design requirements. That, in itself, could cause the size of the switchboard to increase.

If you are using closed transition or soft loading transfer switches, could not the fault current at the generator switchboard be much higher than 10x the current rating of the generator when the utility contribution is factored in during the limited parallel between generator and utility?

Gary Gluck: Absolutely true. My assumption, right or wrong, was that the lion’s share of the ATS’s in use are open transition.

Health care application: fire pump go into the life safety branch or in equipment branch in the generator board?

James Alvers: Fire Pumps are classified under Life Safety for health care applications.

How do you determine the size of the SPD for the switchboard?

James Alvers: Sizing SPDs for emergency switchboards is similar to sizing for standard switchboards.

I also suggest locating switchboard dependent on the geographic areas as well, such as second or upper levels due to flooding.

James Alvers: Yes, this is becoming more common, due to floods and climate concerns.

I am not sure if I have missed the review of the question of hoe to size circuit breaker feeding Fire Pump or do we use tap instead?

James Alvers: The breaker should be sized in accordance with NEC695.4(B)(2)(a). The overcurrent protective device shall not open within two minutes at 600% of the full-load current of the fire pump motor(s). Please refer to the complete section for more details on sizing.

I see a lot of projects where multiple breakers are generator mounted…presumably to save space inside a building. Are there breaker limitations for situations where they are generator mounted? Temperature extremes? Dust/moisture?

James Alvers: Yes, please consult with the generator provider.

I think the ATS is sized for the locked rotor amps.

Gary Gluck: It may be, but it doesn’t have to be. It is not an OCPD and it is not subject to the same rules a breaker is. No different from the cabling.

If a building has a generator, would fire alarm and security electronics be included in NEC 700?

Gary Gluck: It is conceivable that security could be part of 700 if it affected the ability to exit the building. Fire Alarm would also typically be in 700.

James Alvers: Refer to informational note in article 700

If fire pump is an Article 700 emergency equipment, why can it not be in the same vertical section with the other 700 equipment?

Gary Gluck: Our assertion during the presentation was that Fire Pumps can be in the same vertical section.

If I have a distribution switchboard with a full size neutral from utility, do we need the full neutral from generator as well? Does this affect switchboard with any ground fault?

Gary Gluck: The switchboard will have a full size neutral. Please refer to generator manufacturer for the feed from the generator. The situation described should not affect the GFP on the normal side.

If we have the switchboard configured with a main-tie-main to support a redundant power distribution system. What recommendations would you make for the placement of the M-T-M? Should they be located at the center of the switchboard line up or should we separate the Tie CB from one of the mains in the event of a fault on one side we do not bring down the entire switchboard lineup?

Gary Gluck: We see it both ways but we see the M-T-M in the center infrequently. Most of the time the mains are on the ends with the tie-in the center of the lineup.

I’m sorry I did not get why did we not want a GFCI on a generator distribution switchboard? I understand NEC will allow it, but why would we not want it/?

Gary Gluck: I don’t want to get caught up in semantics, but GFCI refers to people level protection which is not part of any discussion here. Ground Fault alarm and trip is discouraged in NEC 700.31 probably for the simple reason that if the emergency system is required to save a building, we are less concerned with the consequences of having a ground fault compared to the saving of human life and assets

I’m working on a 6 story medical office building, but struggling a little determining what the system would be regarded as, optional standby or legally required? Current plan is to have egress lighting and elevators (used for exiting) backed up by genset.

Gary Gluck: Articles 700, 701 and 702 include an informational note listing loads typically assigned to each article.

In ref to p.10 single line example, one too many OCP devices are shown between the generator and the fire pump, correct?

Gary Gluck: We don’t believe that was the issue that was discussed. The point that we were trying to make was that the fire pump is part of the emergency system and, therefore, could be consolidated into the section with the Emergency, thus eliminating a switchboard structure

In your generator switchboard, will a 400/3 branch coordinate with the 700/3 main?

Gary Gluck: With solid state breakers and proper selection these breakers can be coordinated.

Is a circuit breaker outside the building required if the generator is more than 50 feet away from the building?

James Alvers: Yes, the generator needs to be 50 feet or less from the building to be considered within sight or a breaker is required.

Is camlock for temp generator required?

James Alvers: No, Standard or Type W lugs can be used as well. Camlocks offer convenience and are compatible with most temp generators from commercial providers.

Is there a typical amperage size you would prefer to use a switchboard over a gutter/breaker system?

Gary Gluck: Normally, the preference lies with the contractor. Above 400amps, things start to get costly with the separate components.

It was mentioned that barriers must be provided between 700 and 701/702 feeds. Do this apply for the health care feeds as well?

James Alvers: Yes, barriers are required for 517.33 compliant switchboards.

Legally required and optional loads can be on the same section per NEC 700.10 (B)(5) – vertical aeparation? Diagram shows them on the same section.

Gary Gluck: The two can be combined. Our example did not have any optional standby but if it did, it would have been in a single section

Must ATS for fire pump be 4-pole ATS or 3-pole ATS? Is SPD required for each panel on the load side of the ATS?

Gary Gluck: It will most likely simplify grounding if all the ATS’s are specified 4 pole with switched neutrals. SPDs are only required on emergency panels after the ATS.

On the first example one line, it showed a main breaker on the generator, a main breaker on the main emergency distribution board, and a breaker feeding the fire pump. Doesn’t this violate NEC 695 and the number of overcurrent protection devices that are allowed in front of a fire pump?

Gary Gluck: I don’t believe that the rules that apply on the normal side for fire pump feeds also apply on the emergency. Is there a specific code section that you are citing here?

Please clarify section barriers. Are these intended to isolate arc flash energy?

Gary Gluck: Nope. They are designed to keep cabling within the confines of a single section

Please clarify whether automatic throwover system can be used rather than automatic transfer switch.

Gary Gluck: They functionally operate the same way. If UL1008 is a requirement, then stick with Russelectric type switches. If it’s not a requirement, then an “ATO” may be appropriate.

Regarding Busbar ratings, what does Heat Rated means that should be considered from an engineering designer point of view?

James Alvers: Perhaps, what is meant is “heat rise”? That is a method of rating switchboard busing based on temperature rather than amps per square inch. Most UL891 switchboards use this method of bus rating.

Sizing a circuit breaker for a fire pump. How is this circuit breaker sized in the emergency side?  Is it required to size at 6 times FLA of fire pump?

James Alvers: The breaker should be sized in accordance with NEC695.4(B)(2)(a). The overcurrent protective device shall not open within two minutes at 600% of the full-load current of the fire pump motor(s). Please refer to the complete section for more details on sizing.

Slide 11 suggests the specified 100kAIC could be reduced to 8kAIC, and the suggested design within Slide 22 suggests 22kAIC rating. But, isn’t calculated available fault current via the generator switchboard much higher than 8kAIC because the riser diagram within Slide 9 identifies available fault on the generator terminal at each ATS (presumably via the generator switchboard) to be calculated above 44kAIC. Specifically, ATS-EL (44,442 amps), ATS-LS (53,635 amps) & ATS-LR (51,835 amps).

Gary Gluck: The fault current at the ATS will always be higher than the GDB because it also interfaces with the normal power. The GDB does not.

Slide 28: Please clarify 1200A generator breaker does NOT need arc flash mitigation?

Gary Gluck: Yes, it does. All 1200amp and larger breakers need to comply with NEC 240.87

Suppose a switchboard has OCPDs rated at most 800A (in frame and trip), as well as 800AT upstream OCPDs, but the switchboard is rated for 1200A (in terms of housing and bussing). Does this pull in the 1200A requirements of article 110?

Gary Gluck: With some jurisdictions, no but a lot will. We run into this issue with residential metering where the cross bus is 1200amps. Most lineups are not rated for 1200 amps but that’s the metering says

The FPN for Definition of “Emergency” in NEC 701 includes elevators. I’d say this is definitely an NEC 700 load.

Gary Gluck: We completely agree with you, but the reality is that most designs we see have the elevators in 701.

James Alvers: The classification is determined by the intended use of the elevator. See informational note 2 in Article 620.91, which usually puts the elevator in 701.

The one-line indicates that the SPD is mounted outside of the switchboard, which is actually preferable from a maintenance standpoint.

Gary Gluck: This is acceptable, however, keep in mind SPDs perform better when factory installed. We also offer a full line of externally mounted SPD’s.

The presenters discussed generator switchboard configuration as “main lugs only” omitting a main breaker; but, if the switchboard is supplied from a generator located exterior to the building with a feeder entrance into the building, then, will the maximum number of breakers that can be incorporated into the switchboard be limited to six (6) total in order to comply with NEC Art 225.37?

James Alvers: Generator switchboards are not usually classified as service entrance switchboards so the six-circuit rule, Article 230.71, does not apply. A main disconnect is not required, but depending on application may be recommended. Article 225.37 concerns identification of multiple services and feeders.

For a switchboard, do the incoming conductors need to be terminated in their own section or can there be both incoming terminations and CB distribution in the same section?

James Alvers: The code is not clear. However, most AHJs accept distribution breakers in a main lug section while requiring the main generator breaker to be in its own section.

Transfer switch with breaker for temporary generator shall be 4-pole or 3-pole?

James Alvers: This is application dependent. Often a 3-pole ATS is used if the temp generator is not separately grounded.

Two breakers on the generator. Breaker 1 for emergency loads, Breaker 2 for everything else? Is that an option?

Gary Gluck: Sounds good to me with one concern. If coordination is needed, we need to instruct whoever is supplying the generator to include breakers from the same manufacturer of the rest of the gear. Usually, they supply whoever they feel like.

What about security electronics?

James Alvers: Security electronics can be classified as 700 or 701 loads based on application. For example, if needed for egress, they are classified as a 700 load.

What does “MEDS” stand for?

Gary Gluck: It is the switchboard designation.

What if the application was 702 optional standby only? Would GFP (LSIG) be required above 1000A?

Gary Gluck: This is a “it depends on who you talk to” kind of question. In my mind, I would never use LSIG in an emergency switchboard. I would be OK with ground fault ALARM but not ALARM and TRIP.

What if the whole building is on the generator? Service entrance rated ATS?

Gary Gluck: The emergency loads still need to be physically separated from the normal source. Therefore, most likely an emergency switchboard is required.

What is (of any) advantage of an ATS versus. microprocessor-controlled and properly interlocked pair of breakers located in separate-powered gears (e.g., the very breakers that would feed an ATS “normal” & “emergency” sides, and do codes allow such breaker pair to substitute for ATS (where it is currently required)?

James Alvers: Mechanically interlocked breakers may not be substituted for an ATS since they require manual operation. Even if they can be electrically operated, the code is specific about

ATSs being listed for emergency use. This almost always means specifying a UL1008 ATS. Most breaker pairs are typically not listed or tested for emergency use. What is desirable for switching? ATS or STS?

James Alvers: Article 700 requires ATSs listed for emergency use. This almost always means specifying a UL1008 ATS. If the ATS is selected properly, UL1008 ensures both short circuit and short circuit closing ratings can be met.

What is modified differential ground?

Gary Gluck: It is a method for accounting for the possibility that in a system that has two sources (utility and generator) there may be the possibility of two paths to ground. MDGF takes care of this. We have a white paper that we can send you on this topic.

What is more preferable: 3-pole or 4-pole ATS?

James Alvers: Normally an ATS with 4 switched poles is preferable. This allows the neutral to switched usually avoiding multiple grounds. Regardless of ATS used, grounding needs to be reviewed.

What is working distance use for switchboard in model the arc flash study?

James Alvers: The working distances presented reflect code requirements for room configuration. Arc Flash boundaries are application dependent and require a study that is beyond the scope of this presentation.

What special precaution are needed when more than 1 generator is installed in a facility?

James Alvers: From an equipment vendor perspective the most important precaution is designing a proper grounding system that takes into account the generators, main service and any temporary generator inputs.

What type of PQM metering for the circuit breakers do you recommend?

James Alvers: Emergency switchboards do not require any special power quality monitoring. Consider the need for power quality monitoring as you would for other applications. When assessing an electrical room, when the path of egress is not in compliance, what can you suggest to comply with path of egress when the switchboard is 1600 amps and larger. As discussed, an electrical room is a team effort. In this case the only solution may be to work with the architect to either add access or enlarge the electrical room. Refer to Article 110 for room requirements.

When does the NEC or other NFPA codes require a separate space for emergency electrical distribution? I think this is often viewed as a best practice.

Gary Gluck: NEC 700 clearly states that that the emergency system (700) conductors will be in its own barriered section. Please refer to the white paper that Siemens published that CSE provided access to for the specific code sections

When elevators shall be on the emergency branch and not legally required?

Gary Gluck: Our understanding is that it has everything to do with whether first responders are likely to need the elevators in the course of their business.

When must a generator be considered a separately derived system?

James Alvers: If the generator is solidly grounded.

When should we use a switchboard versus a metal enclose switchgear?

Gary Gluck: It often has to do with selectivity. Switchgear is rated for 30 cycles and is perfect for applications where downtime would be very costly. Switchboards are 3-cycle rated and their mission is to clear faults as quickly as possible. Downtime is secondary.

When we have an SPD incorporated into switchboard should there be a fuse or circuit breaker ahead of the SPD? Does the switchboard manufacturer size the protection device ahead of the SPD?  We want to have the ability to service the SPD without powering down the entire switchboard.

Gary Gluck: Based on the last sentence, you want the SPD separate and that’s great. You will need to size a breaker for the feed to the SPD.

When we have switchboards in corrosive environments (i.e., near an ocean/sea, WWTP, industry, etc.) what additional protection can we specify to protect the bus, circuit breakers, controls, enclosure)?

James Alvers: If switchboards are near corrosive environments they should be indoors, in their own room, with purged air. Specialty manufacturers can offer stainless steel enclosures and more corrosive resistant parts. Special attention should be paid to proper bus material and possible lubrications needed.

While the generator max fault current allows the short circuit rating of the switchboard to be smaller, having the normal feed going through this board will mean that will be based on the utility?

Gary Gluck: If that should occur, then yes. That was not happening in our project and is not typical.

Why is zone interlocking the preferred method for meeting arc flash mitigation?

James Alvers: Zone selective interlocking allows downstream breakers to trip first, so selective coordination is preserved while still complying with Article 240.87. ERMS solutions are most likely be uncoordinated during a maintenance event.

Will Siemens consider get into ATS with temporary connection for temp. generator and load bank? What is the typical cost between the switchboard design and transfer switch design for such application?

James Alvers: Yes, we would consider a configuration which includes an ATS to switch between a temporary generator and a load bank if properly designed. Cost can vary widely based on amperage, voltage, short circuit current and specification.

Would the switchboard see a higher maximum fault current from the normal utility feed?

Gary Gluck: In general, yes.

Would you expect the EOR to design and specify an MDGF system or would the equipment manufacturer be expected to design and specify it if called for by the EOR?

James Alvers: It is the consensus among manufacturers that MDGF is mandatory when separately derived systems exist and 4 pole devices are not being utilized.

Would you explain the arc flash versus trip style requirements?

James Alvers: If the breaker frame is 1200A and the trip unit is interchangeable or adjustable to 1200A, arc flash mitigation is required per 240.87.

You have mentioned that at times GFP may be undesirable. Can you elaborate as well as explain when it is required?

Gary Gluck: Ground Fault protection is undesirable in an emergency generator distribution switchboard, because emergency loads need to stay powered for egress and responders. Ground fault monitoring, however, may be desired.

What is the working definition of a generator switchboard? are we talking about a switchboard that is protected by a generator? or is it a switchboard that accumulates multiple generators for load sharing?

Gary Gluck: Any switchboard fed by a generator that provides distribution to loads that are subject to NEC 700/701/702 or 517.33

James Alvers: The emergency generator switchboard needs to be in a different room so that a fault or fire in the normal equipment will not affect the emergency equipment. They can’t be in the same room unless separated by a fire-rated wall. Refer to NFPA 110: Standard for Emergency and Standby Power Systems and your AHJ.

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