Your questions answered: Electrical distribution systems

Questions left unanswered during the "Electrical distribution systems" webcast are answered here

By John Yoon and Jeff Thomas June 22, 2021

It is useful for both electrical and nonelectrical engineers to understand basic features when selecting, specifying and applying electrical distribution systems. To narrow the broad scope of electrical distribution, this discussion will focus on practical considerations for specifying electrical distribution systems.  

To learn more, attend the webcast “Electrical distribution systems,” originally presented on June 15, 2021, and then review the questions and answers below. Expert presenters are:  

  • John Yoon, PE, LEED AP, lead electrical engineer, McGuire Engineers, Chicago
  • Jeff Thomas, PE, CEM, CEA, CHC, vice president, business group director, Lockwood, Andrews & Newnam, Houston 


While the codes and standards that were included in this presentation are certainly important to proper electrical design, would the presenters please also address the concept of prevention through design and the hierarchy of controls recommended by National Institute for Occupational Safety & Health to reduce electrical hazards to operations and maintenance personnel.  

John Yoon: The Occupational Safety and Health Administration publishes and enforces federal regulations. NIOSH does not publish regulations or enforce them. While the holistic NIOSH PtD guidelines do represent best practices, compliance has to start with the regulations that are actually enforced by the authority having jurisdiction. 


How do you reduce harmonics? 

Jeff Thomas: As we know, harmonics collect in the neutral conductor. There are a couple of strategies for handling harmonics. In all cases IEEE-519 is the standard we want equipment and systems to meet: 

  • Use delta-wye transformers to isolate the harmonics. 
  • Use harmonic mitigating transformers but do the research and see if you agree they’ll do what they advertise. 
  • Install harmonic traps (not very common, usually to correct a very specific problem).


How is high and ultrahigh voltage class defined? 

Jeff Thomas: Again, it depends on who you ask. One source indicates for AC voltages, HV is 100-230 kilovolt, EHV is 230-800 kilovolt and UHV is above 800 kilovolts. UHV DC is typically 1,000 kilovolts. 


Is zone selective interlocking something you specify in the drawings? If so, how? Or is that something the manufacturer will include in their submittals with communication wiring/conduit provided by the subcontractor and they would install it accordingly? 

John Yoon: Yes, it must be specified. The manufacturer should be able to help you with the selection. 


Do disconnects also have to meet the working space requirements? 

Jeff Thomas: Yes. 


ASHRAE Standard 90.1 has requirements around metering different types of loads: HVAC, lighting and receptacle. Do you have suggestions on how to lay out distribution panels to be able to meter these loads on a building? 

Jeff Thomas: As you point out, ASHRAE wants us to group the loads. One way is to have dedicated panels for the various types of loads. This can be a maintenance issue as the building ages and people aren’t strict about adding and changing things. Another method might be to use small “internet of things” sensors on the individual loads and aggregate the results to meet the requirements of ASHRAE 90.1. Lighting inverters can help with the lighting portion by providing control and metering.  

John Yoon: Beyond segregating different load types on dedicated panels so that they can be separately metered, the available options for commercial applications are limited. However, the 2021 IECC has also incorporated 90.1’s metering requirements. When jurisdiction start adopting the 2021 IECC, I would expect that there would be much more pressure on the electrical metering manufacturers to develop simpler, more cost-effective solutions. I’m not aware of any requirement that this metering have utility grade accuracy. This would open up the possibility of using other emerging metering technologies that may not have the same level of accuracy. There is a type of DSP based metering for the residential market that can distinguish the individual “electrical fingerprints” associated with different load types sharing a single panel. I would expect that something like this would be a good fit.  


Please confirm: An LED retrofit can use the fluorescents’ former 277-volt panel if LED fixture 277-volt drivers are specified. 

Jeff Thomas: Most LED retrofits contain multivoltage drivers. As long as you verify the driver supports 277 it should be a simple change out. 

Is it reasonable to use both a circuit breaker and a fuse in conjunction with each other? 

John Yoon: One of the primary advantages of fuses is when they are used on utility services with high available fault (i.e., network bus configuration with multiple transformers) is that they can significantly reduce fault current to downstream devices. But when you have fuses and circuit breakers connected in series, you can’t necessarily count on the current limiting characteristics of the fuse to limit fault current to downstream devices. As the contacts open on a circuit breaker during an overcurrent condition, the impedance of the circuit will change. Because of this uncertainty regarding how the fuse with react to this dynamic impedance, you need to verify with the circuit breaker manufacturer to see if they have a series rated combination with the specific type of fuse that you want to use. 


Speaking of future-proofing, are electric vehicle charging requirements becoming more prevalent in design? 

John Yoon: The majority of EV owners currently charge at home. However, as EV adoption expands and are purchased by people who do not have their own charging stations (i.e., apartment dwellers), the need for shared public charging stations will expand. Many jurisdictions and the 2021 IECC have already incorporated new requirements for charging stations. The IECC requirements is that at least 20% of all parking spaces be “EV ready” EV ready means a 40A 208V or 240V circuit reserved for that use. 40A equates to a 6.8- to 7.6-kilowatt charger. This is the tip of the iceberg. The expectation is that EV drivers will want the convenience of level 3 DC fast charging will requires power an order of magnitude greater than that. 

Jeff Thomas: Especially if you’re chasing LEED certification. Vehicle charging stations are a good way to get LEED points. 


What is the voltage used in residential distribution systems in the USA? 

Jeff Thomas: Most residences receive 240V/120V 2-phase power. The 240V is typically for electric heat, electric water heating and the electric clothes dryer and cooking range. Convenience outlets throughout the residence are 120V. The voltage distributed throughout the neighborhood, i.e., the high side of the distribution transformer, varies greatly. 


Can you please discuss 120/240 volt three-phase system? 

John Yoon: 120/240 volt, 3-phase, 4-wire systems are referred to by several names; high-leg delta, red-leg delta, deadman delta, etc. The last one is probably the most fitting since these systems can be very dangerous to the uninformed electrician. The configuration of the service is similar to most delta configured services, but with key different the third transformer/winding is center tapped to derive a neutral (see diagram) for 120V loads. This type of service was typical for smaller light industrial users that had a significant amount of 240V 3-phase load, but only a handful of 120V loads. The safety concern with this type of service is that the B phase to neutral voltage is 208V, not 120V. NFPA 70: National Electrical Code 110.15 requires that the B-phase conductor be identify with orange coloring. NEC 408.3(F)(1) also requires warning labeling. If the labeling isn’t present, there are two common clues that this voltage is present;  

For overhead utility services, one of the 3 single-phase transformer is larger than the other two 

In 3-phase, 4-wire panelboards with single pole branch breakers, every third circuit breaker is missing. 

Concerns related to zone selective interlocking without proper CM control? Any experience with the communication between the different breakers failing and basically defeating the selectivity benefit of this method? 

John Yoon: Yes, it is possible that control link between the upstream and downstream circuit breakers could be interrupted. In that case, each circuit breaker would respond per its individual trip settings. With ZSI, the key point is that the breaker closest to the sends an inhibit signal to initiate a time delay in the other breakers. Without that control link, your coordination is no better or worse than any other distribution system using circuit breakers, assuming that the LSI setting have been properly configured for each breaker. 


With the uncertainty at the dog leg region of a circuit breaker, how do you design using circuit breakers for selective coordination? Do you just try to achieve selection coordination to a certain time (i.e., 0.1 secs)? 

John Yoon: Yes, selective coordination in the instantaneous region is a problem. The selective coordination requirements have been in the NEC for a while. For whatever reason, while Article 507 allows that 0.1 second coordination limitation that you mention, but the NEC requirements for coordination as they relate to COPS and life-safety loads still don’t. ZSI has usually been the workaround for this problem but the cost and complexity associated with ZSI are significant. Circuit breaker manufacturers recognize that this is an issue. As such, they often have tested specific combinations of breakers for selective coordination purposes. Talk to your preferred circuit breaker manufacturer for additional information. 


Are AFCI breakers the same as arc flash reduction? 

John Yoon: No, AFCI breakers are intended to address a sputtering arc fault in a low ampacity downstream branch circuit that typically would not trip a circuit breaker.  


What is the actual current that the fuse can sustain without melting (relative to rating, 1.15x, 2.0x, etc.?) 

John Yoon: It varies depending on fuse class. Refer to NEMA FU1 or the individual manufacturers TCCs. 


Is it OK to use a mixture of fuses and circuit breakers in an upstream to downstream line? Or should it all be the same kind of device? 

John Yoon: See above. Coordination is possible but it may not be possible to depending on a fuses current limiting characteristics unless it is series rated by the manufacturer. 


What is recommended use of grounding system to use per facility type? 

John Yoon: This is a presentation in of itself. Recommend that you review the old IEEE Green Book or standard 142. 


Separate normal and emergency electric rooms: What is the general design approach? When can sub-panel coexist in a room? 

John Yoon: Depends on the application. First, what are the AHJ’s requirements? After you determine what those basic Code requirements are, you have to ask yourself some basic hazard risk analysis questions. What is the likelihood of a catastrophic failure of one damaging the other? If it is damaged and becomes nonfunctional, what impact will it have?  Are you directly and immediately impacting health and safety? What is the medium voltage circuit breaker type? 


What is driving the arc flash label requirements on equipment currently? 

John Yoon: Labeling is required by Occupational Safety and Health Administration and NFPA 70E. 


I believe the maximum panel is 1,200-amp panelboard, is this still correct? 

John Yoon: The largest size of wall mounted panelboard depends on the manufacturer. 


Protective Coordination for an MCC – If the instantaneous is turned off, what is the time rating typical for the STPU setting (as related to the 3-cylce withstand)? 

John Yoon: Refer to diagram in the beginning NEC Article 430. You still require short circuit protection somewhere in that motor starter circuit. You have to compare those current thresholds listed in NEC to whatever the 3-cycle rating is for the other equipment. That 1,700% threshold may cause significant damage. Again, it comes down to comparing the TCCs. 


Does using breakers as switches wear the breaker or fixtures out faster? This seems to be a common approach for some parking garages we build and other industrial projects. 

John Yoon: Yes. Switching duty (SWD) type breakers is appropriate in that case. 


What would a good resource for learning about faults other than ground faults, since they aren’t defined by the NEC (i.e., SLG, 3-phase, etc.)?  

John Yoon: The old IEEE Buff Book. 


Would you interpret the panic hardware requirement to apply to all doors within 25 feet from the working space (including corridors, exterior doors, etc.) or just the doors immediately exiting from the working space if within 25 feet?  

Jeff Thomas: My understanding is you just have to exit the room, not the entire building. 


Explain the series breakers, does this mean the main breaker matches the branch breakers? 

John Yoon: A listed combinations of circuit breakers installed in series. The downstream breaker can be used when higher available fault is present than if it was applied on its own. 


TCC of current limiting fuse versus normal fuse? 

John Yoon: Refer to the manufacturer’s documentation. 


Author Bio: John Yoon, PE, LEED AP, lead electrical engineer, McGuire Engineers, Chicago Jeff Thomas, PE, CEM, CEA, CHC, vice president, business group director, Lockwood, Andrews & Newnam, Houston