Your questions answered: Electrical systems: Designing electrical rooms

Unanswered questions from the Nov. 16, 2017, webcast on designing electrical rooms are addressed by the presenters.

By Mario Vecchiarello, CDM Smith; Rich Vedvik, IMEG Corp. November 22, 2017

Switchboards, switchgear, transformers, generators, and UPSs require space for installation, maintenance, heat dissipation, and possible future expansion. And the wiring, busways, and raceways that distribute the electrical power must be accounted for—now, and in the future. Documentation and monitoring of electrical system’s equipment and how it connects to the rest of the facility must be accurately maintained.

Questions not addressed during the Thursday, Nov. 16, 2017, webcast on Electrical systems: Designing electrical rooms are addressed by the presenters here.


  • Mario Vecchiarello, PE, CEM, GBE, Senior Vice President, CDM Smith, Boston
  • Rich Vedvik, PE, Senior Electrical Engineer and Acoustics Engineer, IMEG Corp., Rock Island, Ill.

Question: Which exact code section defines battery light requirement inside electrical room? Does this rule apply to the main electrical room only?

Rich Vedvik: Not all electrical rooms are required to have battery emergency lightings. NFPA 110 7.3 defines the requirement for Level 1 and Level 2 emergency power supply (EPS) locations (this is echoed in NFPA 70: National Electrical Code (NEC) 517.33 (E). It is recommended to provide battery-powered emergency lighting in main electrical rooms, rooms containing emergency power supply system (EPSS) equipment, and electrical rooms without emergency power.

Question: What are the minimum dimensions needed for electric room for proper ventilation?

Mario Vecchiarello: Room size (and layout) are not prescriptive and depend on the equipment installed within the room, the arrangement available, and the type(s) of ventilation provided. Room and ventilation design, however, should take into consideration equipment that produces more heat gain than others.

Question: In a residential high rise: What is the maximum distance tap for the fire pump switch location? Is it required to be located in a dedicated space in the electrical room?

Vedvik: NEC 695 references NEC 230.82 for service tap requirements. NEC 695 contains fire pump requirements and note the 2-hour rating requirement. It is expensive to route 2- hour fire rated conductors in a building that promotes some coordination with locations of power source and equipment. Also note the requirement in 695.3(A)(1) for the fire pump tap to be in a separate vertical section and also physically separated from the electrical source; this requirement increases the size of the gear and thus the electrical room. The 2015 edition of the International Building Code (IBC) 403.3.4 identifies fire pump room requirements.

Question: What is the lighting requirement in electrical rooms?

Vecchiarello: See NEC Section 110.26(D) for normal lighting and other areas of the NEC and NFPA 101: Life Safety Code for emergency egress lighting requirements.

Question: With regard to a platform for a platform having a 6.9 kV switchgear, it is mounted on an ISO footprint where the distance would not meet NEC requirements as per distance/workspace. Can I add a flip-down structure to get to the 5 or 6 ft needed?

Vedvik: One concern with this method is providing fall protection on the flip-out platform. I have seen these platforms used before, but they usually lack fall protection. The NEC 110.26 clearances are relative to physical surfaces and not specifically working space for equipment. If a flip-down structure is added, the fall protection would need to comply with working clearances in NEC 110.26. NEC 408.18 references 110.26 for clearances around switchgear.

Question: What are the required room conditions for an oil-filled transformer to be in the room (such as an indoor substation)?

Vedvik: Refer to NEC 450.26 for vault construction requirements.

Question: How do you estimate heat gain of equipment?

Vecchiarello: Calculate the total heat dissipation for all the pieces of equipment, such as breakers, starters, VFDs, transformers, batteries, and battery chargers, etc. (should review the information data sheet on all of this equipment).

For double-ended equipment, we would typically account for only one transformer in a unit substation operating at rated load and the other operating at no load. Therefore, we would include the total losses for one transformer, which includes the active full load losses and the no load losses, and the no load losses for the other transformer.

Question: You said NFPA does not permit automatic transfer switch (ATS) and panels in the uninterruptible power supply (UPS) room. Did you mean in the generator room?

Vedvik: NFPA 110.7.2 does permit EPSS equipment to be located within the EPS room. Thanks for helping to clarify this.

Question: NFPA 70E would seem to affect clearances more than other codes. Comments?

Vedvik: The 2015 edition of NFPA 70E only contains five instances of the word “clearance.” NFPA 70E defines requirements for the execution of work while NFPA 70 defines requirements for equipment and spaces containing equipment. NFPA 70E Tables 130.4(D)(a) and (b) identify approach boundaries to live conductors for personnel. These requirements do not change the distances in NFPA 70 110.26 and 110.34 for clearances around electrical equipment. NFPA 70E Annex C.2.1 references NEC 110.26 and 110.34 for working spaces.

Question: What have you seen go wrong in electrical rooms?

Vecchiarello: Several things, including:

·        Equipment purchased is larger than anticipated.

·        Systems foreign to the electrical installation encroaches on the dedicated equipment space and working space.

·        Water leaking on electrical equipment.

Question: How do you best to provide the vertical separation between life safety/critical/normal power?

Vedvik: My preferred method is separate vertical sections of switchgear or switchboard construction. This allows for a common horizontal bus for tap integrity and standard equipment selections. I identify the separate vertical sections on the one-line diagram and also show the separate sections on the floor plan to coordinate with room size. I have also used a tap enclosure made from switchboard construction and bus to allow conductor taps (90°C) for separate distribution panel feeds, but only when equipment depth is of concern.

Question: Is drywall considered a combustible?

Vecchiarello: Per NEC NFPA 70 Section 314.20, gypsum is considered noncombustible for flush-mounted device box installations. For transformers, see informational note to Section 450.21(B), which references ANSI/ASTM E119-15. 

Question: Are there any U.S. Green Building Council LEED requirements for LEED-rated buildings?

Vedvik: LEED has points available for metering, and metering sections will increase equipment size as the location of the current transformers (C.T.) will add physical size to the distribution equipment.

Question: Is an oil-type transformer allowed in U.S. Green Building Council LEED-rated buildings?

Vecchiarello: Designer should review current version of LEED that is being applied to project. At present, LEED does not provide any prerequisite requirements regarding electrical transformers.

Question: Are there reductions of requirements with gas-insulated switchgear (GIS)?

Vecchiarello: If you are asking about working clearances, there are no exceptions for GIS in the NEC that allows a reduction of working clearances. However, GIS-type switchgear generally has a smaller footprint than air-insulated switchgear.

Question: What rules of thumb can be used for determining when switchgear is to be in the middle of the room versus along a wall as you showed in your examples?

Vedvik: The examples identify coordination with structure for footings and beams, which may require the equipment be pulled away from the wall if bottom-fed. This coordination is best done early in design so the location of the electrical room and any structural impacts are identified early. Some types of switchgear require front and rear access. When rear access is required, the switchgear should be in the middle of the room and the requirements of NEC 110.26 and 110.34 apply to both front and rear. Usually, switchboards are front connected and switchgear is front and rear connected. High-voltage switchgear usually requires front and rear access depending on the type. Unit substations may or may not require rear access as it is a combination of the items above. It is important to understand the equipment type desired by the owner early in design.

Question: Where and how are the fire ratings of electrical rooms determined?

Vecchiarello: It’s a combination of the NEC and building codes, and is dependent on the kind of electrical equipment and the rating of equipment, fire protection means, and building type.

Question: Can you briefly discuss the criteria/determining factor for selecting arc fault resistant gear?

Vedvik: Consulting-Specifying Engineer magazine just had an article discussing arc fault resistant gear.

Question: Is there a rule of thumb that can be used to determine a rough electrical equipment room size based on total building square footage?

Vedvik: I discussed sizing electrical rooms based on watts/sq ft as published in RS Means. I also noted other large electrical loads to take into consideration, such as mechanical equipment, kitchens, data centers, electric heating, etc. Determining the service size (voltage and amperage) early in design requires some knowledge of the building use and intended systems. Sizing the electrical room early in design encompasses the rest of the presentation as well. 

Question: Can 112.5 kVA transformers and larger be installed out on the building open floor, not in a room?

Vecchiarello: Per NEC Section 450.21(B), transformers rated larger than 112.5 kVA shall be installed in a transformer room with a minimum of 1-hour fire rating.  However, a transformer that meets the requirements of either Exception 1 or 2 of Section 450.21 (B), is not required to be installed in a transformer room or vault.

Question: Even a small electrical room with branch circuit panels requires a door to egress outward?

Vecchiarello: Personnel doors are required to open in the direction of egress if the equipment is 1,000 V or less and rated greater than or equal to 800 amps, per NEC Section 110.26(C)(3), or if the equipment is rated more than 1,000 V, per NEC Section 110.33(A)(3).