Your questions answered: How to design electrical distribution systems

In the webcast “How to design electrical distribution systems,” some items were left unanswered. Tom Divine tackles several here

By Tom Divine, PE, LEED AP July 19, 2022
Courtesy: Consulting-Specifying Engineer

Electrical and other engineers should 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.

During the June 30, 2022, webcast “How to design electrical distribution systems,” some items were left unanswered. Presenter Tom Divine, PE, Senior Electrical Engineer, Johnston, LLC, Houston, answers several questions here.

Question: What do you think about engineers who use demand factors (or diversity factor) that are not from NFPA 70: National Electrical Code to size service ampacity?

Divine: I think that they’re well-advised to proceed with caution. I don’t find any support in the NEC for the notion that engineers can substitute their judgment for the load calculations prescribed in Article 220 and elsewhere. I don’t find that authorities having jurisdiction generally subscribe to that notion, either.

Question: Is there an equivalent to lighting panelboards, but with fuses? Are they actually cheaper than breaker panels?

Divine: They’re available, but they’re not cheaper. At least one manufacturer provides fused panelboards with up to 42 branch positions. They are considerably more expensive than equivalent circuit breaker panelboards.

Question: How do you specify an interrupting rating if the service utility will not know its transformer’s impedance until it is installed?

Divine: A conservative approach might be to calculate interrupting ratings with the utility transformer sized commensurate with the rating of the service equipment. That would result in a very high calculated fault current, probably much higher than would be achieved over the life of the facility.

Question: What’s the pros and cons of providing main breaker on a sub-panel if upstream feeder already has a breaker?

Divine: Some owners want main circuit breakers in a panel remote from their feeds in order to provide a local means of deenergizing the panelboard for maintenance or additions. It’s worth noting that a main circuit breaker can deenergize a panel’s bus, but can’t entirely de-energize the panel’s interior — the main circuit breaker will still be hot on its line side. An argument against this usage would be that circuit breakers aren’t intended to be used as switches and may fail early if operated often. A potential solution is to provide molded case switches — essentially circuit breakers without a trip function — at panelboards, in lieu of main breakers. Whether that use will improve longevity will depend on details of device construction.

Courtesy: Consulting-Specifying Engineer

Question: Does a transformer need working space?

Divine: It’s often debated among engineers and contractors. NEC 110.26(A) declares that working space is required for equipment that’s “likely to require examination, adjustment, servicing or maintenance while energized.” Some claim that transformers are unlikely to require servicing while energized; others note that thermal imaging requires that equipment be energized during examination. An owner may prohibit opening a transformer while it’s hot, but AHJs are unlikely to accept an ephemeral workplace rules as valid reasons to abrogate the working space requirement. In my opinion, transformers require working space for the highest voltage present, in front. It wasn’t asked, but it’s worth noting that transformers don’t require a dedicated space above or below.


Tom Divine, PE, LEED AP
Author Bio: Tom Divine is a senior electrical engineer at Johnston, LLC. He is a member of the Consulting-Specifying Engineer editorial advisory board.