Are you ready to chase your tail and keep going around and around the way a dog or cat does? That may happen when it comes to figuring out the rules for multiwire branch circuits on construction sites. The last sentence of Sec. 590.4(D)(1) seems pretty straightforward. It states, “Receptacles on construction sites shall not be placed on any branch circuit that supplies temporary lighting.” Some would say that this also includes multiwire branch circuits because the Art. 100 definition defines a multiwire branch circuit as a single circuit. Others, however, would point to Sec. 210.4(A), which clearly states a multiwire branch circuit can be considered multiple circuits as well. So where does this leave us for applying the rules in Sec. 590.4(D)(1)? Is a multiwire branch circuit a single circuit or is it multiple circuits?
The last sentence in the following OSHA rules seems to imply that using a multiwire branch circuit would be permitted if arranged correctly. “1926.405(a)(2)(ii)(C): Receptacles shall be of the grounding type. Unless installed in a complete metallic raceway, each branch circuit shall contain a separate equipment grounding conductor, and all receptacles shall be electrically connected to the grounding conductor. Receptacles for uses other than temporary lighting shall not be installed on branch circuits which supply temporary lighting. Receptacles shall not be connected to the same ungrounded conductor of multiwire circuits which supply temporary lighting.”
What about when a fault occurs? Breakers with independent-trip poles are available. Single-pole breakers with handle ties are permitted by Sec. 240.15(B)(1). Here’s something else to consider: Using a common trip breaker on a multiwire branch circuit would “essentially” be the same as having the receptacles on the same phase as the lights since both would be shut off in the event of a fault. However, some would say using that type of breaker is not prohibited by the NEC or even the OSHA rules, so long as the receptacles and lights were not on the same phase. Yet others will take the position that Sec. 590.4(D)(1) states: “any branch circuit”...including multiwire!
I think this rule (as currently written) could easily have “a line drawn in the sand” with strong arguments on both sides due to the present wording. It would probably be a good idea to discuss this issue with your AHJ to come up with a circuit arrangement that works best for all parties involved.
