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I am planning on similar MWBC wiring in a new shop we are constructing (40x30 pole barn). I plan to use a double pole 20 amp GFCI breaker for each circuit in a 200 amp panel, and put at least one double gang box with one 15-20 and one 6-20 receptacle in each 8 foot bay along the interior walls, so on My question is: how many of these double gang boxes can I put on each circuit?

I will be the only person working in the shop, so only one tool that draws substantial power on a circuit will be on at a time.

I could put each double gang box on a dedicated circuit with a homerun to the panel, but I'd rather not run all that extra 12/3 wire unless required by code. I will have separate dedicated circuits for dust collection, air filtration, possibly for an air compressor, lights, overhead doors, and HVAC heat pumps.

Thank you in advance for any guidance.

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If you want to do it that way, it's a good method. But I'm /cringe at the thought of the cost of multiple 2-pole GFCI breakers.

If your state is still on NEC 2017, I would run non-GFCI 240V circuits. For the 120V circuits, MWBC is fine and I would use GFCI receptacles because of cost.

If on NEC 2020 or if I wanted GFCI, I would do probably one 240V circuit on one GFCI, and then MWBCs and GFCI receptacles on the 120V circuits.

I'm not particularly worried about breaker economy because panel spaces are dirt cheap (when you're buying the panel) so there's no reason to have scarcity around that. I would use a 24-30 space for a shop that size. (48-60 "circuits" which is a nonsense number).

Anyway, the answer to your question is there is no limit to the number of receptacles on a 20A circuit, unless you're in a commercial space, then the answer is twelve. For purposes of this count, this picture contains 1 receptacle

enter image description here

and this picture contains 4. (all allowed on 20A circuits).

enter image description here

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