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Code check for MWBC to shed

Posted by Whiterabbitttttt (My Page) on
Tue, Jun 11, 13 at 19:09

Hello,

I have some questions about a wiring project I want to make sure is code-proper. As background for me, I work with and design electrical for industrial equipment. So ampacity, signals, complex wiring, 3-phase, it's all cake for me. But we all know a little knowledge is a dangerous thing, and that proper for light manufacturing is not proper for residential. So bear with me please. I have personal and fire safety covered, but it basically ends there. Thank you!

My shed is a simple affair, with normal stud construction but sits on concrete plugs. It is not a permanent structure. Inside this shed I will locate a small compressor (30 gallon) and a dust collector (1.5 HP) and nothing else. It's dedicated for these appliances. I figure roughly 12 amps draw for each device, peak. Maybe less than 10 for the compressor.

What I want to do is run some 12-3 from a 20A 2-pole normal breaker in the main panel to the shed to power two 20A GFCI outlets in a 2-gang box inside the shed. Plug one appliance into each line. Maybe plug an incandescent bulb fixture to the compressor side too.

I know there are normally no issues if I were putting in a light and a dorm fridge, but given this is two pieces of heavier machinery than a 60 watt bulb I thought I'd run it by the experts.

I should be conceptually set via code, yes? Or am I?

Could I also do something crazy like run the 12-3 from the main to a J-box through two 20A light switches THEN to the shed, so I could (theoretically) turn on the compressor or DC independently from inside the house?

Thanks!

-S


Follow-Up Postings:

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RE: Code check for MWBC to shed

Auxilliary buildings at or below grade require GFCI.
Other than that you are fine.


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RE: Code check for MWBC to shed

Rats, so I can't get away with GFCI at the shed? I have to go to a GFCI breaker? I ask because GFCI receptacles will cost 20% of what a 2-pole GFCI breaker will cost in SqD QO.

Are there other lower cost alternatives to ensure the line is GFCI-ok in a code friendly way? Or am I stuck paying the $104 for a 2-pole beaker?


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RE: Code check for MWBC to shed

"I figure roughly 12 amps draw for each device, peak. Maybe less than 10 for the compressor. "

Sounds a little low for start-up, even if you have them wired for 240 V (a good idea if they support it and you want a longer life from them).


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RE: Code check for MWBC to shed

You might be right. I'm confident the compressor jobber will be more than OK, but I have no idea what start-up draw is on the DC, and it could be high. That's just one of a couple reasons I had planned on running a 20A circuit out there rather than a 15.

Another option is rather than routing to a J-box wtih two outlets, running to a small sub-box with three breakers and re-wiring the DC to 240 (I DO have that option). I don't want to because of the added expense of the sub-subpanel, particularly if a 20A line will cover me.

The DC is Penn, I do assume the motor ratings are a bit liberal. Still, 1.5hp is 1.5hp. But it's currently on a 12ga 20amp line inside and no issues though.

Given that, and assuming I am less concerned that 120 vs 240 may result in shorter life, do you believe I'm good to go?


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RE: Code check for MWBC to shed

OK, I have the design on paper and I am happy with it. I just need to solve the GFCI question. Two questions:

Does GFCI need to be at the source (load panel) or can it be at the destination?

And if it has to be at the source, can I use a standard 2-pole breaker, route to two GFCI like this:

http://www.homedepot.com/p/t/202026837?storeId=10051&langId=-1&catalogId=10053&productId=202026837&R=202026837

then continue on it's way out to the shed? does that satisfy the GFCI code requirement? (better if I can put the gfci right at the shed)

I ask because the $15 home depot GFCI devices save me $44 (almost half) over buying a 20A 2 pole GFCI breaker.

Thanks guys :)


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RE: Code check for MWBC to shed

Two pole GFCI at the house end.

It is there to protect the lines since there is no panel in the out building or ground rods (the typical requirement for a separate structure).


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RE: Code check for MWBC to shed

Sounds like a std breaker then an inline GFCI would be OK then. All that would be inside the house. Would not leave the house except protected by GFCI, then in the shed as per normal. Only difference is GFCI outside the load panel as opposed to inside.


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RE: Code check for MWBC to shed

Sounds like a std breaker then an inline GFCI would be OK then. All that would be inside the house. Would not leave the house except protected by GFCI, then in the shed as per normal. Only difference is GFCI outside the load panel as opposed to inside.


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RE: Code check for MWBC to shed

A regular GFCI receptacle will not work on a MWBC.

You need a 2 pole GFCI breaker to support a MWBC.


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RE: Code check for MWBC to shed

Either a GFCI with load neutral in the panel can be used or a receptacle after you split it from a MWBC to single circuits. That is, the protected side of the single pole GFCI can not share a neutral with another circuit (even the other side of a MWBC).


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RE: Code check for MWBC to shed

You cannot feed two circuits to a separate structure.

The MWBC counts as one circuit, but you need the GFCI protection on both the 120 and 240 pair.

I think an AHJ would be well within his rights to reject using separate GFCIs and then trying to claim a single circuit.


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RE: Code check for MWBC to shed

The requirement is not to provide GFCI for the entire circuit. Only the receptacles (15/20A 120V) are required to be protected. He can run a MWBC to the structure and place GFCI protected receptacles on it.


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RE: Code check for MWBC to shed

Would this situation be made easier if I install a load center inside the shed (seeing as the cost of a very small load center is minimal)? or would this change nothing about the GFCI requirement on the home load center side?


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RE: Code check for MWBC to shed

Eh?

The only issue with the one-circuit requirement is whether it excuses you from installing a grounding system at the out building.

You need GFCI's regardless. They can be in the panel or as receptacles. Other than being careful what you do with the load side (i..e, to protect further recetpacles in the circuit), there's no real issue with MWBC versus separate circuits.

I disagree with BRICK with regard to splitting the MWBC into two circuits downstream.


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RE: Code check for MWBC to shed

"I disagree with BRICK with regard to splitting the MWBC into two circuits downstream. "

You can split once it gets into the separate structure, just not before.


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RE: Code check for MWBC to shed

"I think an AHJ would be well within his rights to reject using separate GFCIs and then trying to claim a single circuit"

The NEC describes a multi wire circuit as being a line to neutral load only configuration - then giving exceptions as to what needs to be done to allow line to line loads. Since the NEC doesn't require GFCI protection on a 240 volt circuit, a 240 volt receptacle (or two) could be installed ahead of two 120 volt GFCI receptacles.

Either way, the multi wire circuit is defined as one circuit whether the 120 volt loads are two single GFCI receptacles or a combination of other stuff. Once you get passed the disconnect in the building (needs to be a two pole switch) you can split things to your heart's content.


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RE: Code check for MWBC to shed

wait, I was under the impression that the disconnect could be two single pole switches? I was misinformed? I was told the BREAKER had to be two pole to be a MWBC, but the disconnect inside the house could be two single pole switches.


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RE: Code check for MWBC to shed

OK, I see now that double pole snap switches are relatively easy and inexpensive to source. So I am all set there. But now I have a REALLY dumb question.

So, I'll have J-box in the garage feeding a conduit body with .75 EMT 6" down to shed with another conduit body to a JB with a blankoff. From there it'll get piped in EMT to a mid height JB with a SINGLE double pole switch, 20A. From there piped by EMT to yet another JB with a dual 20A outlet.

Here's the dumb question. The wiring that feeds this from the garage. Can I run it through a J-box through two separate 20A single pole switches? This would act as a secondary disconnect from inside the house. It would let me turn appliances inside the shed on and off remotely.

allowed?


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RE: Code check for MWBC to shed

NEC 225.38(B) Simultaneous Opening of Poles.

Each building or structure disconnecting means shall simultaneously disconnect all ungrounded supply conductors that it controls from the building or structure wiring system.

The breaker needs either handle ties or to be a 2 pole breaker, and the switch used for a disconnect in the shed needs to be 2 pole. Beyond that you can pretty much do what you want.


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