Two Circuit Shed Wiring

TXTroyJuly 18, 2011

Hey All,

I'm wiring a small outdoor workshop (Structure separate from the house) with two 20 amp circuits. Now, I know I need to have a master cut-off power switch through which all my power enters the workshop.

My question is.....If I have two 20 amp circuits entering the shed, do I have to have one or two switches for the master switch(s) upon entering the workshop? If just one, does anyone know what type of capacity or type of switch can handle two 20 amp circuits?

Thanks in advance, Troy

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kurto

Well, the first problem is that you're only allowed a single branch circuit (or feeder) to your workshop. If you need two circuits (or more), you'll need to put in a small subpanel. If you have 6 or fewer breakers in the subpanel, they can serve as your disconnect. If more breakers, then use a panel with a main to serve as your single disconnect.

    Bookmark   July 18, 2011 at 5:27PM
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TXTroy

Hmm, Interesting. Do you know the logic behind this and how confident are you in your notion that two circuits cannot be used without a subpanel?

I had an electrician out to estimate the job and he only mentioned a master switch and nothing regarding a possible subpanel.

Thanks for the response

    Bookmark   July 18, 2011 at 6:11PM
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saltcedar

There are no electrical codes in many unincorporated
areas in TX. So if this applies to you that may explain
the discrepancy. The incorporated areas are under NEC
and Kurto's advice applies.

    Bookmark   July 18, 2011 at 6:56PM
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brickeyee

Two 120 V, 20 amp circuits can be run if they are part of a multiwire branch circuit since that counts as a single circuit.

it means that each of the 20 amps circuits are on different legs of the 120/240 V service, and a single double pole breaker is used to feed the circuit.

A single double pule switch can be used if you want a single switch to kill all the power to the shed (even in the shed).

You should be aware theta the branch circuit to the shed would no be a 120/240 V circuit.

Any loads from either hot leg to the neutral will be 120 V, while 240 V would be available from hot-to-hot.

    Bookmark   July 18, 2011 at 6:59PM
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TXTroy

Ah,

So I have to use two 12/2 wires on a single double pole breaker connected to a single double throw switch.......

and the possibility of using a single 12/3 cable to feed both circuits isn't code "wise" right? I've seen wiring diagrams illustrating this possibility.

Thanks!

    Bookmark   July 18, 2011 at 7:09PM
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kurto

A multiwire circuit is okay per the NEC, but I wouldn't recommend it as my only source of power in a workshop. Since it's a single circuit, the breaker handles must be tied together. That means that if you lose power on either leg, it takes out both legs. The problem in a workshop is that when your saw works too hard and throws the breaker, the lights will go out as well. Nothing like waiting for a saw to slow down in the dark.

    Bookmark   July 18, 2011 at 9:11PM
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TXTroy

Thanks Kurto,

Let me clarify a bit....this will be more of a craft-y workshop and not so much a wood or metal type workshop in the traditional sense of what most people may think of. Basically, two twenty amp circuits is more than plenty when it comes to usage here for my situation. There are only going to be 6 outlets and 3 light fixtures on both these circuits. In fact, I may have been able to get by on only one 20 amp circ.

Having said that, would you still recommend a subpanel? I'm really trying to keep this as simple as possible and wondering the best possible wiring scenario.

To recap.....I'm wanting to run two 20-amp circuits to an outdoor shed/workshop and wondering what type of breaker, wire, and master cut-off switch to use.

    Bookmark   July 18, 2011 at 9:57PM
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hendricus

What brickeye said;
3 wire #12 with a 20 amp double pole breaker at the main panel.
Run another 20 amp double pole breaker in the shed.

    Bookmark   July 18, 2011 at 10:08PM
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kurto

Given the additional information, I guess I'd save the money and effort by installing the multiwire branch circuit. The trade off is that if you decide to do something in the future like install a table saw that requires 240V, you'll be starting over with your electrical plans.

Be sure to include a GFCI protection in the circuit.

    Bookmark   July 19, 2011 at 8:23AM
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brickeyee

"and the possibility of using a single 12/3 cable to feed both circuits isn't code "wise" right? I've seen wiring diagrams illustrating this possibility. "

Perfectly acceptable.

Since the hots are on opposite legs they can share a neutral.
The neutral only carries the difference in the current on the two hot lugs.

You do need to make sure the "cable" is rated for wet locations since underground conduit is wet by definition and use in a conduit system.

Separate THWN wires are a lot easier to pull.

    Bookmark   July 19, 2011 at 10:02AM
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TXTroy

One more follow up here.....

Because I've got a lot of elbows in my conduit run from the house to the shed, I'm thinking about using THHN or THWN wire instead of UF (I'm pulling the wire through conduit under a deck for the first portion of the run from the house and then going underground from the deck to the shed for which I'll use UF for that). Does anyone know which would be best for this application?

Thanks

    Bookmark   July 19, 2011 at 11:52AM
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hendricus

The trade off is that if you decide to do something in the future like install a table saw that requires 240V, you'll be starting over with your electrical plans.

With a multi-wire branch circuit you have the 240 right there. I wired my previous garage with a MWBC and wired up a 240v outlet between the garage doors so in the summer I could take my radial arm saw out in the sunshine.

    Bookmark   July 19, 2011 at 11:53AM
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hendricus

W for wet

    Bookmark   July 19, 2011 at 11:56AM
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TXTroy

Sorry Brick, didn't see your last post before I posted mine. Question is still there though.

The electrician who estimated the job said he would use THHN for the run under the deck, then transfer to UF. My only idea for using THHN would be for the higher temp rating as compared to THWN because the wire isn't underground (therefore subject to more heat build up). But I read THHN is rated for damp locations, just not wet like THWN.....Any ideas?

Would THWN be the best all around for the "under deck" run as well as the underground portion (i'm attaching the conduit to the deck, not in the gound, under the deck)? Does it matter at all (wire choice)if I'm installing a small junction box at the outer edge of the deck?

    Bookmark   July 19, 2011 at 12:02PM
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normel

Use THWN for the entire run. There is no need to switch to UF. Just means the conduit will have to extend all the way from the house panel (or junction box) to the disconnect at the shed.

    Bookmark   July 19, 2011 at 3:50PM
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