Electrical run to shed/garage

shaneb · 01-02-2013, 06:16 PM

Hi folks, first off, let me say that I am pretty handy, but my no means a pro electrician. I am in the middle of building a small 8x16 office shed (office to me.... shed to the city) in the backyard. The building is a prebuilt Tuff-Shed that sits on grade. Inside there will be basic lighting, a computer (~3amps) and accessories and a small AC (5000BTU, ~6amps).

When we moved in the house we re-pulled the electric to the detached garage. We have 100 amp service at the house for a 1200 square foot home. It originally had electric stove, but we replaced with gas. At the same time we took down the aerial wires to the garage (three non-code 14 ga conductors) and replaced it with a 10/3 UF cable buried to the garage and put in a subpanel with 2 20 amp breakers. So there are two hot wires (one to each breaker) a neutral and a bare ground running from the house to the detached garage. In the garage there are two circuits. One feeds just the bench where a small saw and sander are located. The other feeds four fluorescent lighting fixtures. In the main panel, my handyman (who no longer works for me) convinced me at the time to use the existing 50 amp breaker from the stove to feed the garage through the 10/3. I'm uncomfortable with this.

So.... what I am looking to do is connect the office to the subpanel at the garage (this run would be less than 20 feet).

Questions:
1. I would like to replace the 50 amp breaker with something more appropriate. I think I read code to say that I need to put a double pole 20a breaker in place. 20amp on one 10ga hot, 20a on another 10ga hot. One 10ga neutral.

2. I can easily run the entire garage on one 20a circuit. It is all new 12/2 wiring BTW. There is rarely more than one appliance used at the time. And even if I did the load would be under 20a. We built in a lot of overkill there when I thought I might buy a large air compressor and never did. That would leave me an empty 20amp circuit to run to the shed office in the existing garage subpanel.

2. Do I need to install an equipment ground rod at the garage? There is not one currently, although it is grounded to the house equipment ground through the 10/3 bare ground. Do I need one at the shed. My understanding is that I do not since there is no subpanel.

3. My thought is to run a 10/2 UF feed to the shed from the 20a breaker in the garage subpanel. Would this be too much? Would I be better off running 12/2 UF? The distance is so short that the money wouldn't be much different.

Sorry for the long post... there's a lot to consider.

Thanks in advance,
Shane

Jim Port · 01-02-2013, 06:44 PM

The 50 amp breaker needs to be replaced with a 30 amp two pole breaker. If you do this you will need a subpanel in the shed to install the branch circuit breakers for the lights and receptacles. If you install a subpanel you will need a ground rod.

You could also install a 2 pole 20 amp breaker and skip the subpanel since the branch circuits would already be protected. With the 30 amp you need to reduce the branch circuit ampacity to 20 amp maximum.

The 30 amp feeder will give you more options down the road along with capacity for future equipment.

shaneb · 01-02-2013, 08:54 PM

Thanks Jim. So if I installed the 30a double pole at the house then I would need a second subpanel at the shed? I mean there is already one at the garage. Maybe I missed something.

If I put in the 30a at the house, then two 20s at the subpanel in the garage (one to shed, one to garage) it seems to me they would be protected?

And that's what I thought on the subpanel ground. I will install a rod at the subpanel ASAP.

Thanks,
Shane