This doesn't seem up to code / safe (small gauge wire to 100 amp breaker in an outbuilding)

greeneggsandcrack · Jun 28, 2023

Last year we purchased a home that was built in the late 1990s It has one outbuilding, sort of a barn/shed about 160 feet from the house (and probably closer to 280' from the utility connection on the far side of the home), which has a breaker box in it supplying three 20A circuits (lights on one, and two sets of GFCI protected outlets).

I added some storage units on the far side of the barn. There are no outlets there, so I ran some 12-2 Romex to the area and fed it through the existing conduit into the breaker box. And there's where I noticed something that seemed off to me.

I am not an electrician but feel I'm a reasonably competent DIY'er for basic things like adding circuits for outlets, lights.

The main breaker says 100 A but the wires supplying the breaker box seem awfully small. They look like they're about 10 g to me, though I didn't put calipers on them. Looks like a direct burial line with two hot, a neutral, and a ground.

There's a photo below of the breaker box. The new Romex not connected to anything is the line I just ran.

Can someone explain to me how this might be OK, or if it's definitely wrong? I don't see how it's possible those wires could supply several 20A loads at the same time. Especially considering the distance that underground line has to go. It doesn't seem to originate from the home's breaker boxes (at least, I don't see anything with a label there). I'm not sure where it connects to utility power.

Overall the house is exceptionally well built. We did some renovations right after we bought it, and the electrician commented on the quality of the work, how meticulous the organization was, etc. It seems weird that there'd be a big error in the way the outbuilding was wired. The existing 20A outlets work just fine. I haven't really put any serious loads on them though, just battery chargers for a few outdoor tools.

Edit - I tried embedding a higher resolution photo but the forum has a size limit, and wouldn't let me embed/hotlink one from another server. Hopefully this is good enough.

SteveInMex · Jun 28, 2023

Greeneggsandcrack,

You can find your answer by using a "voltage loss/voltage drop" calculator. Basically, such a calculator will help you to determine how large/small wire is required for any combination of distance, diameter, type of wire and expected/maximum load.

Note, some of the different sites have more or less complicated calculators, look at several to find one that suits you needs/information available.

You may find one that you input the size and type of wire, voltage and length, and it will tell you the maximum safe load in amps.

Another will let you input the maximum amps/load you will put on the circuit, the length and type of wire and it will tell you the size of wire you need.

Another will tell you the amount of loss of voltage over the distance.

Here are some examples you might try...

Voltage Drop Calculator | Southwire - https://www.southwire.com/calculator-vdrop

Voltage Drop Calculator - https://www.omnicalculator.com/physics/voltage-drop

Voltage Drop Calculator - http://wiresizecalculator.net/calculators/voltagedrop.htm

electrician2.com voltage drop calculator - http://www.electrician2.com/calculators/vd_calculator_initial.html

greeneggsandcrack · Jun 28, 2023

Thanks, I've seen and used those calculators.

What I can't get my head around is the incongruity between the size wire the calculators tell me is needed, and what I see, in what appears to be a functioning system.

The outlets work, and I measured 123V at them (no load, and with ~300W of lights going). So it seems that everything is functioning well.

I think I just need to have an electrician come and physically look at it, but I was hoping someone here would be able to tell me with some certainty that yes it's OK or no it's not.

Thanks again.

Megawatt · Jul 1, 2023

greeneggsandcrack said:
Last year we purchased a home that was built in the late 1990s It has one outbuilding, sort of a barn/shed about 160 feet from the house (and probably closer to 280' from the utility connection on the far side of the home), which has a breaker box in it supplying three 20A circuits (lights on one, and two sets of GFCI protected outlets).

I added some storage units on the far side of the barn. There are no outlets there, so I ran some 12-2 Romex to the area and fed it through the existing conduit into the breaker box. And there's where I noticed something that seemed off to me.

I am not an electrician but feel I'm a reasonably competent DIY'er for basic things like adding circuits for outlets, lights.

The main breaker says 100 A but the wires supplying the breaker box seem awfully small. They look like they're about 10 g to me, though I didn't put calipers on them. Looks like a direct burial line with two hot, a neutral, and a ground.

There's a photo below of the breaker box. The new Romex not connected to anything is the line I just ran.

Can someone explain to me how this might be OK, or if it's definitely wrong? I don't see how it's possible those wires could supply several 20A loads at the same time. Especially considering the distance that underground line has to go. It doesn't seem to originate from the home's breaker boxes (at least, I don't see anything with a label there). I'm not sure where it connects to utility power.

Overall the house is exceptionally well built. We did some renovations right after we bought it, and the electrician commented on the quality of the work, how meticulous the organization was, etc. It seems weird that there'd be a big error in the way the outbuilding was wired. The existing 20A outlets work just fine. I haven't really put any serious loads on them though, just battery chargers for a few outdoor tools.

Edit - I tried embedding a higher resolution photo but the forum has a size limit, and wouldn't let me embed/hotlink one from another server. Hopefully this is good enough.

View attachment 109088

You are correct about that wire being # 10 and it actually needs to be # 3 copper for a 100 amp breaker. At the footage you explained you definitely have a voltage drop also.