electric wire gauge help

[dug]

Here is the situation-

Power comes to our property at a pole with meter and load center. I am using 2 gauge aluminum wire (comparable to 4 gauge copper) 160 feet underground to a sub panel at my house. We thought there would be enough room at the main load center on the pole to install a 100 amp breaker to power the house but found out we couldn't.

After going over some options I decided to remove 2 breakers at the load center that currently power a trailer with two 20 amp circuits. I am then going to back feed to the sub panel at the trailer from the sub panel at the house (also about 160 ft.), using the same trench I dug for the 2 gauge wire. Currently the trailer is fed with 8 gauge wire to a large outlet receptacle, and the sub panel plugs into that using 10 gauge wire.

My friend and mentor, a licensed electrician I work for, said I could use UF 10 gauge to feed back to the trailer. I didn't think much about it and bought all the supplies but after looking at it the next day I think he may have been mistaken about the wire size. He is very competent and experienced but was under a lot of stress that day because he was trying to wrap everything up in preparation for a month long vacation starting the next day.

So now he is gone for a month, and I wouldn't dare interrupt someone's vacation for a work related question but I really need to finish this and fill in the trench, and I don't want to mess up on the wire size!

One other thing- my friend says the power we receive in our particular area is closer to 135 volts, almost 15% above the norm. At that, my best (and most likely poor) calculation says I should get just over 120 volts back to the trailer- so maybe he was right.   

O.k., last thing (feeling a little like Columbo  )  I understand that sub panels are usually fed with 2 hot wires, but the one at the trailer plugs into a receptacle fed with one hot. Is this normal? Or should I be running ?/3 wire back to the trailer rather than ?/2 wire? Trailer was wired by a (different) licensed electrician about 5 years ago.

[muldoon]

I am trying to follow, does this look about right?



You are asking if you can use the subpanel in the house to feed the subpanel in the trailer? 
Also, it sounds as though the trailer is wired for 110 volts and not 220. 

Is that correct? 

[dug]

That's it exactly, except the trailer is right near the pole. Trailer is wired for 110.

[muldoon]

Almost clear, you say the trailer is wired with a single hot to a receptacle; and elsewhere mention a subpanel in the trailer, then you also mention there were originally two 20 amp breakers for the trailer.  Perhaps I am not following, can you confirm there is an actual panel in the trailer?  Do you have two wires/circuits coming in? 

[dug]

Almost clear, you say the trailer is wired with a single hot to a receptacle; and elsewhere mention a subpanel in the trailer, then you also mention there were originally two 20 amp breakers for the trailer.  Perhaps I am not following, can you confirm there is an actual panel in the trailer?  Do you have two wires/circuits coming in? 

That's pretty much it. There is a large receptacle on the outside of the trailer that is connected to the main panel on the pole, one hot, one neutral, and ground. a standard outdoor sub panel plugs into this receptacle (not hard wired) using 10 gauge cable. There are two 20 amp breakers in this box that feed power to the trailer. I'm not sure why they didn't just wire from the sub panel to the pole and eliminate the plug in receptacle.   

So basically I'm unplugging the trailers sub panel from the main box in order to free up space for the 100 amp breaker to the house, then running power from the house sub panel back to the trailer.

[dug]

Thought I'd post a few photos, can't hurt.

Crappy main breaker box on pole-




receptacle that trailer sub panel plugs into-



trailer sub panel-



ditch to the house and sub panel-

[muldoon]

Dug,

From the description at the trailer I interpret this as you have a single 110 circuit feeding the subpanel in the trailer. 

The original post says "After going over some options I decided to remove 2 breakers at the load center that currently power a trailer with two 20 amp circuits. "

If you have two breakers at the main pole feeding the trailer, I do not understand how you end up with a single 110v circuit:

"one hot, one neutral, and ground. a standard outdoor sub panel plugs into this receptacle (not hard wired) using 10 gauge cable. "

What did the other breaker attach to? Were both breakers in the load center on the main pole in use by the trailer? 

[ScottA]

I was going to say the same but muldoon beat me to it. 1 hot is 1 hot. Why 2 breakers? That looks like a 240 plug.

[dug]

O.K. I messed up.  I went out and looked and also just noticed that I can see it on the picture posted here that The wire going to the trailer is only connected to one 20 amp breaker. The bottom one is empty.

I suppose my #1 question would be is will that 10/2 UF wire do the job from the house back to the trailer.

[muldoon]

ok, that makes sense. 

http://www.okonite.com/engineering/nec-ampacity-tables.html

NEC ampacity table says #10 wire is good for 30 amps, in alum is good for 25 amps.  So yes, #10 feeder is appropriate for a single 110v 20 amp run of that distance. 

As for 10/2 being appropriate, since you are only landing a single circuit, it should be no problem. 

[ScottA]

I agree.

[dug]

Thanks!  Now I can bury without worrying about it!