Hot Topics: Wiring Sub Panel Questions

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Original Post: Wiring sub panel questions

Isaac Rummell - Member

The house we moved into has a little cattle barn with separate power coming in from the road with its own meter. I have a box on the outside of the house with 100 amp main that just feeds the outlets in the basement and the furnace. There's a 60 amp 220 breaker that is not being used. I talked to an electrician friend of mine and he said I could use that breaker to run power to my barn and just hook it into the main breaker in that panel. I know I have to remove the bonding screw in the sub panel in the barn—do I need to put a separate ground bar on it?

Also, he said to use 4-gauge wire but the closest I can find locally is 2-gauge that's put together in a bundle of three 2-gauge wires and four bare for ground. It's aluminum wire. Is there anything special I need to do since it's aluminum? I read you're supposed to put some kind of coating on it whatever you strip off. All I'm going to run in the barn is lights and a couple outlets for battery chargers.

pattenp - Member

Check the type of wire. If it's URD direct bury it is not to enter the structure and has to be terminated on the outside. Which calls for the question if you are direct burying the cable or using conduit underground?

Mobile Home Feeder (MHF) is commonly used and is direct bury approved and is allowed to enter the structure because of having the proper insulation requirements to be inside. If using conduit it is best to use either THWN or XHHW wire. When using aluminum wire it's advised to use antioxidant paste on the bare wire ends at the connections. #2 Al is good for up to 90 amps, so it sounds like that maybe overkill for your needs.

Isaac Rummell - Thread Starter

The wire is not direct bury. I am going to run it through PVC conduit. The wire is just under one inch diameter overall. I'm assuming I'll need to use like 1.5 conduit.

AFJES - Member

How many feet is it from where the power would come from the house panel to the barn? There may be voltage drop to consider.

pattenp - Member

If the cable is SER it is not to be installed underground in conduit. Please post the identification printed on the wire that you are thinking of using.

Isaac Rummell - Thread Starter

I think about 70 feet of wire will do it. It does say SER.

So I need wire that has the bare ground wire insulated? That's what I seem to find online. Lowe's has 3 wire cable twisted together so I'm assuming I would need to run a separate fourth wire.

pattenp - Member

Just to be clear, is SER the wire you planned on using underground in conduit? If so, then you need different wire for the portion that's underground.

If it's aluminum, yes the ground needs to be insulated. In SER the ground is bare and the outer jacket does not protect it from water and when underground it will turn to mush. You need wire that is approved for wet locations in conduit underground.

You can use SER above ground and transition via a junction box to individual wires of THWN or XHHW in conduit to go below ground.

Isaac Rummell - Thread Starter

I can get three wire twisted together for underground use. I'm assuming I'll need to run a fourth wire

pattenp - Member

Be careful of what the 3-wire is. Most of the twisted 3-wire cable is URD for power distribution and is not fire rated to enter the structure. So if you are planning to have this wire run into the structure it needs to be approved for interior use. And you do need the feeder to be four wires, two hots, one neutral, one ground. Check the three wire insulation rating that it's just not USE-2, but also carries the RHH/RHW ratings for interior use.

Isaac Rummell - Thread Starter

I found the wire at another local supplier. Thank you for pointing me in the right direction. Is there anything else I need to know for hooking up the sub panel. I know I have to remove the bonding screw in the sub panel.

Isaac Rummell - Thread Starter

The wire would have less than a foot inside. The panel is right on the wall.

pattenp - Member

All I can advise you of is the NEC states USE only rated cable shall be terminated outside of the structure.