Planning Electrical for a Barn!

[Mike Gay]

I'm hoping to build a barn this year on our 30 acres for our Prevost coach, tractors boats, hunting gear, and other toys. It will probably be around 60'X40' with 16' walls. Instead of running additional 600 feet of underground high voltage to a transformer at that site, It occurred to be that I can trench from my home transformer to the point of entry for the barn.

My late father was an electrician, but he's not longer around. It looks like I'd need to run cable about 400-420' underground to the barn, and I could trench most of that. The transformer option would probably require using a rock saw for that 600' route, so this is looking like a better option.

We moved our service entrance and meter in 2019, so we have a new transformer and 3 new 200-amp panels at the transformer.

For the barn, I'll have some lighting and normal 120 volt outlets scattered around with 3 electric garage doors. I'll install 2 50-amp RV plugs, one indoors and one outdoors in case we have a party and want to move our coach outdoors. That would also help if we have a guest who needs a 50 amp.

If any of you are knowledgable on cable sizes and calculating voltage drop, I'm wondering what size wire will be needed on such an underground run. Thanks!

 

[pattenp]

Shooting from the hip I'd say 100A feed using 4/0 MHF.

 

[13mo]

I'm hoping to build a barn this year on our 30 acres for our Prevost coach, tractors boats, hunting gear, and other toys. It will probably be around 60'X40' with 16' walls. Instead of running additional 600 feet of underground high voltage to a transformer at that site, It occurred to be that I can trench from my home transformer to the point of entry for the barn.

My late father was an electrician, but he's not longer around. It looks like I'd need to run cable about 400-420' underground to the barn, and I could trench most of that. The transformer option would probably require using a rock saw for that 600' route, so this is looking like a better option.

We moved our service entrance and meter in 2019, so we have a new transformer and 3 new 200-amp panels at the transformer.

For the barn, I'll have some lighting and normal 120 volt outlets scattered around with 3 electric garage doors. I'll install 2 50-amp RV plugs, one indoors and one outdoors in case we have a party and want to move our coach outdoors. That would also help if we have a guest who needs a 50 amp.

If any of you are knowledgable on cable sizes and calculating voltage drop, I'm wondering what size wire will be needed on such an underground run. Thanks!

420 feet is a very long way. I would carefully calculate your anticipated loads as voltage drop will be a big issue. In particular, knowing how much power an RV actually draws from "shore power" would be helpful as even though you may connect it to a 50 amp receptacle, it may draw close to that much power, or it may draw far less.

Best practices are to keep voltage drop to less than 3% between the meter/transformer and the receptacle. A previous poster mentioned using the widely available and inexpensive 4/0 mobile home feeder cable. Normally this is rated for 180 amps for an outbuilding (and it will do that just fine out to about 150 feet) but at 420 feet, you will have a voltage drop of more than 3% if you draw more than about 70 amps at 240 volts. I suspect you'd probably be fine with that setup as generally actual power draws are far less than what we all guesstimate, but a good accurate load calculation will be crucial here.

 

[theoldwizard1]

I'll install 2 50-amp RV plugs, one indoors and one outdoors in case we have a party and want to move our coach outdoors. That would also help if we have a guest who needs a 50 amp.

Shooting from the hip I'd say 100A feed using 4/0 MHF.

Pattenp is correct if you want TWO 240V @ 50A outlets. Dropping down to TWO 120V @ 30A outlets would save a lot of $$$ ! Then you would only need a 60A feed (2/0 ?).

You could wire ONE of them as 240V @ 50A, but when you have guests, you won't be able to run your second A/C !

 

[theoldwizard1]

Best practices are to keep voltage drop to less than 3% between the meter/transformer and the receptacle.

In a perfect world ! 7% is "acceptable".

 

[pattenp]

At a full 100A load on the 4/0 AL at 420ft should be around 3.6% VD which is no big deal.

 

[alfredeneuman]

7% is "acceptable".

What leads you to think that? Have you some secret inside information that the rest of us don't have access to?

 

[dcg9381]

420' is pretty long. There are calculators online that will indicate wire type for a run that long. I'd target 90-100A (whatever reasonable cable is closest). Here, we run it in 3" conduit... I'd have a plan to pull it that far and make sure it's straight.

I have 90A at my 40x60 shop. I have 4 RV connections, 4 tons of AC in the shop. As you probably know, a 50A RV is really (120V * 50A = 6000 watts peak), but 50A RV connections are 240V * 50A = 12,000 watts, so you can really get 2 50A RVs on a typical pedestal.... We've run 3 RVs here, no problem with the shop AC...

 

[theoldwizard1]

What leads you to think that? Have you some secret inside information that the rest of us don't have access to?

POCOs ! You be lucky to get +/- 10% from them !!

 

[theoldwizard1]

... but 50A RV connections are 240V * 50A = 12,000 watts, so you can really get 2 50A RVs on a typical pedestal.... We've run 3 RVs here, no problem with the shop AC...

I think the RV industry went crazy when they "standardized" on 240V@50A. Not many large Class A owners are going to be running a BIG welder or a compressor and plasma cutter !

 

[wyliesdiesels]

420 feet is a very long way. I would carefully calculate your anticipated loads as voltage drop will be a big issue. In particular, knowing how much power an RV actually draws from "shore power" would be helpful as even though you may connect it to a 50 amp receptacle, it may draw close to that much power, or it may draw far less.

Best practices are to keep voltage drop to less than 3% between the meter/transformer and the receptacle. A previous poster mentioned using the widely available and inexpensive 4/0 mobile home feeder cable. Normally this is rated for 180 amps for an outbuilding (and it will do that just fine out to about 150 feet) but at 420 feet, you will have a voltage drop of more than 3% if you draw more than about 70 amps at 240 volts. I suspect you'd probably be fine with that setup as generally actual power draws are far less than what we all guesstimate, but a good accurate load calculation will be crucial here.

not quite.

the NEC recommends 3% VD for feeders and 5% for branch circuits.

 

[dcg9381]

I think the RV industry went crazy when they "standardized" on 240V@50A. Not many large Class A owners are going to be running a BIG welder or a compressor and plasma cutter !

I guess it covers the million dollar rigs that are running 4 ACs.

It's very confusing because "mere mortal" RVs aren't 240v rigs - nor do their onboard generators provide 240v... They're just 2 x 25A x 120V rigs... More than enough power in a 50A RV pedestal to run 2 big RVs.