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Old 02-20-2011, 07:30 PM   #1
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electrical service to new shed


want to run a subpanel to new machine shed from shop. Shop has 100 amp service. New shed is 140' from shop. How big of underground line do I need. Will only need some lights and plug-ins. Also small 220 v air compressor.

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Old 02-20-2011, 09:20 PM   #2
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electrical service to new shed


how much power do you use in your shop? 40 amps?
how many amps do you need for your compressor?
what other tools do you plan on running?
the problem is that you have a subpanel feeding a subpanel and what you have running to the first subpanel is as large aas you can go on to the second
You also need four wires a 2 hots,ground,neutral, and the ground an neutral have to be isloated in a subpanel they cannot be on the same bar like a reg service panel


Last edited by bluebird5; 02-20-2011 at 09:27 PM.
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Old 02-20-2011, 09:31 PM   #3
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electrical service to new shed


use a voltage drop calculator. You are allowed 3 percent drop to reg circuit and 5 percent to a to a circuit through a feeder panel
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Old 02-20-2011, 09:46 PM   #4
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electrical service to new shed


IIRC minimum sub panel size is 30a on a 4w 120/240 service.


@OP the "size" is the final calculation you make when designing a service.

You start with your Load requirements at the sub panel. Compressor, lights, outlets etc. Also figure for possible expansion, you dont want to redo this do you? What ever you figure your load is or will be, I STRONGLY suggest either rounding up to the nearest 10 amps OR adding 10 amps and rounding to the 10s place.

Example: 32 amp load, I would do 40 amp service. 38 amp load, I would do 50.....

Then figure your circuits and spaces required. (At this point you can shop for your service panel and equipment)

Next you get your distance. 140 is what you said; this is a feasible system that leaves you do reasonably put in whatever amperage you want to up to 100. (as you posted your origin panel main is)

So now you calculate voltage drop with your Main and standard wire size for that amperage. If it is greater than 5% upsize your wire ONE size and recalc. Repeat till you are under 5%

make sure you calculate using the CORRECT wire temperature/jacket.

Table for COPPER UF, direct bury or in conduit are as follows:

30A: #10
40A: #8
50A: #6

You wont get UF much larger than that. If you need more than that go with Aluminum USE/URD OR run THHN inside conduit.

IMO UF is your easiest route, but it wont be the cheapest necessarily. You are looking about $1 a foot or so on #10.........and up from there.....

Just remember, If you want it done RIGHT, size your WIRE to the SERVICE, unless it is not feasible, at which point you size your SERVICE to the WIRE......
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