Choosing wire size to run to a sub panel


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Old 08-28-16, 02:39 PM
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Choosing wire size to run to a sub panel

Hi yall, I'm going to be running g a 100amp sub panel in my pole barn. Its going to be coming off of my 200 amp panel In my house the total run from Main panel to sub panel is about 220 feet. I ran 2" conduit last weekend. I would like to run alum wire, now my question is what size wire would be big enough for my sub panel, I know its something like a 10% drop for every 100 feet of the run. Also is it 3 wire or 4 wire. thank you
 
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Old 08-28-16, 05:00 PM
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You need 4 wire. I suggest using #2/0 Mobile Home Feeder at that distance for a full 100A. Such as this... https://www.wireandcableyourway.com/...der-cable.html

Oh but the problem is going to be 2" conduit may be too small. You need 2.5". Using aluminum you need to drop down to #1/0 using AL THHN/THWN or XHHW. https://www.wireandcableyourway.com/XHHW-2/

Edit: After looking at the MHF 2/0-2/0-1-4 it may be okay in 2" conduit because of being compacted conductors, but it will still be a bear to pull.
 

Last edited by pattenp; 08-28-16 at 05:16 PM.
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Old 08-28-16, 06:18 PM
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Why do you need a 100 amps? That is generally enough for a whole house. Just because the subpanel is 100 amps doesn't mean you need to feed it with 100 amps.
 
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Old 08-28-16, 09:23 PM
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When I got the conduit they said 2inch would be fine for what I would be doing I hope I didn't get screwed, but I need 100amp service because I have welders and stuff that are going to be ran out there
 
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Old 08-29-16, 06:26 AM
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Why do you need 100 amps?
Incidentally, after distance voltage drop considerations have called for upsizing of the feed conductors campared with feeding a nearby building, the load analysis out at the destination may treat continuous loads (e.g. heaters) as intermittent, possibly resulting in a lower total wattage figure for choosing wire size.

Outbuiilding master breaker and supra panel breaker would still be selected using continuous loads where applicable.

(Branch circuits in the outbuilding still need appropriate continuous/intermittent calculations unless they, too, were upsized for voltage drop in a very large building.)
 
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Old 08-29-16, 06:50 AM
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1/0 aluminum will give you a 3.82% voltage drop at 100 amps, and 240 volts.

Voltage drop will depend on load. The lower the load, the lower the voltage drop.
I suggest adding up what you think you will have for equipment in the pole shed (saw, welder, heater, etc.) and then figure your your wire size. Remember only figure what equipment will be running all at one time.

For example, if your load was only 50 amps you could use #4 aluminum. That would be quite a savings over 2/0 aluminum.
 
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Old 08-29-16, 06:54 AM
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When I got the conduit they said 2inch would be fine for what I would be doing I hope I didn't get screwed,
The 2" conduit if using copper shouldn't be an issue for 100A. The use of aluminum is where it becomes a possible issue due to needing larger wire for the 100A, plus the additional size for possible voltage drop at the long distance. I believe you'll be okay with the aluminum, you just need to decide on the type wire and size and do an accurate conduit fill calculation to verify if it will fit in the 2" conduit and not exceed the max NEC fill requirement. My suggestion of using 2/0 AL was based on a full load of 100A.
 
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Old 08-29-16, 07:02 AM
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For example, if your load was only 50 amps you could use #4 aluminum. That would be quite a savings over 2/0 aluminum.
I do not suggest #4 Al for a 50A load at 220 ft. You should try to stay at a voltage drop of no more than 3% which would require #2 Al.
 
 

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