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Old 04-03-2013, 01:23 PM   #1
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100 Sub Panel


I am building a 380 sq ft detached casita (one big room) with a bath. It will have standard outlets and lighting. The only appliances will be an electric hot water heater 30 gal, and a 24,000 btu mini split HVAC unit. I am planning on installing a 100 amp sub panel, fed from my 225 main panel of the house. The sub panel will be approx 125' away. I plan on using Conduit & THHN to run the circuit. My question is what size wire can I use and what size conduit is code for the needed wire? I am thinking
3-#3 w/#6 gnd. all THHN. (but can i get away with #4 and even if I have to use #3 can I use a #8 gnd going back to the main panel?) Code is requiring a ufer ground attached to a 20' piece of rebar in the slab, stubbed up and connected at the sub panel in addition to a ground running all the way back to the main panel also.
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Old 04-03-2013, 02:24 PM   #2
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1 1/4" schedule 80 pvc conduit is good for (5) #3 awg wires. I don't have my chart with me but I believe the #8 ground is good for a 100 amp sub panel.

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Old 04-03-2013, 02:40 PM   #3
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I would love to run 1" conduit, will code allow 3-#3 w/#8 in a 1"?
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Old 04-03-2013, 11:51 PM   #4
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You could use 1" but with a long run you will appreciate the extra room that will make the wire pull thought the conduit much easier.
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Old 04-04-2013, 05:00 AM   #5
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I would love to run 1" conduit, will code allow 3-#3 w/#8 in a 1"?
While using a 1" conduit is Code-compliant, I would advise against it on such a long run. You'll regret it once you start pulling in those conductors. I recommend a 1" for this, although a 1" would suffice.
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Old 04-04-2013, 11:31 PM   #6
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So everyone understands, it's not 125' straight pull, the longest part of the run is about 45' between LB's and only 2 - 90's after that.
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Old 04-05-2013, 12:53 AM   #7
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So everyone understands, it's not 125' straight pull, the longest part of the run is about 45' between LB's and only 2 - 90's after that.
Pulling wire is a two person job. The other person is lubing and feeding, sometimes pushing, the wires. Stop being stupidly cheap, listen to the experts and upsize the conduit. BTW, below 18" underground is sch 40 not sch 80 and outdoor conduit is always a Wet location THWN wires.

If money is tight use aluminum. You might be able to find #1 AL mobile home feeder cable. Just protect it in sch 80 above ground and to 18" depth. (Trench 27" deep, 24" from top of cable to grade.) Or use AL individual conductors, but then 1 1/4" sch 40 is code minimum.

Always torque to spec AL connections.

Doing a residential load calc would save you money too.

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Old 04-05-2013, 01:46 AM   #8
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If using Aluminum conductors, we use #1 XHHW for the hots, #2 for the neutral, and #4 for the ground (Although #6 is compliant). All in a 1' PVC conduit.
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Old 04-05-2013, 08:23 AM   #9
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I have been on a wire pull where the conduit was sized to code but now I size the conduit to common sense. It's much easier on your and less chance of having trouble.
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Old 04-05-2013, 10:59 AM   #10
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if your expecting to reach close to a 100a draw you should up size the wire so you don't have a significant voltage drop.

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