Wire size for 20 amp at 125 feet

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Slick8810
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Wire size for 20 amp at 125 feet

Post by Slick8810 » Sat Sep 28, 2019 9:23 pm

We just moved a shed to the back of our yard and it’s going to have a 20 amp breaker and about 100-125ft distance. What size wire should I use? It will be buried and i will be using basic power tools and compressor in the shed so nothing that’s going to draw too much. Thanks

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emtnut
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Re: Wire size for 20 amp at 125 feet

Post by emtnut » Sun Sep 29, 2019 6:43 am

Slick8810 wrote:
Sat Sep 28, 2019 9:23 pm
We just moved a shed to the back of our yard and it’s going to have a 20 amp breaker and about 100-125ft distance. What size wire should I use? It will be buried and i will be using basic power tools and compressor in the shed so nothing that’s going to draw too much. Thanks
Tough question to answer without knowing the actual load (Amps).
A skil saw and compressor (if used at the same time) 'could' be over 20A.

Voltage drop is calculated based on load (ie, not the 20A). Since you shouldn't be going over 16A on that circuit, #10awg would be 4.1% drop at 16A.
#10awg would be 3% drop at 12A (not sure if your NEC or CEC, but CEC would require the 3%, NEC doesn't specify)

If you ran #10/3 (240V circuit), it cuts the voltage drop in 1/2 and doubles the power available.
(2% drop @240V). For the price difference of 10/2 vs 10/3, I'd say it's worth it.
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emtnut
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Re: Wire size for 20 amp at 125 feet

Post by emtnut » Sun Sep 29, 2019 10:39 am

Now that I think about it, If you ran 12/3 on a 240V circuit, you'd be at 3% at 15A !

You would run from a 20A double pole, and use a multi wire branch circuit in the shed (shares the neutral with the 2 hots for 2 separate circuits)

Food for thought
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Aaron
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Re: Wire size for 20 amp at 125 feet

Post by Aaron » Sun Sep 29, 2019 4:30 pm

Yeah I'd do 12/3, especially if the compressor motor is capable of being rewired to run at 240V instead of 120V. Doesn't that cut the load in half, approximately?

Not to mention just having two 120V circuits out there, for just a nominal increase in cable cost.

He needs Type UF-B cable in the USA, or Type NMWU in Canada. Trench should be at least 18".

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emtnut
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Re: Wire size for 20 amp at 125 feet

Post by emtnut » Mon Sep 30, 2019 8:23 am

Aaron wrote:
Sun Sep 29, 2019 4:30 pm
Yeah I'd do 12/3, especially if the compressor motor is capable of being rewired to run at 240V instead of 120V. Doesn't that cut the load in half, approximately?

Trench should be at least 18".
240V on the compressor is better 'if' possible. Load (watts) stays the same, but the current is half.
But even at 120V, he would have the other circuit for other loads.

I think you can get away with a 12" trench in NEC if the circuit is GFCI'd ... CEC does not allow that thou.
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Aaron
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Re: Wire size for 20 amp at 125 feet

Post by Aaron » Mon Sep 30, 2019 3:47 pm

Yeah that makes sense. I think I heard 240V is better for motors too, they run cooler and more efficiently.

Yeah I don't agree with that 12" depth provision in the NEC, that's too shallow in my opinion so I just tell people 18" for everything. Covers NEC and CEC.

I think that's a modern provision, too, it's not in older NEC revisions.

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