[NA]Household wire size calculator

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Dave Taylor
As a part of my other thread about what size wire for my pump I was saying I want to put in a 120v supply for the light and heater, receptacle for tools in the pumphouse and looking at the wire size calculators got me scratching my (greatly underslept) head...

125'; individual copper solid wires (2 conductors plus ground) in underground plastic conduit (it will be VERY loosely loaded ie not packed with wires)
The calculators were saying 8AWG which seemed enormous to me. I have 100' extension cords which carry 15A+ to my tools ( ie circular saw).
Anyone want to revisit this with me, seems like 12AWG intuitively should be plenty? I have a building which has 120v runs longer than 125' and no conductor is >12AWG, all rated at 15-20A
 
Let'sgoflying! said:
As a part of my other thread about what size wire for my pump I was saying I want to put in a 120v supply for the light and heater, receptacle for tools in the pumphouse and looking at the wire size calculators got me scratching my (greatly underslept) head...

125'; individual copper solid wires (2 conductors plus ground) in underground plastic conduit (it will be VERY loosely loaded ie not packed with wires)
The calculators were saying 8AWG which seemed enormous to me. I have 100' extension cords which carry 15A+ to my tools ( ie circular saw).
Anyone want to revisit this with me, seems like 12AWG intuitively should be plenty? I have a building which has 120v runs longer than 125' and no conductor is >12AWG, all rated at 15-20A
Click to expand...
What inputs are you using? How many amps is the heater? #8 does seem quite heavy.
 
Southwire gives the same result. I'm surprised. Screen Shot 2020-04-30 at 12.22.57 PM.png
 
When using extension cords, by definition, you are temporary use, and in typical practice, you are monitoring if voltage drop is causing problems for your tools. You ease the load on your skill saw, or turn off the compressor that won’t hot start and feels overheated. You won’t be providing the same manual oversight for your rather expensive submersible pump.
 
-KLB- said:
When using extension cords, by definition, you are temporary use, and in typical practice, you are monitoring if voltage drop is causing problems for your tools. You ease the load on your skill saw, or turn off the compressor that won’t hot start and feels overheated. You won’t be providing the same manual oversight for your rather expensive submersible pump.
Click to expand...
This is not for the pump. That calculated out to #12 wire. This is a 120V 15A circuit.
 
Let'sgoflying! said:
125'; individual copper solid wires (2 conductors plus ground) in underground plastic conduit (it will be VERY loosely loaded ie not packed with wires)
The calculators were saying 8AWG which seemed enormous to me. I have 100' extension cords which carry 15A+ to my tools ( ie circular saw).
Anyone want to revisit this with me, seems like 12AWG intuitively should be plenty? I have a building which has 120v runs longer than 125' and no conductor is >12AWG, all rated at 15-20A
Click to expand...

First of all, it doesn't matter how "loosely loaded" your conduit is. If it has four current carrying conductors in it, per the NEC, they must be derated to 80%.

Secondly, using 3% voltage drop to calculate the wire size is overly conservative. Use 5%. A heater and power tools will work fine at 95% nominal voltage. Using your estimate of 15 amps load doesn't matter if the circuit will see other non-continuous (defined as less than three hours by the NEC) loads in excess of that, they are allowed, with the breaker then being the limiting factor. The breaker must be sized at 125% of the continuous load or the next larger size.

Using the 80% deration factor, #10 AWG wire is adequate for this circuit. Use a 20 amp breaker.

If I were you, I would use #10 for the pump too. With the deration factor, #12 is very close to being inadequate.
 
How about the breaker size on the pump supply wires?
The spec sheet on the pump says it will draw 11A, but without mention of transient loads such as startup.
A ganged 30A breaker at the main disconnect would protect the 10awg wires but it seems to me I should put a lower rated breaker in the pumphouse to protect the pump? Or just rely on the pump's internal protection.
 
The proper breaker size for the pump is a 15 amp. NEC Section 430 says motor overcurrent devices for 1 HP and above and a service factor greater than 1.15 shall be no greater than 125% of the full load current. Your pump's FLC is 11.2.

As I said above, #12 wire is barely adequate for the pump circuit, but using #10 is better. The math is #12 ampacity = 20 amps, 20% derating for four conductors = 16 amps. Pump FLC 11.2 X 125% = 14 amps.

I'd use the larger wire to ensure the longevity of the pump, the voltage drop will be less.
 
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