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# Stranded Wire / Portable Cord Voltage Dropoff

#1
01-06-13, 08:44 AM
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Join Date: Jul 2009
Location: NYC
Posts: 302
Stranded Wire / Portable Cord Voltage Dropoff

For a 20 amp circuit, let's say 100 ft 10AWG extension cord versus 100 ft 12/3 extension cord, how much voltage dropoff will I see.

I'm not sure what the derating is, but for example, I was looking at a 8/4 SOOW cord. In my *head* I knew as a general guideline 8AWG was good for 50 Amps. The Ampacity in the portable cord showed that 8AWG was good for 35 amps.

Was the derating due to stranded and portable cord.
If I take the same 8AWG stranded and let's say use AC stranded 8AWG, would it be 35 amp rated or 50 amp capable.

#2
01-06-13, 09:30 AM
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Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 12,288
Voltage drop is based on voltage, distance of the circuit, material of the conductor, and load in amps. Search Google for "voltage drop calculator" Here is one I like: Voltage Drop Calculator

120 volts at 100', with #10 copper conductors, and a load of 20 amps, your voltage drop would be 4.8 volts, or 4%.

#8 THHN wire is rated at 55 amps (THHN is 90 degree wire) SOOW cord is rated biased on table 400.5(A) of the NEC (2008). 8/4 cord is rated at 35 amps under column A. (3 current carrying conductors)

It does not matter if the conductor is stranded or solid.

#3
01-06-13, 09:48 AM
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Join Date: Jul 2012
Posts: 70
Edit: Doh, Tolyn beat me to it

The derating is a function of the number of conductors in the cable, the insulation temp rating, and the length.

SOOW cord is typically rated at 90C temp, compared to the 105C you might find in THHN. Your cord assembly also counts as having four conductors bundled in an area so there is a greater derate than three conductors in a metal conduit.

For your 12AWG cable, you can expect about 8.5 volts of voltage drop vs 5 volts for the 10 AWG.

#4
01-07-13, 06:21 PM
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Location: Twin Cities, MN
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THHN is only rated at 90 degrees.

#5
01-07-13, 11:07 PM
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Join Date: Jul 2012
Posts: 70
90C in building wire. Up to 105C in chassis wiring. But then it wouldn't technically be THHN, it would be AWM. Even though it is really the same wire.

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