Wire Ampacity for DC
- Thread starter gojoeba
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jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Why would DC ampacity be different than AC? I would think the wire size would be the same.
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
I am under the same understanding.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
Actually, without reactance, a given conductor has slightly less impedance with DC, so you'll end up with a hair greater ampacity.
jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
Understood( XL and XC). Question is: are DC amps different from AC amps? I do see how? Am I wrong?
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
Are you saying that the heat build up in a conductor is less with DC current in the same size/insulation/distance/etc. conductor?
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
I think he is talking about capacitive reactance, XC, and inductive reactance,XL, in a AC circuit vs a DC circuit. It is a frequency dependent calculation.
The impedance of a AC circuit is greater than a DC circuit given the same values for components.
Crap, I hope I got this right.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
No, RMS AC amps and DC amps have the same heating effect.
For a given current, that and less voltage drop.
Close enough.
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
I guess I'm getting old and slow, it seems to me you are saying in the 1st sentence the heating effect is the same and in the second sentence that they are not.
Since our meters are measuring RMS AC amps, then I always understood any AC amps to be RMS.
The peak current in an AC circuit is the square root of 2 x the nonimal
AC value . A 10 amp AC current has a peak value of 14.142 amps.
The average heating value (I^2 x R) of a 60 cycle current having a peak value of 14.142 amps over one cycle is the same as the heating value of a 10 amp DC current.
That works for pure sine waves. The new electronic meters can calculate the root mean current values of lots of different waveforms ( harmonics, sawtooth, etc).
IF you have VFD drive and your reading 20 amps with your Simpson 260 you may get a higher reading with a good Fluke. The old meter won't respond to the higher harmonics. The higher harmonics will heat the wire more than the 260 would tell you.
reddy kw
Member
- Location
- Mudville, upstate New York.
RE: Ampacity in DC wiring-Any first year 'Introduction To DC Fundamentals' textbook should have all the info you need. Mine was 'Delmar's Standard Textbook of Electricity (Third Edition)', FWIW.
At what voltage and with what load?
Amps is amps as far as wire is concerned. 12 awg THHN carries 20 amps at any AC or DC voltage-for the first ten feet, anyway. The lower the voltage, the higher the voltage drop over distance (worse with DC then AC).
Are you running photovoltaic panel wires from roof down to batteries/inverters? Try to keep power supply as close as practicable to load.
Low voltage DC suffers much more voltage drop then AC. In DC, RETURN path is counted in voltage drop at any voltage. 50 feet of 'hot' from supply to load and 50 ft. of return back to power source equals 100 ft. when figuring wire size relative to voltage drop.
For example, if you need 10 awg for a 40 ft total (20 ft from supply to load and back again) run, you'll need 4 awg for 80 ft (40 ft supply to load).
Then (if we're talking motors) there's the type of load to consider. Dig out your textbook from trade school, it's trigonometry time. That's why my response was-DC used for what, at what voltage?
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