[Sunour]

I need somebody to show me proof that the national electrical code amperage ratings of thhn copper wire are based on 120 vac please.

[stanfortyman]

I need somebody to show me proof that the national electrical code amperage ratings of thhn copper wire are based on 120 vac please.

Sorry, they are not.

Here is the beginning of that part of the code: "Ampacities for conductors rated 0 to 2000 volts........"


See 310.15, 310.16, 310.17, etc

[Tev]

The best I can do is give you what evidence I know of that it is not based on voltage.

310.15(A)(1) allows for the use of a formula to find conductor ampacity under engineering supervision. That formula is found in 310.15(C) and it does not include variables or constants relating to voltage.

Also the last sentence in the paragraph at the top of the tables read "based on ambient temperature of 30 degrees C"

Between that and the formula variables I would say that the ampacities are based on heat dissipation more than anything else.

[Sunour]

Thank you both for your timely input. In case you are wondering where this is all coming from, I will provide a link to another Q&A forum.
Wind Turbine wire size at The Alternative Energy Store
Sincerely, Thomas Allen Schmidt.

[stanfortyman]

Thank you both for your timely input. In case you are wondering where this is all coming from, I will provide a link to another Q&A forum.
Wind Turbine wire size at The Alternative Energy Store
Sincerely, Thomas Allen Schmidt.

That Ken Hall dude is all wrong. The only thing that is even close to right is the fact that voltage drop IS reliant on the circuit voltage like they are referring to.
To say the tables are based on 120v is completely bogus.

[Sunour]

Yeah, that was my first thought too. That it was bogus, I mean. But I've been reading Ken's posts for a long time and I've learned that Ken is very knowledgeable of RE. I did a double take the first time I read it. It didn't "sound" like him.
I believe that for someone who works with THHN a lot where electricity is supplied from the utilities, will think of those NEC amperage ratings as relating to 115, 230, 460 vac. Where as someone working primarily with low voltage dc will think in terms of the high amount of voltage drop and, there for way lower amperage "ratings" for a given size and length of wire and percent of efficiency.

In otherwords, the NEC is about maximum fuse or breaker ratings for a given wire size.
RE, being low voltage dc is about voltage drop.

#12 awg thhn per nec is good for 20 amps.
#12 awg thhn per RE is only good for 5 amps at 12 vdc for 20 feet of wire run considering a 3% margin...

You say tomato, I say tomato kind of thing.

[stanfortyman]

#12 awg thhn per RE is only good for 5 amps at 12 vdc for 20 feet of wire run considering a 3% margin...

See, this I don't get either.
Amps are amps, volts are volts, watts are watts.
#12THHN is good for 20 amps, regardless of voltage. The allowable wattage is what changes depending on voltage.

#12 can handle 2400 watts @ 120v
#12 can handle 240 watts @12v
Both are 20 amps

[Sunour]

#12 can handle 2400 watts @ 120v
#12 can handle 240 watts @12v
Both are 20 amps

Yes. That's it. Its all relative. Only the length of wire and efficiencies at 12v are of more importance than at 120v
That is what the little quip I put in about tomato and tomato is about. They are both spelled the same only its pronunciation sounds different.
If the wattage is lower then therefore ether the voltage or the amperage or both will be lower and efficiency factors will come into play
When utilizing 12 vdc, voltage drop is dramatically worse than at 120 vac per say.
So when I wrote that #12 is only good for 5 amps at 20 foot with a 3% voltage drop it's a, "I say tomato, you say tomato." kind of thing.

A 50 watt 12 vdc incandescent light bulb will burn brighter with only 20 foot each + and - of #12, than that same bulb at 60 foot each + and - of #12.
Someone working with low voltage dc circuitry, like those found in certain RE electrical systems, and who is concerned with efficiency, will think in terms of what #12 wire is good for in those systems because of the worse amount of voltage drop.
Where as someone more apt at utilizing 3 phase 460 is a lot less likely to even consider voltage drop.
I am not sure how else to put it. Maybe...
A 90 pound man would not even think about using a 2x4 as a bridge to cross a gap between to distances, he would just us it. But a 300 pound man would not even consider a 2x4 to start with.

This was something that I did not fully appreciate when I read Kens first reply.