Ampacity of 12 THHN conductor

[tom baker]

is the ampacity of a 12 THHN conductor 20 amperes?
If not where does it say otherwise.
Why do so many electricans feel it is 20 amperes

 

[George Stolz]

Re: Ampacity of 12 THHN conductor

Um...240.4(D)?

 

[iwire]

Re: Ampacity of 12 THHN conductor

Originally posted by tom baker:
is the ampacity of a 12 THHN conductor 20 amperes?
If not where does it say otherwise.
Why do so many electricans feel it is 20 amperes

The ampacity of 12 AWG THHN is 30 amps per table 310.16.

However 110.14(C) generally limits its rating to 25 amps

240.4(D) does not change the rating of 12 AWG THHN but often requires it to be protected at 20 amps.

Table 240.4(G) lists the types of circuits where 12 AWG can be protected above 20 amps.

[ March 26, 2005, 10:43 AM: Message edited by: iwire ]

 

[dillon3c]

Re: Ampacity of 12 THHN conductor

Hummm,
The (Ampacity) of #12 THHN table 310.16,is 30 amp,listed in the 90 degree C.Are we leading upto a trick question..?

(sorry Bob, u beat me to the button)

[ March 26, 2005, 10:47 AM: Message edited by: dillon3c ]

 

[electricmanscott]

Re: Ampacity of 12 THHN conductor

As pointed out already by the really smart guys, 240.4(D) Is probably what everyone automatically thinks of. But as we all know there are exceptions to every rule. This is the NEC after all!

 

[jimwalker]

Re: Ampacity of 12 THHN conductor

If they ever print NEC in clear english they would sell millions of copies

 

[physis]

Re: Ampacity of 12 THHN conductor

Jim, you can't use plain English in court.

 

[james wuebker]

Re: Ampacity of 12 THHN conductor

It's a trick! Remember until 100 amps you use 60degree C table and 100 amps or more you can use the 75 degree C table. Now when you use THHN wire it's in the 90 degree C table. That doesn't mean you go by what amp the wire is rated for. This table you can use as derating circuits. We need this to be able to hold 20 amps. So if there was 6 current-carrying conductors in this pull and the was #12 THHN. If we now look at the 90 degree C the rating is 30 amps. Derating on this would adjust to 80%. Now the wire is only good for 24 amps. Now if we were to use TW wire we would have to use #10. That's some of the advanages of have THHN or some of the other 90 C insulation. Good for derating of your circuits. I don't know why I wrote this. You all probalay know this already.

 

[iwire]

Re: Ampacity of 12 THHN conductor

Originally posted by james wuebker:
It's a trick! Remember until 100 amps you use 60degree C table and 100 amps or more you can use the 75 degree C table.

Most equipment is rated 75 C even under 100 amps.

As long as the equipment is marked 75 C you can use the 75 C column with THHN or THWN. If not marked you are stuck with 60 C under 100 amps.

However NM is always based on 60 C.

 

[johndeereman]

Re: Ampacity of 12 THHN conductor

Most equipment is rated 75 C even under 100 amps.

As long as the equipment is marked 75 C you can use the 75 C column with THHN or THWN. If not marked you are stuck with 60 C under 100 amps.

However NM is always based on 60 C.

Dito

 

[johndeereman]

Re: Ampacity of 12 THHN conductor

Most equipment is rated 75 C even under 100 amps.

As long as the equipment is marked 75 C you can use the 75 C column with THHN or THWN. If not marked you are stuck with 60 C under 100 amps.

However NM is always based on 60 C.

Dito I agree with "iwire"

 

[sandsnow]

Re: Ampacity of 12 THHN conductor

Why?.....Hold over from the old days of the beloved TW

 

[coulter]

Re: Ampacity of 12 THHN conductor

Originally posted by johndeereman:

As long as the equipment is marked 75 C you can use the 75 C column with THHN or THWN. If not marked you are stuck with 60 C under 100 amps.

Help me out here, I need a little education.
Unmarked - used 60 deg C column
Marked 75C - use 75C column with 75/90C wire

I understand that.

The question: What conductor temp rating do you use if the device is marked 60/75C?

My buddy, the second best EE in Alaska, says it means to use 60C column with 60C wire and 75C column with 75C wire. I say that is the same thing that being marked 75C means. The best EE in the state agrees with me - He doesn't know what or if there is a difference between the two markings.

Anybody got any clues?


carl

 

[iwire]

Re: Ampacity of 12 THHN conductor

Originally posted by coulter:

The question: What conductor temp rating do you use if the device is marked 60/75C?

Anybody got any clues?


carl

The UL whitebook provides the answer.
CIRCUIT BREAKERS (DHJR)
GENERAL

2. Circuit breakers with a current rating of 125 A or less are marked as being suitable for 60?C, 75?C only or 60/75?C rated conductors. It is acceptable to use conductors with a higher insulation rating, if the ampacity is based on the conductor temperature rating marked on the breaker.

3. Circuit breakers rated 125 A or less and marked suitable for use with 75?C rated conductors are intended for field use with 75?C rated conductors at full 75?C ampacity only when the circuit breaker is installed in a circuit breaker enclosure or individually mounted in an industrial control panel with no other component next to it, unless the end-use equipment panelboard, switchboard, service equipment, power outlet, etc.) is also marked suitable for use with conductors rated 75?C.

 

[coulter]

Re: Ampacity of 12 THHN conductor

Originally posted by iwire:
The UL whitebook provides the answer.

iwire -

Thanks for the post. I'm still not getting the difference between the 60/75C marking and the 75C marking. It looks like they are both limited by the enclosure/panel rating and adjacent equipment, as well as conductor rating. I've got a UL subscription at work, I'll look it up Monday.

carl

 

[shortcircuit2]

Re: Ampacity of 12 THHN conductor

In Massachusetts, the ampacity of type NM cable is allowed to be based on the 75 degree column when not installed in thermal insulation...

shortcircuit2

 

[physis]

Re: Ampacity of 12 THHN conductor

I think I remember there being some reason for the 60?/75? C. marking but I can't think of what it might be. Is there a possible installation where everything is rated 75? C. or more and the 60?/75? C. marking would force everthing to be treated as 60? C.?

 

[sandsnow]

Re: Ampacity of 12 THHN conductor

Device marked 75'C only then you cannot use 60'C wire because the device operates at a higher temp than the wire insulation will withstand. (conditions per the white book also)

Device marked 60/75'C You can use 60'C wire at it's ampacity or 75' wire at it's ampacity. The device will not operate at a temp high enough to damage the 60'C wire insulation.

My distillation and interpretation and opinion of the NEC and White Book rules and past articles in IAEI mag and UL newsletters.

 

[james wuebker]

Re: Ampacity of 12 THHN conductor

iwire, you wrote:
Most equipment is rated 75 C even under 100 amps.

As long as the equipment is marked 75 C you can use the 75 C column with THHN or THWN. If not marked you are stuck with 60 C under 100 amps.

However NM is always based on 60 C.

So you are saying if I use #8 THWN and the equipment is marked 75 C I can put a 50 breaker on this. I don't agree if this is what your saying. Alot of people don't understand the theory about these tables and how to use them correctly. I remember that it took me a while to learn how to understand the correct way. After Easter I'll try to explain what I learned about these tables. Wow, That was a how time ago.

 

[mc5w]

Re: Ampacity of 12 THHN conductor

One reason for the 20 amp limitation for #12 copper is that binding screw terminals have more than their fair share of contact resistance. That is, binding screws heat up more than pressure plate lugs and box lugs. There is also the matter of keeping voltage drop reasonable. Also, when you run wires hot you are paying for electricity and not getting it. With a larger wire size more electricity reaches the load.

SquareD rates their 15 amp to 100 amp circuit breakers and panels as suitable for both 60 and 75 degree Celsius application.

I have also installed industrial 20 amp and 30 amp locking receptacles that had 90 degree Celsius rated terminals. Turned out that because I had the wireway between the conduits and the panelboards I could do derating of the wires in the conduits using 90 degree Celsius and since the circuit breakers could handle 60 or 75 Celsius I was fine.