I was under the impression all wiring device terminations (including circuit breakers) with ratings under 100 Amps must be treated as 60C terminals unless otherwise marked. I am looking at Spa instructions which call for a 50 amp circuit using #8 THHN/THWN. Using the 60C column only allows for 40 Amps. Are the instructions wrong or is it me?

BigB...maybe 430.53(C)applies to your installation?

shortcircuit

Big B you are remebering only part of the code section.

Part of 110.14(C)(1)


Look at your breakers, I would bet they are marked 60/75 C or just 75 C

[This message has been edited by iwire (edited 01-12-2005).]

That's just it iwire, the breaker offers no help. It is a GE THQL 2 pole 50 amp. There is a circle on it with "40C" printed inside the circle, which I think must indicate the ambient temperature it is calibrated at. I tried to research breaker specs but couldn't come up with the info.

OK I just went out to my truck and examined a bunch of breakers and sure enough, they do say 60/75C on most all of them. I had to get out a magnifying glass for some. I don't have that GE here, I wonder if I just didn't see the fine print.

That and good lighting...it is frustrating to find the markings on some equipment.

Bob

If I wire a 12kw range in a dwelling with 8-3 copper romex and it hooks up to a 50 amp breaker...how is this OK if the copper wire amperage is 40 amps based on the 60C column?

shortcircuit

Big B said


Which makes me think he is running a raceway so the 75 C rating could apply.

If it is NM then you are stuck with the 60 C rating.

But romex comes with 90C rated conductors.This is confusing me here.Help me out?

shortcircuit

Look in 334.80 Ampacity. Ampacity of NM type cables shall be 60 degree with the 90 degree rating allowed for derating purposes.

Shane

Shortcircuit, Shane is correct.

Go to this Southwire page and scroll dow to NM-B for an explanation.

Roger

Its been so long since I learned why I run #8 romex to ranges that I forgot about 220.19

The demand factor of 1 range of 12kw is 8kw and thats why I run #8 to my ranges.Which code rule allows that #8 to be on a 50 amp breaker?

shortcircuit

I do not understand.


310.16 rates 8 AWG copper 50 amps at 75 C

As long as we are not using NM and the wire, breaker and other terminals are all 75 C listed we can use a 50 amp breaker with 8 AWG copper.

NM is different regardless of the THHN inside it.

iwire...every freestanding range I've wired for the last 25 years has been wired with # 6 aluminum or # 8 copper connected to a 50 amp breaker.Thats what I was taught to use and thats the way I see electric ranges connected.I used to use #6 SEU until we had to use 4 wire and then I switched to 8-3 romex and I have never been sited for a violation.?

shortcircuit

Shortcircuit, in the case of "I used to use #6 SEU (AL per above) until we had to use 4 wire and then I switched to 8-3 romex and I have never been cited for a violation". The inspectors simply have not spotted the violation.

Inspectors are like Inspectees, they both make mistakes.

Roger

[This message has been edited by Roger (edited 01-13-2005).]

So 8-3 romex protected with a 50 Amp breaker for a Range (up to 12KW) is a violation, but can you wire the range with 8-3 romex and use a 40Amp braeaker per Table 220.19 Note 4:
----------------
Table 220.19 Note 4. Branch-Circuit Load. It shall be permissible to compute the branch circuit load for one range in accordance with Table 220.19.
-----------------
Per Table 220.19 Column C:
compute branch circuit load at 8KW
8KW/220V=36.33Amps
8-3 romex with 40Amp breaker OK?

Iwire,Electric Ian,

A quote from the 2002 Massachusetts Electric code:

"334.80. Revise to read as follows:

334.80 Ampacity. Type NM, NMC, and NMS cable shall have conductors rated at 90C(194F). Where installed in thermal insulation, the ampacity of conductors shall be that of 60C(140F) conductors. The ampacity of Types NM, NMC,and NMS cable installed in cable trayshall be determined in accordance with 392.11."

This revision seems to allow #8-3 copper romex to be protected by a 50 amp breaker where the wire isn't installed in insulation...here in Massachusetts.

shortcircuit

I agree and that is a good point for us MA folks.

Bob

shortcircuit ---

It is not clear if the ampacity is before or after final derating.

The intent of the code could be to derate from the 60 degree colume within insulated walls.

George...It is my understanding, for derating purposes the 90C column is used.A 8-3 romex cable run by itself needs no derating.

I interpet the MA code (334.80) that I posted to mean if the 8-3 copper romex I run in a dwelling is imbeded in thermal insulation that I must use the 60C column which restricts the wire to 40amps.

If the wire is run through the basement(which most houses in New England have)along the sill and up through the floor to the freestanding range, NOT imbeded in thermal insulation, then the wire will be good for 50 amps as listed in the 75C column in 310.16

Also take note of the temperature correction factor table below 310.16. If the wire is run in the basement where the average tempature will be 70F-77F or lower around here, I can apply a correction factor of 1.05...or 50amps x 1.05= 52.5amps

On the other hand if the wire is run up through the ceiling embeded in thermal insulation exposed to attic tempatures in excess of (lets say) 114F-122F, then I would figure the amperage of that #8 copper romex from the 60C column with a tempature correction factor of .58...40amps x .58= 23.2 amps.

Quite a swing in allowable ampacity

shortcircuit