90C terminations

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jim dungar

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Staff member
Location
Wisconsin
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Retired Electrical Engineer - Power Systems
I was sent a private message asking about the use of 90C insulation and why it must always be applied to equipment based on the 75C column of NEC tables. I think the answer would be of value to many people, so I am replying in the Form.

The primary reason is the way UL tests temperature rise at terminations. The conductor is actually used as a heat shink during their tests.

The following quote is taken from a Square D publication that cover wire terminations.

"For electrical equipment rated for 600 V and less, terminations are typically
rated at 60 ?C, 75 ?C or 60/75 ?C. No distribution or utilization equipment is
listed and identified for the use of 90 ?C wire at its 90 ?C ampacity. This
includes distribution equipment, wiring devices, transformers, motor control
devices, and even utilization equipment such as HVAC, motors, and light
fixtures."

For more information go to:
http://www.squared.com/us/products/gendoc.nsf/07a0210021262d45862564b5006e4f84/7e3f70f8decb27da85256b8a00700e48/$FILE/0110DB9901R2-02.pdf
 

txsparky

Member
Location
Conroe, Texas
Re: 90C terminations

As far as I know,( and I ain't the sharpest knife in the drawer ), there are no terminations made by the manufacturers that are rated for 90C. They are always 75C. The 90C can be used for calculating voltage drop and derating when sizing wire.

[ May 01, 2003, 06:22 PM: Message edited by: txsparky ]
 

ron

Senior Member
Re: 90C terminations

Lots of terminations (on their own) are rated at 90 deg. But as stated by Square D, the manufacturers have not tested them at 90 deg in the distribution equipment, so you must apply them at 75 deg (or 60 deg) as listed by UL and the manufacturer of the equipment (not the lug itself).
 

templdl

Senior Member
Location
Wisconsin
Re: 90C terminations

Yes, temperature rise test are conducted so that any heat generated by the wire doesn't derate the breaker by increasing the breakers temperature. And, as such, the wire acts sort of like a heat sink by taking heat away from the breaker or in so much as not adding heat to the breaker. The test are done with, as I can recall, 4' lengths of 75degC rated wire. The only time 90degC rated wire is address is with the application of 100% rated breakers. However, the 90degC rated wire still must be applied at 75degC.
 

mark

Member
Location
Illinois
Re: 90C terminations

I believe Burndy lists some of their mechanical lugs @ 90 degrees. However, I agree, a conductor's ampacity should be de-rated for 75 degrees when initially sizing for loads due to termination limitations (unless listed otherwise). When de-rating for more than (3) current carrying conductors in a raceway, 310.15(B)(2), you can utilize the 90 degree column if your insulation allows (i.e. THHN).
 

jim dungar

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Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
Re: 90C terminations

The temperature rating of the lug is not the temperature rating of the termination. The lug rating must always be equal to or higher than the conductor insulation rating. The lug is only one component (the others include the bolt, conductor, landing pad, etc).

UL does not list any equipment (600V and less) that has a termination rating higher than 75C. This means that regardless of the conductor insulation rating, after all derating factors are applied, the resulting conductor size must not be smaller than if you had used the 75C (or in some cases 60C) tables.
 

gwz2

Senior Member
Location
Indiana
Re: 90C terminations

It is my understanding that if a non thermo medium is used such as bus bar splice as large , amperage-wise, or larger than the 90? conductor, then the 90? rating could be used.

Example:

A conductor leaving a 75? terminal of a 500A CB (terminals suitable for two 250 Kcmil's) must meet the 75? column of Table 310.16, thus 2 paralleled 250Kcmil (510 Ampacity) could be used to a junction or terminal splice box and then 1 700 Kcmil (520 ampacity) could be spliced to the paralleled 250 Kcmil's for a long cable run and then spliced back to 3 - 2/0's paralleled (525 ampacity) so as to connect to a 75? termination which is designed for the 3 - 2/0's.

Depending on the installation this could be cost effective.
 

smenser

Member
Location
Indiana
Re: 90C terminations

I haven't seen anyone address the specific code article that mentions these rules you're discussing, so I thought I would throw in my 2 cents worth. 110.14(C)(1)a&b list the rules for conductor ampacities in relation to the terminations. I always say if one is using THHN wire rated 90 degrees C the folowing rules apply in general. Under 100A or the termination is marked for wires 14 through 1 base the ampacity on the 60 degree C column. If over 100A, or larger than 1 use the 75 degree C column. Never use the 90 degree C unless all terminations are rated 90 degree C continuous.
 
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