What amp rating to use when using 90C conductors. 3

[edryder]

If you seclect 75C conductors to feed a 400A panelboard two 3/0 (200A from table 310-16) per phase should work disreguarding all but the amp rating. My question is if 90C conductors were use instead would the amp rating be taken from the 90C column (225A) or the 75C column (200A). From what I understand from the NEC you would still use the amp rating from the 75C column if the equipment is rated for 75C conductor terminations.
Thank You,
Ed

 

[mpparent]

You are correct...you have to be careful of the lug ratings. Most equipment is rated for 75deg C,...so you would use the 75deg C column for your conductors. Remember, anything rated 100A or less, you have to use the 60deg C column.

Mike

 

[edryder]

Thanks Mike

 

[busbar]

One qualifier for ANSI applications… 99NEC110-14

(2) Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than No. 1, shall be used only for
(a) Conductors rated 75°C (167°F), or
(b) Conductors with higher temperature ratings provided the ampacity of such conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors.

 

[wareagle]

mpparents Posts
"Remember, anything rated 100A or less, you have to use the 60deg C column."
If the equipment is rated for 75C regardless of its amp rating you can use 75C conductors. You do not use the 60C col. Check 110.14.C.(1)a3.

 

[mpparent]

wareagle,

I still maintain what I said is correct regarding 60deg C conductors and their use. For equipment, rated 100A and below, requiring connections, I've really only seen 60deg C, except for motors. Besides that, using the 60deg C table for 100A and less on all equipment is good, conservative engineering practice.

Mike

 

[wareagle]

New equipment is rated at 75C, therefore you are not required to use the 60C column. You may if you want but you
are not required by the NEC. Using the 60C rating will likely cause you to use a larger conductor than is necessary. Cost may be a factor.

 

[RRaghunath]

I thought 90 or 75 is to do with the type of insulation that the conductor is provided with (for example if the insulation is PVC we cannot exceed conductor temperature of 75 and the same can go up to 90 if the insulation happens to be XLPE).
Further, in case of bare conductors or bus bars, if the conductor joints (in case of busbars in switchgear) and other current carrying hardware like lugs are tinned, the conductor temperature limit is 105degC. Same is limited to 75degC if not tinned.

 

[wareagle]

rraghunath
"I thought 90 or 75 is to do with the type of insulation that the conductor is provided with (for example if the insulation is PVC we cannot exceed conductor temperature of 75 and the same can go up to 90 if the insulation happens to be XLPE)."
You are correct in that you can not exceed the temp ratings for the cable. However you can not exceed the terminal ratings of the equipment.
Take for example a 100 amp breaker panel rated at 75C.
You would need to choose #3 thhn rated at 100 amps at 75C for the feeder to this panel. If you were to pick a conductor form the 90C column, you might choose #4 rated at 95 amps. If you have a continous load of 95 amps the temp of the #4 would reach 90C which would exceed the ratings of the panel terminal rating of 75C. You may choose the 90C column but the amp rating of the conductor may not exceed the 75C column rating. In this case the 75C rating of #4 is 85 amps and would not be adequate. See 110.14.C.
Comments?

 

[timohearn]

The selection of conductor ampacity is a lot more complicated than is being described. Other factors such as derating due to raceway configuration, ambient temperature, short circuit withstand, voltage drop, etc.. all of these issues affect the selection of the conductor size and therefore must be taken into account to select the proper ampacity rating.

 

[mpparent]

wareagle said, "Cost may be a factor."

If you take the cost of #2 to #3 say, you're only looking at about 10 cents a foot difference. IMHO, I don't think that would cause a client heartburn if you decided to choose #2 (60deg C) to #3 (75deg C).

Mike

 

[REDDOG]

Use conductors with 90 degree temperature rated insulation.
Use the 60/75 degree column for the conductor size to meet the termination requirments for temperature. Use the 90 degree rating for derating purposes ie. fill and ambient temperature.