Cable Ampacity Rating Based on Termination Temperature Rating (NEC)

[eg6791]

In summary per NEC 110.14, you must base your cable ampacity on the temperature rating of the conductor equivalent to the temperature rating of the terminal that the cable would be terminating to (I.E. motor, molded case breaker, switchgear, etc.). Here is my scenario:

I am trying to provide power to a 3-ph 460V 125HP motor. The FLA rating of this motor is 148A. Taking a 125% increase on the FLA for safe practices brings it to 185A. Per ampacity charts that we have standarized from IEEE&IPCEA industry standards, 3 - 1/C #2/0 cables rated at 90degree C are rated for 204A. The design firm we are contracting this work to is suggesting we increase our conductor size to a 3/0. Their reason is based on since the motor which is of NEMA design B, and the molded case breaker we are feeding this motor from, their terminals are rated at 75degree Celsius, we need to correct the sizing of our conductor and rate it at 75degree C. In turn, the #2/0 cable rated at 75degree C ampacity rating of 178A is less than the 125% FLA rating of 185A.

Can anyone provide a logical reasoning as to why there is such an importance on going off the rating of the terminals? It has been standard practice at my company to base everything off of a 90degree C cable in a 40degree C ambient; we have never worried about terminal ratings in the past. I understand the fact of proper heat dissipation with sizing your cable according to the terminal rating. Typically motors arent sized to run at 100% full load anyway so the chances of getting to those high of currents would be rare.

What implications would arise, if any, if we were to use a 2/0 rated at 90degree C connected to a 75degree C rated terminal, and we did achieve 100% FLA on the motor? I dont suspect the terminal would fail. In my judgment I feel we would be okay using the 90degree C as we have continuously done in the past. Any explanations/justifications about this matter would be greatly appreciated. Thanks.

 

[jumper]

In my judgment I feel we would be okay using the 90degree C as we have continuously done in the past. Any explanations/justifications about this matter would be greatly appreciated. Thanks.

Simple. It is wrong and a code violation as you already know.

 

[Johnnybob]

Yup, your conductor is only as good as it's termination! If your lug's etc. are only rated at 75, so's your conductor! Or 60 C for that mater. Only as strong as the weakest link, yeah? :happyyes:

 

[eg6791]

Yup, your conductor is only as good as it's termination! If your lug's etc. are only rated at 75, so's your conductor! Or 60 C for that mater. Only as strong as the weakest link, yeah? :happyyes:

I guess what puzzles me is as to why are these lugs/terminals based off 60C and 75C...is it that they didnt have a means of testing them at 90C? I dont see a metal lug melting at 90C (196F).

 

[iwire]

I guess what puzzles me is as to why are these lugs/terminals based off 60C and 75C...is it that they didnt have a means of testing them at 90C? I dont see a metal lug melting at 90C (196F).

There are plenty of 90c terminals available.

What you will not find is 600 volt or less electrical equipment that is listed for use at 90c.

For instance you may see a 90 c terminal on a breaker but the breaker itself was only tested and listed for operation at 75 c.

 

[jim dungar]

I guess what puzzles me is as to why are these lugs/terminals based off 60C and 75C...is it that they didnt have a means of testing them at 90C? I dont see a metal lug melting at 90C (196F).

Blame the 'standards groups' like NEMA and UL.

But, the answer is: the conductors are often used as heat sinks.
According to a 1986 edition of NEMA AB1, for molded case circuit breakers, device terminals are limited to a maximum heat rise of 50C based on an ambient of 25C.

 

[Smart $]

... I dont see a metal lug melting at 90C (196F).

It's not about the lug or even conductor insulation melting. What is tested at the rated termination temperature is the integrity of the connection through multitudes of expansion and contraction from changes in current through the connection. A proper termination will maintain its integrity as long as the conductor does not exceed the rated temperature.

And there are ways to use the 90°C rated ampacity of a conductor... but not as long as the equipment being connected to has a lower termination temperature rating.

http://www2.schneider-electric.com/...7/en_US/Wire Terminations 0110DB9901R2-02.pdf and see page 4

 

[Mike Furlan]

Simple. It is wrong and a code violation as you already know.

Yes I would agree, this is a clear code violation as stated.

As I understand it the code is mostly a Heuristic, a "Rule of Thumb". It will not always "make sense" it just "is."

However, at least in one case, as in 310.15 (C) Engineering Supervision, you can use an "engineered approach to a code issue, in this case conductor ampacity."

 

[Smart $]

...However, at least in one case, as in 310.15 (C) Engineering Supervision, you can use an "engineered approach to a code issue, in this case conductor ampacity."

Yes, but even going that route, the method of calculating conductor ampacity is proscribed. I have no idea whether you gain or lose, on average, by going that route... but I don't see a lot of engineers taking that route, so that in itself tells me it isn't generally worth the added effort.

 

[don_resqcapt19]

...
However, at least in one case, as in 310.15 (C) Engineering Supervision, you can use an "engineered approach to a code issue, in this case conductor ampacity."

While that may change the ampacity of the conductors themselves, it has no effect on the rule for the terminations.

110.14(C)(1) Equipment Provisions. The determination of termination provisions of equipment shall be based on 110.14(C)(1)(a) or
(C)(1)(b). Unless the equipment is listed and marked otherwise, conductor ampacities used in determining equipment termination provisions shall be based on Table 310.15(B)(16) as appropriately modified by 310.15(B)(7).

 

[Smart $]

While that may change the ampacity of the conductors themselves, it has no effect on the rule for the terminations.

And the reason 110.14(C)(1) specifies Table 310.15(B)(16) is because it just uses the values in the Table. It has nothing to do with actual ampacity of the conductor and more to do with the temperature of the conductor at rated current. Even when you use another table for conductor ampacity, such as Table 310.15(B)(17) when run in cable tray, you still have to use Table 310.15(B)(16) for terminations.

Many do not understand why, and their skepticism is warranted to a degree, but the issue I point out to them is that the different tables are for conditions of use beyond the confines of the termination enclosure (i.e. the wiring method). There is no wiring method inside a termination enclosure. The NEC treats the inside of all enclosures the same regardless of conditions of use beyond its confines. A #4 copper wire is the same to a termination whether it is run in cable tray or a member of direct buried cable.

 

[Julius Right]

First of all according to 430.6 Ampacity and Motor Rating Determination.
one has to state the motor current according the HP following the suitable
table. In this case from Table 430.249 for 460 V and 125 hp current has to be only 135 A. Then, as per 430.22 Single Motor.125% it has be 168.75 A and so 2/0 awg copper Table 310.15(B)(16) 75oC allowable 175 A.
All that in order to comply with NEC.
I don’t know if the cable will run underground but for exposed in free air NEC Table 310.15(B)(16) is very oversized.:weeping:

 

[david luchini]

First of all according to 430.6 Ampacity and Motor Rating Determination.
one has to state the motor current according the HP following the suitable
table. In this case from Table 430.249 for 460 V and 125 hp current has to be only 135 A. Then, as per 430.22 Single Motor.125% it has be 168.75 A and so 2/0 awg copper Table 310.15(B)(16) 75oC allowable 175 A.
All that in order to comply with NEC.
I don’t know if the cable will run underground but for exposed in free air NEC Table 310.15(B)(16) is very oversized.:weeping:

You've got the wrong table. You want 450.250 which says 156A for 125HP. 125% of that is 195A, so #2/0 is too small.

 

[Julius Right]

You've got the wrong table. You want 450.250 which says 156A for 125HP. 125% of that is 195A, so #2/0 is too small.

Thank you, David. I forgot, in USA an odd supply system exists [4 wires, two phases].:ashamed1:
However, still I don't understand why two-phase supply system requires only 135 A [instead of 240 A as an European system will require at two-phase 460 A- 125HP.]:?