De-rating of conductors

[tahir mehmood]

Hi all;

If i have a conductor of 55 amps, the terminal lugs at both side are rated at 60 centigrade, and the insulation is THHN. shall i select the conductor from 90 centigrade column or 60 Centigrade column. kindly clarify as THHN insulation comes under 90 column, and the rule says if amperes are less than or equal to 100 A, use 60 centigrade column.

 

[ActionDave]

You use the ninety degree column for any adjustments for number of current carrying conductors in a conduit or temperature but your final amperage can't be larger than the sixty degree column.

 

[infinity]

Hi all;

If i have a conductor of 55 amps, the terminal lugs at both side are rated at 60 centigrade, and the insulation is THHN. shall i select the conductor from 90 centigrade column or 60 Centigrade column. kindly clarify as THHN insulation comes under 90 column, and the rule says if amperes are less than or equal to 100 A, use 60 centigrade column.

As Dave stated you would use the 60° C column. If the terminals on both ends and the conductor were rated for 75° C then you could use the 75° C column even if 100 amps or less.

 

[winnie]

Think about it this way: you have to evaluate the circuit as a set of segments, and then use the minimum ampacity of any of the segments.

The _wire_ has 90C insulation, but the terminals have a 60C rating. So for the segment of the circuit right near the terminals you need to use the 60C ampacity value.

But say you have a bunch of wires for different circuit in a single conduit. For this segment of the circuit, which is not near the breaker terminals, you get to use the 90C rating of the wire itself.

Say in another part of the circuit you only have wires for this circuit, but it runs through a 45C hot room. In this section, you again get to use the 90C rating of the wire to adjust for ambient temperature.

After you do all of your derating and adjustments for each different segment, you pick the lowest value. That is the ampacity of the _circuit_.

-Jon

 

[Carultch]

Think about it this way: you have to evaluate the circuit as a set of segments, and then use the minimum ampacity of any of the segments.

The _wire_ has 90C insulation, but the terminals have a 60C rating. So for the segment of the circuit right near the terminals you need to use the 60C ampacity value.

But say you have a bunch of wires for different circuit in a single conduit. For this segment of the circuit, which is not near the breaker terminals, you get to use the 90C rating of the wire itself.

Say in another part of the circuit you only have wires for this circuit, but it runs through a 45C hot room. In this section, you again get to use the 90C rating of the wire to adjust for ambient temperature.

After you do all of your derating and adjustments for each different segment, you pick the lowest value. That is the ampacity of the _circuit_.

-Jon

If the equipment is in a hot 45C room and has 60C rated terminals, is there any temperature correction that applies to the 60C termination ampacity? I know 110.14(C) says otherwise, but conceptually it seems like this is an unintended gap in the code.

 

[kwired]

If the equipment is in a hot 45C room and has 60C rated terminals, is there any temperature correction that applies to the 60C termination ampacity? I know 110.14(C) says otherwise, but conceptually it seems like this is an unintended gap in the code.

That is where 310.15(B)(2) comes into play, for the conductor, there is no adjustment for the termination.

If the room were normally over 60C ambient - you wouldn't put much in there that wasn't directly acting on something in that environment, and it would probably have more then 60C terminations, maybe even more then 90C conductors and terminations in many instances.