Branch Circuit Sizing Temperature Correction

[hill900505]

Hello,

Can somebody explain when sizing branch circuit conductor size considering temperature correction for a THHN wire, why do we correct the ampacity per 90 C column in 310.15.b.2.a BUT not based on 60 degree C column if the terminal is only rated for 60 degree C. I thought no matter what type of insulation the wire has, if equipment current is 100A or less, we need to go to 310.15.b.16, 60 degree C column to size the wire and de-rate the ampacity using the 60 degree C column for temperature correction at table 310.15.b.2.a.

Greatly appreciated
Jim

 

[iwire]

Hello,

Can somebody explain when sizing branch circuit conductor size considering temperature correction for a THHN wire, why do we correct the ampacity per 90 C column in 310.15.b.2.a BUT not based on 60 degree C column if the terminal is only rated for 60 degree C.

We can use the 90 C column for derating wire that is 90C rated but if after derating the result is higher than the 60c rating of a terminal we would be stuck using the 60c rating.

In other words when it's all said and done we are still limited by the terminal temperature.

I thought no matter what type of insulation the wire has, if equipment current is 100A or less, we need to go to 310.15.b.16, 60 degree C column to size the wire

Unless the equipment is marked 75 C which it often is.

 

[Dennis Alwon]

Hello,

Can somebody explain when sizing branch circuit conductor size considering temperature correction for a THHN wire, why do we correct the ampacity per 90 C column in 310.15.b.2.a BUT not based on 60 degree C column if the terminal is only rated for 60 degree C. I thought no matter what type of insulation the wire has, if equipment current is 100A or less, we need to go to 310.15.b.16, 60 degree C column to size the wire and de-rate the ampacity using the 60 degree C column for temperature correction at table 310.15.b.2.a.

Greatly appreciated
Jim

110.14(C) states 100 1mps or less we must use 60C unless we know the rating of the terminations. Thus if the breakers and terminals are rated 75C and the wire is 90C then we can use the 75C rating.

Now the correction factor is based on the conductor insulation. We can use 90C for de-rating purposes as long as the final overcurrent protective device is not larger than 75C or 60C (depending on terminations) rating of the wire

 

[Dennis Alwon]

This is what 110.14(C) states

(C) Temperature Limitations. The temperature rating associated
with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.
Conductors with temperature ratings higher than specified
for terminations shall be permitted to be used for ampacity
adjustment, correction, or both.

 

[hill900505]

Thanks Dennis and iwire. But if we use 90 degree C ampacity for temperature correction, after derating, we get a ampacity value, how do we know if the conductor carrying this amount of current will be hotter or cooler than 60 degree C of the terminal rating?

 

[iwire]

Thanks Dennis and iwire. But if we use 90 degree C ampacity for temperature correction, after derating, we get a ampacity value, how do we know if the conductor carrying this amount of current will be hotter or cooler than 60 degree C of the terminal rating?

I am going to make numbers up and exaggerate them for an example.

Let's say you have a wire rated at 20 amps @ 60C and 40 amps @ 90C.

You have equipment rated 60C

You derate from 40 amps and the result is 30 amps.

That 30 amp result cannot be used because 30 amps is more than the 60C rating of the wire. and you would be stuck at the 20 amp rating of the 60c column.

 

[Smart $]

Thanks Dennis and iwire. But if we use 90 degree C ampacity for temperature correction, after derating, we get a ampacity value, how do we know if the conductor carrying this amount of current will be hotter or cooler than 60 degree C of the terminal rating?

We do not know... but Code has us assume when the wire is conducting the amount of current in the 60°C column, that the wire will not exceed 60°C. To provide a bit of reassurance, Code pads the value we cannot exceed by adding an extra 25% of the continuous portion of the load.

For the sake of discussion, let's say the 90°C derated value is equal to the 60°C column value. The hard part to understand is at this same level of current, Code has us believe the conductor temperature will be 60°C at the terminal while it'll be 90°C in the raceway or cable.

Consider that terminals are typically in enclosures with substantially more volume per length of wire than the same wire in a raceway (typically). This allows for better heat dissipation, so the wire could theoretically be 60°C at the terminal and 90°C in the raceway.

 

[kwired]

Thanks Dennis and iwire. But if we use 90 degree C ampacity for temperature correction, after derating, we get a ampacity value, how do we know if the conductor carrying this amount of current will be hotter or cooler than 60 degree C of the terminal rating?

When selecting minimum conductor size needed you basically have to calculate two things.

1 - minimum size required based on termination temperature rating, which will usually be 60 or 75 C.

2 - minimum size required based on insulation temperature rating, which for most general instances with today's conductors will be 90C.

your minimum size conductor is the larger of those two calculations. Unless you have temperature or number of conductors in a raceway to consider it will usually be the termination temp that yields the larger conductor.