Wire Size Using 90C or 75C Columns

[JimmysLimeade]

I am having trouble understanding which ampacity to use when sizing AC wires and I can't wrap my head around it for some reason. Here's an example of what I am having issues with:

I have an inverter with an output current of 42A in an area with an ambient temperature of 35C. There are 3 THWN-2 CU conductors in the conduit from the inverter to MSP. What size of conductor would you use and how do you get to that answer.

The way I have previously done it was take the conductor ampacity in the 90C column and use the derate factors for temperature and current-carrying conductors to get the max conductor ampacity. That number has to be larger than the output current * 1.25.

Using that method, I would get a #6 conductor for this example. But if I used the ampacity at 75C, I would get a #4 conductor. After reading a lot of different ways people get their sizing, I feel like this is the wrong way to do it.

What should I change here? Could someone help me understand what is missing?

Thank you!

 

[wwhitney]

Here's what it boils down to, although tracing this through the various NEC sections and citations is fairly complicated, so I omit the references. You need to do three checks:

1) Termination check: for this you are limited to the termination temperature rating (equipment is basically always 60C or 75C, although if you have a wire segment that just runs from junction box to junction box and is joined with 90C connectors, you can use 90C for that wire segment). And you need to use the 125% factor for continuous loads (since that factor is due to OCPD performance only, and OCPD is a termination). You do not need to use ampacity adjustment (for number of CCCs, as presumably this is in an enclosure where the conductors are spread out) or correction (for temperature; this omission strikes me as odd on the physics, but perhaps the idea is that the equipment has specified acceptable ambient temperatures and the termination temperature ratings take that into account.)

2) Mid-run check: For this you don't need the 125% continuous factor, but you do need to do ampacity adjustment and correction.

3) OCPD check: the answer from (2) is the actual wire ampacity. That ampacity has to be high enough to be properly protected by the minimum OCPD required for the circuit.

So take your example of an inverter with a maximum continuous output current of 42A. That's considered a continuous "load", so 125% * 42 = 62.5A. Suppose you are using 90C wire from the inverter to the OCPD, both of which have 75C rated terminations.

For 1), you'll need the 75C column table value to be at least 62.5A.
For 2), you'll need the 90C column, times adjustment values for CCCs and times correction values for temperature, to be at least 42A.
For 3), you'll need to use a minimum 62.5A OCPD, which means a 70A OCPD, the next standard size up. As 60A is the next lower size, your ampacity from (2) actually needs to be 61A, which can be protected by a 70A breaker per 240.4(B).

Thus based on (1) only you can use #6 Cu, which has a 75C base ampacity of 65A and a 90C base ampacity of 75A. But based on (2) and (3) the final ampacity needs to be at least 61A, so if the adjustment and correction factors multiplied together end up less than 61/75 = 0.81, you'll need to bump up the size.

Cheers, Wayne

 

[infinity]

The way I have previously done it was take the conductor ampacity in the 90C column and use the derate factors for temperature and current-carrying conductors to get the max conductor ampacity. That number has to be larger than the output current * 1.25.

Using that method, I would get a #6 conductor for this example. But if I used the ampacity at 75C, I would get a #4 conductor. After reading a lot of different ways people get their sizing, I feel like this is the wrong way to do it.

Use the ampacity rating of the conductor based on the insulation type.

 

[wwhitney]

The way I have previously done it was take the conductor ampacity in the 90C column and use the derate factors for temperature and current-carrying conductors to get the max conductor ampacity. That number has to be larger than the output current * 1.25.

As per my earlier description, that's a comparison you never have to do. That is for (1) you use the column of the termination temperature, and a 125% factor, but don't derate. While for (2) and (3) you use the column of the insulation temperature, no 125% factor, and you do derate.

Cheers, Wayne

 

[JimmysLimeade]

Here's what it boils down to, although tracing this through the various NEC sections and citations is fairly complicated, so I omit the references. You need to do three checks:

1) Termination check: for this you are limited to the termination temperature rating (equipment is basically always 60C or 75C, although if you have a wire segment that just runs from junction box to junction box and is joined with 90C connectors, you can use 90C for that wire segment). And you need to use the 125% factor for continuous loads (since that factor is due to OCPD performance only, and OCPD is a termination). You do not need to use ampacity adjustment (for number of CCCs, as presumably this is in an enclosure where the conductors are spread out) or correction (for temperature; this omission strikes me as odd on the physics, but perhaps the idea is that the equipment has specified acceptable ambient temperatures and the termination temperature ratings take that into account.)

2) Mid-run check: For this you don't need the 125% continuous factor, but you do need to do ampacity adjustment and correction.

3) OCPD check: the answer from (2) is the actual wire ampacity. That ampacity has to be high enough to be properly protected by the minimum OCPD required for the circuit.

So take your example of an inverter with a maximum continuous output current of 42A. That's considered a continuous "load", so 125% * 42 = 62.5A. Suppose you are using 90C wire from the inverter to the OCPD, both of which have 75C rated terminations.

For 1), you'll need the 75C column table value to be at least 62.5A.
For 2), you'll need the 90C column, times adjustment values for CCCs and times correction values for temperature, to be at least 42A.
For 3), you'll need to use a minimum 62.5A OCPD, which means a 70A OCPD, the next standard size up. As 60A is the next lower size, your ampacity from (2) actually needs to be 61A, which can be protected by a 70A breaker per 240.4(B).

Thus based on (1) only you can use #6 Cu, which has a 75C base ampacity of 65A and a 90C base ampacity of 75A. But based on (2) and (3) the final ampacity needs to be at least 61A, so if the adjustment and correction factors multiplied together end up less than 61/75 = 0.81, you'll need to bump up the size.

Cheers, Wayne

WOW! Thank you so much for taking the time to write all that out! It helped a ton