Wire Size

[Steelhead]

Hey Guys,

I want to make sure I calculated my wire size correctly. I have a 75A continuous load, 9 CCC in the same conduit. Seventy-five degree terminals. Minimum ampacity would be 75A x 1.25=93.75A. Nine CCCs in the same conduit derates wire to 70%. 1AWG THHN rated 145A @90C = 145 x 0.70=101.5A

So I should use 1AWG wire that is derated to 101.5A to satisfy 9 CCCs in the same conduit with a minimum ampacity of 93.75A?

Thanks!

 

[charlie b]

Yes, though I would have gotten there a different (but equivalent) way.

• 75 x 1.25 = 93.75
• 93.75 / 0.7 = 133.9
• Smallest wire with a minimum of 133.9 ampacity in the 90C column is #1.

 

[david luchini]

Hey Guys,

I want to make sure I calculated my wire size correctly. I have a 75A continuous load, 9 CCC in the same conduit. Seventy-five degree terminals. Minimum ampacity would be 75A x 1.25=93.75A. Nine CCCs in the same conduit derates wire to 70%. 1AWG THHN rated 145A @90C = 145 x 0.70=101.5A

So I should use 1AWG wire that is derated to 101.5A to satisfy 9 CCCs in the same conduit with a minimum ampacity of 93.75A?

Thanks!

#2 THHN would be OK. It's larger than the min required conductor size (#3) and the ampacity is 91, which is sufficient for the load (75A) and properly protected by a 100A OCPD.

 

[Mustwin351]

Would not #3 thhn be sufficient for the 75 amp load as David stated.

 

[ramsy]

#2 THHN would be OK. It's larger than the min required conductor size (#3) and the ampacity is 91, which is sufficient for the load (75A) and properly protected by a 100A OCPD.

Not legal without engineering supervision, per 310.60(B)

Engineering may show #2 @ 71°C (Pushing 75°C Equipment Limits),
but #2 violates NEC tables. No can do without stamped plans from licensed EE/PE/Architect.

NEC Tables requires 94A available after derating, not 91A with #2cu, which comes up short.
#1awg operates near 63°C under conditions specified, which is NEC's intended margin for 75°C equipment in this case.

 

[ramsy]

Would not #3 thhn be sufficient for the 75 amp load as David stated.

Per derating required, and NEC Table default ambient of 30°C, #3 would burn up the equipment at 82°C

If, under engineering supervision, you could prove an ambient of -40°C all year round, #3 would operate near 73°C.

 

[JFletcher]

To the original poster, your math and David's math in post #2 are correct. That said, this is making the assumption that all of your initial parameters are correct, and that no data is missing that would or could affect your calculations, such as ambient temperature.

If this is a test question, then you have the correct answer. If it is a real world application there could be errors in some of the data, like for example thinking the load is continuous when it is not, miscounting the number of current carrying conductors, not factoring in any ambient temperature derating, etc.

 

[david luchini]

Would not #3 thhn be sufficient for the 75 amp load as David stated.

#3 thhn is sufficient for the load, but would not be properly protected by a 100A OCPD.

 

[david luchini]

Not legal without engineering supervision, per 310.60(B)

Engineering may show #2 @ 71°C (Pushing 75°C Equipment Limits),
but #2 violates NEC tables. No can do without stamped plans from licensed EE/PE/Architect.

NEC Tables requires 94A available after derating, not 91A with #2cu, which comes up short.
#1awg operates near 63°C under conditions specified, which is NEC's intended margin for 75°C equipment in this case.

No engineering supervision required. #2 THHN is is larger than the minimum required conductor size, has sufficient ampacity for the load, and would be properly protected by a 100A OCPD which is the minimum allowed size for a 75A continuous load.

 

[Steelhead]

#2 THHN would be OK. It's larger than the min required conductor size (#3) and the ampacity is 91, which is sufficient for the load (75A) and properly protected by a 100A OCPD.

Wouldn't the ampacity need to be 94? Continuous load of 75A x 125%?

 

[Steelhead]

To the original poster, your math and David's math in post #2 are correct. That said, this is making the assumption that all of your initial parameters are correct, and that no data is missing that would or could affect your calculations, such as ambient temperature.

If this is a test question, then you have the correct answer. If it is a real world application there could be errors in some of the data, like for example thinking the load is continuous when it is not, miscounting the number of current carrying conductors, not factoring in any ambient temperature derating, etc.

I assume you mean post #3? There should be no missing data that I'm aware of. Not a test question. Seventy-five amp continuous load with 9 CCCs in conduit. No ambient temp derating needed.

 

[Steelhead]

Yes, though I would have gotten there a different (but equivalent) way.

• 75 x 1.25 = 93.75
• 93.75 / 0.7 = 133.9
• Smallest wire with a minimum of 133.9 ampacity in the 90C column is #1.

Thanks Charlie!

 

[david luchini]

Wouldn't the ampacity need to be 94? Continuous load of 75A x 125%?

No, the ampacity needs to be 75A or greater.

The minimum conductor size, before the application of any adjustment or correction factors, needs to have an allowable ampacity of not less than 75x125%.

 

[Steelhead]

No, the ampacity needs to be 75A or greater.

The minimum conductor size, before the application of any adjustment or correction factors, needs to have an allowable ampacity of not less than 75x125%.

In a previous post I mentioned that the ampacity needed to be 94A (75x125%). I'm confused by your response.

In my mind the ampacity needs to be 94A because of the 75A continuous load. Correct?

Now, I have 9 current carrying conductors in the same raceway so derating is required. Correct? So, how do you figure out what size wire is needed after accounting for the 9 CCCs?

 

[david luchini]

In a previous post I mentioned that the ampacity needed to be 94A (75x125%). I'm confused by your response.

In my mind the ampacity needs to be 94A because of the 75A continuous load. Correct?

No, that is not correct.

Now, I have 9 current carrying conductors in the same raceway so derating is required. Correct? So, how do you figure out what size wire is needed after accounting for the 9 CCCs?

You didn't mention if this was a feeder or branch circuit, but the rules are similar for both. 215.2 & 215.3 cover feeders and 210.19 & 210.20 cover branch circuits.

Your calculated load is 75A. Per 215.2(A)(1) (assuming feeder,) the conductors shall have an ampacity not less than required to serve the load...that is 75A. So you need a conductor that has an ampacity of at least 75A.

The second thing 215.2(A)(1) requires is that the conductor have a minimum SIZE, before the application of any adjustment or correction factor, that has an allowable ampacity of not less than the noncontinuous load plus 125% of the continuous load. This is where the 125% comes in. 75A*125%=93.75A. Table 310.15(B)(16) tells us that the smallest allowable conductor will be #3 AWG.

Next, you are applying an adjustment factor for 9 CCCs, which is 70%. That means you need a conductor that has an ampacity of at least 75A/0.7=107.1A. Since you are using THHN, the 90deg column can be used. T310.15(B)(16) shows that #3 AWG, THHN will have the proper starting ampacity, 115A. (Or going in the other direction, #3 THHN has an allowable ampacity of 115A at 90deg, applying the 70% adjustment factor makes the ampacity 80.5A, which is larger than the load of 75A from the first line of 215.2(A)(1)).

Next, 215.3 says the conductor must be protected against overcurrent per Part I of Art 240, and also says the rating of the ocpd shall not be less than the noncontinuous load plus 125% of the continuous load, or 93.75A, so a 100A ocpd would be the minimum size allowed, so let's assume the ocpd is 100A. The #3 AWG adjusted for 9 CCCs has an ampacity of 80.5, so it would not be properly protected by a 100A ocpd using the next size up rule.

#2 AWG would have an adjusted ampacity of 91A, so #2 THHN would have an ampacity large enough for the load (first part of 215.2(A)(1)), would be larger than the minimum allowable conductor size (second part of 215.2(A)(1)), and would be properly protected by a 100A ocpd (per 215.3 and 240.)

 

[david luchini]

Of course, I would use a shortcut after the minimum conductor size:

I know the minimum conductor size will be #3: 75A*1.25=93.75A = #3 AWG

I know the minimum ocpd size will be 100A: 93.75 next size up = 100A

I know I need a conductor with 75A ampacity to handle the load, but I also know that I need a conductor with greater than 90A to be protected by the 100A ocpd using the next size up rule in 240.4(B).

So 90.1A/0.7 for the 9CCCs adjustment factor = 128.7A. In the 90deg column, #2 AWG is the smallest conductor with an ampacity larger than 128.7.

 

[LarryFine]

You didn't mention if this was a feeder or branch circuit, but the rules are similar for both. 215.2 & 215.3 cover feeders and 210.19 & 210.20 cover branch circuits.

Your calculated load is 75A. Per 215.2(A)(1) (assuming feeder,) the conductors shall have an ampacity not less than required to serve the load...that is 75A. So you need a conductor that has an ampacity of at least 75A.

The second thing 215.2(A)(1) requires is that the conductor have a minimum SIZE, before the application of any adjustment or correction factor, that has an allowable ampacity of not less than the noncontinuous load plus 125% of the continuous load. This is where the 125% comes in. 75A*125%=93.75A. Table 310.15(B)(16) tells us that the smallest allowable conductor will be #3 AWG.

Next, you are applying an adjustment factor for 9 CCCs, which is 70%. That means you need a conductor that has an ampacity of at least 75A/0.7=107.1A. Since you are using THHN, the 90deg column can be used. T310.15(B)(16) shows that #3 AWG, THHN will have the proper starting ampacity, 115A. (Or going in the other direction, #3 THHN has an allowable ampacity of 115A at 90deg, applying the 70% adjustment factor makes the ampacity 80.5A, which is larger than the load of 75A from the first line of 215.2(A)(1)).

Next, 215.3 says the conductor must be protected against overcurrent per Part I of Art 240, and also says the rating of the ocpd shall not be less than the noncontinuous load plus 125% of the continuous load, or 93.75A, so a 100A ocpd would be the minimum size allowed, so let's assume the ocpd is 100A. The #3 AWG adjusted for 9 CCCs has an ampacity of 80.5, so it would not be properly protected by a 100A ocpd using the next size up rule.

#2 AWG would have an adjusted ampacity of 91A, so #2 THHN would have an ampacity large enough for the load (first part of 215.2(A)(1)), would be larger than the minimum allowable conductor size (second part of 215.2(A)(1)), and would be properly protected by a 100A ocpd (per 215.3 and 240.)

Now there's a candidate for a sticky!

 

[Steelhead]

No, that is not correct.



You didn't mention if this was a feeder or branch circuit, but the rules are similar for both. 215.2 & 215.3 cover feeders and 210.19 & 210.20 cover branch circuits.

Your calculated load is 75A. Per 215.2(A)(1) (assuming feeder,) the conductors shall have an ampacity not less than required to serve the load...that is 75A. So you need a conductor that has an ampacity of at least 75A.

The second thing 215.2(A)(1) requires is that the conductor have a minimum SIZE, before the application of any adjustment or correction factor, that has an allowable ampacity of not less than the noncontinuous load plus 125% of the continuous load. This is where the 125% comes in. 75A*125%=93.75A. Table 310.15(B)(16) tells us that the smallest allowable conductor will be #3 AWG.

Next, you are applying an adjustment factor for 9 CCCs, which is 70%. That means you need a conductor that has an ampacity of at least 75A/0.7=107.1A. Since you are using THHN, the 90deg column can be used. T310.15(B)(16) shows that #3 AWG, THHN will have the proper starting ampacity, 115A. (Or going in the other direction, #3 THHN has an allowable ampacity of 115A at 90deg, applying the 70% adjustment factor makes the ampacity 80.5A, which is larger than the load of 75A from the first line of 215.2(A)(1)).

Next, 215.3 says the conductor must be protected against overcurrent per Part I of Art 240, and also says the rating of the ocpd shall not be less than the noncontinuous load plus 125% of the continuous load, or 93.75A, so a 100A ocpd would be the minimum size allowed, so let's assume the ocpd is 100A. The #3 AWG adjusted for 9 CCCs has an ampacity of 80.5, so it would not be properly protected by a 100A ocpd using the next size up rule.

#2 AWG would have an adjusted ampacity of 91A, so #2 THHN would have an ampacity large enough for the load (first part of 215.2(A)(1)), would be larger than the minimum allowable conductor size (second part of 215.2(A)(1)), and would be properly protected by a 100A ocpd (per 215.3 and 240.)

Wow, now I'm really confused. I need to chew on that for awhile. I guess what I thought I understood about sizing wire is incorrect.

 

[david luchini]

Wow, now I'm really confused. I need to chew on that for awhile. I guess what I thought I understood about sizing wire is incorrect.

Example D3(a) in Annex D is a good example for review.

 

[ramsy]

Example D3(a) in Annex D is a good example for review.

Example D3(a) omits Adjustments & Corrections.

1) Adjustment for 8 current-carrying conductors (CCC) @ 70% is omitted from Overcurrent calculation.
2) Continuous Load correction of 125% is omitted from Feeder calculation.