Clarification of Feeder Ampacity Calculation

[Techmagician]

Here?s the question:
What is the minimum size THHN copper conductor required for a feeder supplying a continuous load of 85A and a non-continuous load of 105A while in an ambient temperature of 34C and with a total of 4 current-carrying conductors in the same raceway?
NEC 215.2 The minimum feeder-circuit conductor size conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the non-continuous load plus 125 percent of the continuous load.
Therefore calculated load is
Continuous load 85A x 125% = 106.25A
Plus Non-Continuous load +105A
211.25A
Because there is no specification for temperature rating of the terminal and the load is over 100A the 75C column of NEC Table 310.16 is used in accordance with NEC 110.14(1)(b)(2).
4/0 AWG rated at 230A is sufficient to carry the 211.25A calculated load.
This question requires two deratings, one for ambient temperature and one for multiple conductors in a raceway.
For deration the ampacity for 4/0 THHN Copper is in the 90C column of NEC 310.16 is 260A
Correction factor for 31-35C on NEC Table 310.16 is .96
Therefore 260A x .96 = 249.6A
Adjustment factor for 4-6 wires in the same raceway on NEC Table 310.15(b)(2)(a) is .80
Therefore 249.6A x .80 is 199.68A
This is where I am confused.
Is the derated ampacity required to carry the actual load of 190A or the calculated load for continuous of 211.25A? My interpretation is the actual load of 190A because of NEC 215.2 but I am not sure.

 

[kwired]

When you determine minimum size of conductor needed you usually figure two different values, unless you are using conductors with same temperature rating as terminal temperature rating.

You figured the minimum conductor correctly based on 75C terminations and came up with 4/0 cu. That is first value and is based on minimum size necessary based on termination temperature.

The second value is based on determining what size is needed without excessive heating of conductor insulation. Which I believe you also figured correctly.

The next step is to select the larger conductor of the two values. The 190 amp conductor is fine for insulation temperature but you still need the 211 amp minimum at 75C for the termination temperature rating, so minimum acceptable conductor size is 4/0 for this application.

 

[david luchini]

Is the derated ampacity required to carry the actual load of 190A or the calculated load for continuous of 211.25A? My interpretation is the actual load of 190A because of NEC 215.2 but I am not sure.

The derated ampacity is required to carry the actual load of 190A, so you are correct in that respect.

However, there is a missing link in the question. Yes, the minimum conductor size is 4/0 for non-continuous plus 125% of continuous loads per 215.2(A)(1). And, yes, 4/0 THHN will have an adjusted ampacity of 199.68 which is large enough to carry the load of 190Amps.

BUT, 215.3 requires that the OCPD for the feeder be not less than the non-continuous load plus 125% of the continuous load. This would require an OCPD of 225A. The 4/0 THHN with an ampacity of 199.68 will not be properly protected by a 225A OCPD. You would need a minimum 250MCM THHN feeder to meet the specified conditions.

 

[Techmagician]

Would NEC 240.4(b) allow you to use the next size OCPD 225A since its under 800A?

 

[Dennis Alwon]

Would NEC 240.4(b) allow you to use the next size OCPD 225A since its under 800A?

No because the next size from 199.68 is 200 amps. If the conductor ampacity was 201 amps then you could go to the 225 amp breaker but not when it is below 200 amps.

 

[Dennis Alwon]

Also the ampacity of the conductor must be 211.25. Now if the conductor you use is good for that then you can use 225 amp breaker

 

[Smart $]

No because the next size from 199.68 is 200 amps. If the conductor ampacity was 201 amps then you could go to the 225 amp breaker but not when it is below 200 amps.

Also the ampacity of the conductor must be 211.25. Now if the conductor you use is good for that then you can use 225 amp breaker

Seems like you're contradicting yourself...

Here's how it goes...

1. The conductor's adjusted and corrected ampacity must exceed the next lower standard ocp rating of 800A or less (201A or greater in this case).
2. The conductor's allowable ampacity before adjustment and correction (i.e. its tabled value) must equal or exceed non-continuous plus 125% continuous load value (211A in this case).

Two different assessments.

 

[Smart $]

...
However, there is a missing link in the question. ...

BUT, 215.3 requires that the OCPD for the feeder be not less than the non-continuous load plus 125% of the continuous load. This would require an OCPD of 225A. The 4/0 THHN with an ampacity of 199.68 will not be properly protected by a 225A OCPD. You would need a minimum 250MCM THHN feeder to meet the specified conditions.

BUT the exception permits OCPD rating determination by continuous and non-continuous summed (100% of both).

At this point I going to note that typically question writers do not consider exceptions as relevant in the consideration of their 'correct' answer... but how is one to know for certain?

 

[Techmagician]

Ok I get it. Derated ampacity is 199.68A. Next up is 200A but NEC 215.3 requires at least the 211.25A so upsize wire to 250kcmil and use 225A OCPD. Thank you to everyone for helping me get this straight.:thumbsup:

 

[kwired]

We have been overlooking one thing in the OP - the 34C ambient temp.

This is where it gets tricky, depending on where you decide to round off fractions, and whether or not you are one that thinks that 125% of continuous load applies when considering ampacity adjustments. I have been taught you figure ampacity adjustments at 100% of load, but have seen good arguments that it must be 125% of continuous load.

If you don't apply the 125% when calculation ampacity adjustments you get a minimum needed ampacity of 260 which is right at the max 90C rating of 4/0 copper.


If you do apply the 125% you get a minimum needed ampacity of 290 which is right at the max 90C rating of 250 copper. Round up any fractions in the right place and you are going to be over 290 and will have results needing at least 300 copper.

 

[david luchini]

We have been overlooking one thing in the OP - the 34C ambient temp.

I don't see that anyone has overlooked the 34C ambient temp. Everyone has included it in their calculations.

This is where it gets tricky, depending on where you decide to round off fractions, and whether or not you are one that thinks that 125% of continuous load applies when considering ampacity adjustments. I have been taught you figure ampacity adjustments at 100% of load, but have seen good arguments that it must be 125% of continuous load.

I don't see where rounding off a fractions affects this problem at all. And you are correct that the ampacity adjustments should be figured at 100% of load. 215.2(A)(1) says that the feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV and V of Article 220. 310.15(B) says that the ampacity of the conductors shall be as specified in the ampacity tables as modified by (B)(1) through (B)(6)

If you don't apply the 125% when calculation ampacity adjustments you get a minimum needed ampacity of 260 which is right at the max 90C rating of 4/0 copper.

Here, you would need a conductor with a minimum ampacity (before adjustment) of 247, not 260.

If you do apply the 125% you get a minimum needed ampacity of 290 which is right at the max 90C rating of 250 copper. Round up any fractions in the right place and you are going to be over 290 and will have results needing at least 300 copper.

Here, you would need a conductor with a minimum ampacity (before adjustment) of 275, not 290.

In both cases (whether you adjust the ampacity to 100% of the load or non-cont + 125% continuous load), 250mcm THHN cu would be the minimum size for the feeder, because the minimum feeder OCPD will be 225A. I don't see where rounding of fractions would get you to require 300mcm cu.

 

[Smart $]

We have been overlooking one thing in the OP - the 34C ambient temp.

This is where it gets tricky, depending on where you decide to round off fractions, and whether or not you are one that thinks that 125% of continuous load applies when considering ampacity adjustments. I have been taught you figure ampacity adjustments at 100% of load, but have seen good arguments that it must be 125% of continuous load.

If you don't apply the 125% when calculation ampacity adjustments you get a minimum needed ampacity of 260 which is right at the max 90C rating of 4/0 copper.


If you do apply the 125% you get a minimum needed ampacity of 290 which is right at the max 90C rating of 250 copper. Round up any fractions in the right place and you are going to be over 290 and will have results needing at least 300 copper.

Don't know how you came up with 260 and 290, but just to keep things in perspective, you seem to be using the 'unadjust, uncorrect' method for your values...

85A + 105A = 190A
(85A ? 125%) + 105A = 211.25 minimum Table ampacity

​

190A ? 0.96 ? 80% = 247.40A minimum Table ampacity unadjusted, uncorrected​

211.25A ? 0.96 ? 80% = 275.07A​

Otherwise, I agree that 125% of continuous loading only applies before adjustment and correction (the 211.25A value)

 

[kwired]

I don't see that anyone has overlooked the 34C ambient temp. Everyone has included it in their calculations.



I don't see where rounding off a fractions affects this problem at all. And you are correct that the ampacity adjustments should be figured at 100% of load. 215.2(A)(1) says that the feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV and V of Article 220. 310.15(B) says that the ampacity of the conductors shall be as specified in the ampacity tables as modified by (B)(1) through (B)(6)




Here, you would need a conductor with a minimum ampacity (before adjustment) of 247, not 260.




Here, you would need a conductor with a minimum ampacity (before adjustment) of 275, not 290.

In both cases (whether you adjust the ampacity to 100% of the load or non-cont + 125% continuous load), 250mcm THHN cu would be the minimum size for the feeder, because the minimum feeder OCPD will be 225A. I don't see where rounding of fractions would get you to require 300mcm cu.

How are you coming up with your figures of 247, or 275?[/quote]

I see what I did wrong - I selected 60deg C ambient correction adjustment factor of .91 instead of 90deg C factor of .96. I was attempting to do the right thing but wasn't paying attention to using correct column:slaphead: I still come up with different answer than you have for the 125% continuous load scenario but still within same conductor size range. Rounding off does not have an impact on conductor size in this case but if ambient temp correction factor would have been .91 it would put your result right on the borderline and rounding up/down would have made difference between 260 and 261.

I have:

85 amps continuous load.
105 non continuous load.
4 CCC in raceway = 80% adjustment
34C ambient = 96% adjustment

Using 100% of load for ampacity corrections I have 85+105= 190 amps of load
190 / .80 = 237.5 for number of conductors in raceway
237.5 / .96 = 247.39 for ambient temp adjustment

Using 125% of continuous load I have 85 x 1.25 + 105 = 211.25 amps of load
211.25 / .80 = 254.0625 for number of conductors in raceway
254.0625 / .96 = 264.648... for ambient temp adjustment

What are you doing differently?

 

[david luchini]

I have:

85 amps continuous load.
105 non continuous load.
4 CCC in raceway = 80% adjustment
34C ambient = 96% adjustment

Using 100% of load for ampacity corrections I have 85+105= 190 amps of load
190 / .80 = 237.5 for number of conductors in raceway
237.5 / .96 = 247.39 for ambient temp adjustment

Using 125% of continuous load I have 85 x 1.25 + 105 = 211.25 amps of load
211.25 / .80 = 254.0625 for number of conductors in raceway
254.0625 / .96 = 264.648... for ambient temp adjustment

What are you doing differently?

This is what I'm doing differently:

Using 125% of continuous load I have 85 x 1.25 + 105 = 211.25 amps of load
211.25 / .80 = 264.0625 for number of conductors in raceway
264.0625 / .96 = 275.066... for ambient temp adjustment

 

[kwired]

This is what I'm doing differently:

Using 125% of continuous load I have 85 x 1.25 + 105 = 211.25 amps of load
211.25 / .80 = 264.0625 for number of conductors in raceway
264.0625 / .96 = 275.066... for ambient temp adjustment

I can't even correct myself and come up with right answer:weeping:

I'm pretty sure I double checked it and still got it wrong - this was not a case of going about it wrong either just a math mistake somehow, and I was even using a calculator. I think I probably had right number on calculator but wrote down wrong number and continued with that wrong number.

Maybe I should take the rest of day off before I really goof something up:thumbsup: