motor conductor sizing/derating

[Larry Bohn]

430.21 states that motor conductors must have an ampacity of 125% of the motor full load current, as determined by the NEC tables. My supervisor and I have a disagreement about the required conductor size.
We are installing 75 Hp motors, 480V, 3 phase, the full load current from the table is 96 amps.
96A X 1.25 = 120A (required ampacity of the conductor)

Here's where we disagree. We are running #1 THHN, 6 current carrying conductors in a conduit (two motors). They need to be derated to 80%. The ambient temperature is also 46-50C, so they also need an additional derate to 82% of the ampacity calculated by the CCC derate.
#1 THHN = 145A X .80 X.82 = 95.12 ampacity (after applying the derating factors)

His calculations are as follows. 96FLA X 125% (this is the 80% derate for current carrying conductors) X 122% (this is the .82 derate for temperature)
So, 96 X 1.25 X 1.22 = 146.4 amps, it'll be fine since the nameplate current is less than the book, and we don't have to worry about the 125% conductor sizing required by 430.22 since the table value of a THHN conductor is 145 amps, well over the required 120 amps. The derates only are used to see if the conductor ampacity calculation is large enough to carry the load, which in this case is the motor FLA at 96A. Which as shown above, calculates to 95.12 amps.

How do you guys do your calculations?

 

[infinity]

430.21 states that motor conductors must have an ampacity of 125% of the motor full load current, as determined by the NEC tables. My supervisor and I have a disagreement about the required conductor size.
We are installing 75 Hp motors, 480V, 3 phase, the full load current from the table is 96 amps.
96A X 1.25 = 120A (required ampacity of the conductor)

Here's where we disagree. We are running #1 THHN, 6 current carrying conductors in a conduit (two motors). They need to be derated to 80%. The ambient temperature is also 46-50C, so they also need an additional derate to 82% of the ampacity calculated by the CCC derate.
#1 THHN = 145A X .80 X.82 = 95.12 ampacity (after applying the derating factors)

His calculations are as follows. 96FLA X 125% (this is the 80% derate for current carrying conductors) X 122% (this is the .82 derate for temperature)
So, 96 X 1.25 X 1.22 = 146.4 amps, it'll be fine since the nameplate current is less than the book, and we don't have to worry about the 125% conductor sizing required by 430.22 since the table value of a THHN conductor is 145 amps, well over the required 120 amps. The derates only are used to see if the conductor ampacity calculation is large enough to carry the load, which in this case is the motor FLA at 96A. Which as shown above, calculates to 95.12 amps.

How do you guys do your calculations?

On the surface it would appear that your #1 is no good because your adjusted value is well below the minimum required for the motor conductors, however since you've already added 25% for the derating do you need to add it again for the 125% motor conductor factor?

 

[Larry Bohn]

It appears to me you apply 430.21, then find a conductor with the required ampacity after derates are applied. My supervisor, if he applies derates for current carrying conductors and temperature, he only uses the table FLA value, but if it's a single motor in a conduit, where derates for temperature and number of conductors don't apply, then he'll use 430.21 to calculate conductor size.
I can guarantee this guy knows his code, I'm just not following his reasoning why he doesn't apply 430.21 when he derates conductors.

 

[david luchini]

It appears to me you apply 430.21, then find a conductor with the required ampacity after derates are applied. My supervisor, if he applies derates for current carrying conductors and temperature, he only uses the table FLA value, but if it's a single motor in a conduit, where derates for temperature and number of conductors don't apply, then he'll use 430.21 to calculate conductor size.
I can guarantee this guy knows his code, I'm just not following his reasoning why he doesn't apply 430.21 when he derates conductors.

I agree with you. For a 75Hp, 460V motor, your motor branch circuit conductors need an ampacity of 120, per 430.21. Adjusted and corrected for 6 ccc's and for ambient temp, you would need a 75C conductor with a starting ampacity of 120/0.8/0.75=200 (#3/0) or you would need a 90C conductor with a starting ampacity of 120/0.8/0.82=183 (#2/0).

 

[Gregg Harris]

I agree with you. For a 75Hp, 460V motor, your motor branch circuit conductors need an ampacity of 120, per 430.21. Adjusted and corrected for 6 ccc's and for ambient temp, you would need a 75C conductor with a starting ampacity of 120/0.8/0.75=200 (#3/0) or you would need a 90C conductor with a starting ampacity of 120/0.8/0.82=183 (#2/0).

This is also what I calculated using the THHN

 

[kwired]

Conductor ampacity is determined by starting in 430.22 which says 125% of full load current.

Full load current is determined from 430.6(A)(1) which says NEC table values are to be used (none of the exceptions apply to the motor in the OP unless qualifying information has been withheld from us) 75 hp @ 480 volts is 96 amps.

430 doesn't say much more on determining conductor ampacity so back to general rules in 210 for branch circuits and 310 for conductors.

210.19(A)(1) - continuous load is already accounted for in 430.22, we don't have to adjust by 125% again for continuous load.

Minimum conductor size is (still in 210.19) 125% of full load before application of adjustments or correction factors.

So 96 amps @ 75C termination temp x 125% = 120 amps which 1AWG is smallest 75C conductor rated for 120 amps or greater. In no case after adjustments can the conductor be smaller than 1 AWG - sometimes adjustments can produce a smaller conductor.

Now come adjustments for insulation rating - assuming we have 90C conductor insulation and working it to what some would call backwards (finding a minimum ampacity instead of selecting a conductor and then derating it only to start over again if the selection didn't work) we have an adjustment of 125% (1/.80) for number of conductors in the raceway, and an adjustment of 122% (1/.82) for ambient temp. We make this adjustment from the 96 amps full load as the termination temp is what the 125% full load factor applies to, insulation temp rating only depends on the load itself.

So we have 96 x 1.25 x 1.22 = 146.4. 146 amps is minimum 90C conductor needed in this application.

1 AWG (our minimum conductor before adjustments) @ 90C is only rated for 145 amps so we must go larger.

You must use the larger of :

the minimum size before adjustments
or the size determined after adjustments.

Minimum size conductor for the OP application is 1/0 AWG.

If termination temp rating and insulation temp rating were the same temp these calculations would be simpler, and you could do them all at 75C and not be in any violation of code, but you will end up with larger conductors and more cost than what is permitted.

 

[david luchini]

Minimum size conductor for the OP application is 1/0 AWG.

The ampacity of 1/0 AWG (90C) adjusted for 6 ccc's and corrected for 46-50C ambient would be 112A. This is smaller than the required conductor ampacity of 120A. Minimum size conductor for the OP application is 2/0 AWG.

210.19(A)(1) - continuous load is already accounted for in 430.22, we don't have to adjust by 125% again for continuous load.

Minimum conductor size is (still in 210.19) 125% of full load before application of adjustments or correction factors.

210.19 does not apply to branch circuit supplying motor loads only. See 210.1.

 

[augie47]

I agree. 2/0 minimum

 

[infinity]

Adjusted and corrected for 6 ccc's and for ambient temp, you would need a 75C conductor with a starting ampacity of 120/0.8/0.75=200 (#3/0) or you would need a 90C conductor with a starting ampacity of 120/0.8/0.82=183 (#2/0).

So we have 96 x 1.25 x 1.22 = 146.4. 146 amps is minimum 90C conductor needed in this application.

1 AWG (our minimum conductor before adjustments) @ 90C is only rated for 145 amps so we must go larger.

If I'm reading this correctly, one calculation uses the 125% motor factor and the other does not?

 

[automate0042]

In addition to continuous duty service, ampacity requirements for motor conductors also include the possibility of acceptable overload currents i.e. motors w/ service factors over 1. Another reason for the FLA X 1.25. Derate from there.

 

[david luchini]

If I'm reading this correctly, one calculation uses the 125% motor factor and the other does not?

430.22 requires the motor branch circuit conductors to have an ampacity not less than 125% of the motor FLA. The adjusted/corrected ampacity of the OP conductor must have an ampacity of at least 120.

 

[infinity]

430.22 requires the motor branch circuit conductors to have an ampacity not less than 125% of the motor FLA. The adjusted/corrected ampacity of the OP conductor must have an ampacity of at least 120.

I agree but isn't that what Kwired stated in his calculation? Your calculation used both the 125% for the motor factor and it's inverse 80% for derating. Are both needed?

So we have 96 x 1.25 x 1.22 = 146.4. 146 amps is minimum 90C conductor needed in this application.

1 AWG (our minimum conductor before adjustments) @ 90C is only rated for 145 amps so we must go larger.

You must use the larger of :

the minimum size before adjustments
or the size determined after adjustments.

Minimum size conductor for the OP application is 1/0 AWG.

 

[kwired]

I agree 210.19(A)(1) probably does not apply.

430.22 still gives us 125% just like 210.19(A)(1) would if it did apply. This is for continuous load and termination temperature purposes. So whether we have a motor circuit or not we still have a 125% factor for continuous loading. If the motor is not continuous load 430.22 doesn't apply either and you continue ampacity selection using 100% of full load current.

Adjustments for ambient temp and for number of conductors in the raceway are based on insulation temp and not on termination temp, you base them on actual load on the conductor.

 

[infinity]

I agree 210.19(A)(1) probably does not apply.

430.22 still gives us 125% just like 210.19(A)(1) would if it did apply. This is for continuous load and termination temperature purposes. So whether we have a motor circuit or not we still have a 125% factor for continuous loading. If the motor is not continuous load 430.22 doesn't apply either and you continue ampacity selection using 100% of full load current.

Adjustments for ambient temp and for number of conductors in the raceway are based on insulation temp and not on termination temp, you base them on actual load on the conductor.

So in this example you're saying that you need to add both the 125% motor factor and 125% for 6 CCC (80% derating)?

 

[kwired]

So in this example you're saying that you need to add both the 125% motor factor and 125% for 6 CCC (80% derating)?

No, not at same time.

Just like any other conductor ampacity selection you have to choose the larger conductor from either the termination temp rating or from the insulation temp rating.

The 125% for continuous load applies to termination temp ampacity selection.

Insulation temp ampacity selection is based on 100% of the load no matter what that load may be.

You take motor current and multiply by 125%. This gives you absolute minimum conductor size - no matter what adjustments give you the conductor can not be smaller than this value - usually this will be based on 75C.

You then find out what size conductor is necessary for insulation temperature rating - usually this will be based on 90C conductors. This is not covered at all in 430 and you need to go back to 310 for these adjustments. Nowhere in 310 does it mention factors for continuous loading. The adjustments are for insulation temperature.

You have selected two conductors based on two conditions at this point and must use whichever is larger as your final selection.

I might be able to be sold on the fact that because of what is mentioned in 430.22 that you still need a 125% factor for the insulation temp rating, but am not yet convinced this is what is intended. On non motor circuits you only add 125% for continuous loads because of the termination rating, and if it is a 100% rated device you only use 100% of the load for ampacity selection. I think 430.22 intends you to use 125% even if you have a 100% rated device, but I don't think it is intended to apply to insulation temp rating. What if you have motors and non motor supplying conductors in the raceway? Do you calculate each conductor ampacity based on what kind of load is supplied even though the type of load is not really going to impact the temperature developed in the raceway? IMO, just the current in each conductor is what is contributing heat in the raceway and that is what the adjustments are based on.

 

[infinity]

There are two different minimum conductor sizes (post #9) in this thread #1/0 and #2/0. In your calculation you only used the additional derating value of +25% (6 CCC @ 80%). David uses 125% and 125% (/.80) so the minimum conductor size is larger. Only one method is correct, so which one is it?

 

[david luchini]

There are two different minimum conductor sizes (post #9) in this thread #1/0 and #2/0. In your calculation you only used the additional derating value of +25% (6 CCC @ 80%). David uses 125% and 125% (/.80) so the minimum conductor size is larger. Only one method is correct, so which one is it?

420.22 says that for a motor branch circuit you need a conductor with an ampacity of not less than 125% of the motor full load current. In the OP this would be 96A*1.25 = 120A.

The corrected/adjusted (per the OP) ampacity of #1/0 THHN would be 170*0.8*0.82 = 112A.

The corrected/adjusted (per the OP) ampacity of #2/0 THHN would be 195*0.8*0.82 = 128A.

Clearly, #1/0 does not meet the required conductor ampacity, and #2/0 does.

#2/0 is correct, #1/0 is too small.

 

[david luchini]

If the motor is not continuous load 430.22 doesn't apply either and you continue ampacity selection using 100% of full load current.

430.22 doesn't apply base on what? 430.22 doesn't say anything about continuous vs. non-continuous. It says that the motor branch circuit conductor must have an ampacity which is not less than 125% of the FLA. #1/0 THHN has an ampacity LESS than 125% of the FLA in the OP conditions.

 

[iwire]

430.22 doesn't apply base on what? 430.22 doesn't say anything about continuous vs. non-continuous. It says that the motor branch circuit conductor must have an ampacity which is not less than 125% of the FLA. #1/0 THHN has an ampacity LESS than 125% of the FLA in the OP conditions.

I agree, 125% applies regardless of run time.

 

[kwired]

430.22 doesn't apply base on what? 430.22 doesn't say anything about continuous vs. non-continuous. It says that the motor branch circuit conductor must have an ampacity which is not less than 125% of the FLA. #1/0 THHN has an ampacity LESS than 125% of the FLA in the OP conditions.

Read what I said again, I was talking about a non continuous duty motor in what you quoted. I did not go much further than that but in general the motor will have a duty rating if this is the case.

430.22 Single Motor.
Conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less than 125 percent of the motor full-load current rating, as determined by 430.6(A)(1), or not less than specified in 430.22(A) through (G).

The only thing that will change my opinion of how I determined 1/0 was needed for the OP, is if you can convince me the 125% in 430.22 also applies to determining ampacity based on insulation temp. I do realize there is nothing that says it is allowed to exclude that, but at same time if same current were on the conductor and it was anything but a motor, it could be 1/0, but could also never be smaller than 1 AWG when it comes to termination rating.

Does a conductor carrying 96 amps supplying a motor create more heat in same raceway than same sized conductor supplying something other than a motor also carrying 96 amps? IMO 430.22 is basically saying motors are considered continuous loads and the 125% factor applies to the termination temp rating.