motor conductor sizing/derating

[david luchini]

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.

I did read what you said, and it mentioned "continuous load" not continuous duty motor.

If the motor is not continuous load 430.22 doesn't apply

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.

430.22 says the motor circuit conductor shall have an ampacity not less than...in the OP's example that ampacity value is 120. In the OP example #1/0 THHN would have an ampacity of 112. 112 is less than 120. I don't think it could be any simpler than that.

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.

If there is nothing that says it is allowed to exclude that, why are you trying to exclude that. Yes, if the load was anything else but a motor then it could be 1/0. BUT, the rules for motors vs. other loads are different. You are still trying to apply 210.19 even though you agree that 210.19 doesn't apply. The motor circuit conductors must comply with Article 430, and your approach doesn't comply.

 

[kwired]

I did read what you said, and it mentioned "continuous load" not continuous duty motor.






430.22 says the motor circuit conductor shall have an ampacity not less than...in the OP's example that ampacity value is 120. In the OP example #1/0 THHN would have an ampacity of 112. 112 is less than 120. I don't think it could be any simpler than that.



If there is nothing that says it is allowed to exclude that, why are you trying to exclude that. Yes, if the load was anything else but a motor then it could be 1/0. BUT, the rules for motors vs. other loads are different. You are still trying to apply 210.19 even though you agree that 210.19 doesn't apply. The motor circuit conductors must comply with Article 430, and your approach doesn't comply.

I have no other reply other than why is it worded the way it is? Why do we need the additional 125% for motors? Something besides "that is what it says" is what I am looking for. Every code section has reasons for existence even if those reasons are not logical to all, I just want the reason why we need that extra 25%.

I did mention I was willing to concede, but need a reason to before I will.

 

[david luchini]

I have no other reply other than why is it worded the way it is? Why do we need the additional 125% for motors? Something besides "that is what it says" is what I am looking for. Every code section has reasons for existence even if those reasons are not logical to all, I just want the reason why we need that extra 25%.

I did mention I was willing to concede, but need a reason to before I will.

I can't answer the "why", I didn't write the Code.

The OP's question is what does the Code require, not does this Code section make sense.

Going through the entire Code, I'm sure I would find several sections where I didn't understand the logic. That would not prevent me from doing what the Code says, however.

 

[kwired]

I can't answer the "why", I didn't write the Code.

The OP's question is what does the Code require, not does this Code section make sense.

Going through the entire Code, I'm sure I would find several sections where I didn't understand the logic. That would not prevent me from doing what the Code says, however.

That is fine, I do some things also without knowing why, but most of the time I want to know why, if anyone has any reason as to why your input is appreciated. Maybe what is printed is a mistake as to what the actual intent was - it does happen.

I think there are many people that would have calculated this motor application the same way I did, I also am pretty sure there are some out there whose primary job is instructing that are teaching to do it the way I did, and have justification for why they are teaching the way they are. I was taught to do this the way I did, I can not justify every reason for why without consulting where I learned from, I do recall the 125% in 430.22 being related to the 125% in 210.19 somehow, and applying to termination temp rating, and insulation temp rating being completely independent from termination rating - then pick the one that yields a larger conductor.

 

[automate0042]

I don't see any exceptions in any of the articles pertaining to this that would exempt the ambient temp or conductors in a conduit. I'm probably missing the point but motors can and do pull current for extended periods above their FLA, another reason for the FLA X125%. From there, if the ambient is above, I cant see how it wouldn't be figure in, as well as the number of conductors- "double derating." I don't do residential work but it seems to me if one had a simple 20a receptacle ckt, one would derate for ambient temp. and number in a conduit, right?- the code has already accounted for the 125% by limiting the load to 16a.

 

[kwired]

I don't see any exceptions in any of the articles pertaining to this that would exempt the ambient temp or conductors in a conduit. I'm probably missing the point but motors can and do pull current for extended periods above their FLA, another reason for the FLA X125%. From there, if the ambient is above, I cant see how it wouldn't be figure in, as well as the number of conductors- "double derating." I don't do residential work but it seems to me if one had a simple 20a receptacle ckt, one would derate for ambient temp. and number in a conduit, right?- the code has already accounted for the 125% by limiting the load to 16a.

Starting out with an additional 25% isn't exactly derating, it is increasing the starting point, so there is no "double derating".

The 20 amp circuit doesn't matter if it is residential or not, same rules apply. We also get into additional rules with receptacles - especially where a circuit has multiple receptacles. The load is 20 amps for a 20 amp branch circuit for the purpose of determining conductor ampacity with multiple receptacles, then you mix in the small conductor rules of 240.4(D) and you end up using a 12 AWG where calculations only yield 14 AWG otherwise.

Code does not limit load on a 20 amp general purpose receptacle circuit to 16 amps. Continuous loads yes, but non continuous can be a full 20 amps.

 

[automate0042]

Based on the original post, I see the double derating for no. of wires in a conduit and ambient correction. So I see the calc as follows:
Figure the cond. ampacity before the application of derate factors (based on the coolest termination of 75c) 96X1.25=120a. In the 75c col, that's 1/0 wire. That's the min. "bare copper size to handle term. temp. Now insulate the wire. I think you must derate twice for a final ampacity of 182.5. If the 75c col wont give you that w/o bumping up, try the 90c col at the 1/0 size. That wont work either, it looks like you'd be stuck w/ a 2/0 conductor with the proper insulation from the 90c column.

 

[kwired]

Based on the original post, I see the double derating for no. of wires in a conduit and ambient correction. So I see the calc as follows:
Figure the cond. ampacity before the application of derate factors (based on the coolest termination of 75c) 96X1.25=120a. In the 75c col, that's 1/0 wire. That's the min. "bare copper size to handle term. temp. Now insulate the wire. I think you must derate twice for a final ampacity of 182.5. If the 75c col wont give you that w/o bumping up, try the 90c col at the 1/0 size. That wont work either, it looks like you'd be stuck w/ a 2/0 conductor with the proper insulation from the 90c column.

Look at table again, 1AWG @75C is good for 130 amps. That is minimum size conductor needed before adjustments. Perhaps you were looking at the 60C column or the aluminum part of the table?

There is no question we need to derate at 80% for number of conductors in raceway, and also derate 82% for ambient temp, the debate here is what ampacity do we start with, 96 amps or 120 amps? If we start at 120 amps I wouldn't call it double derating, but instead it is starting with a different minimum.

 

[automate0042]

I was thumbing thru pretty fast but- I admit that I have been skimming over the posts but I see a few things. I thought that the code specified any continuous load of 3 hrs or must have conductors rated at 125% of full load amps- no matter the load? For motors, this 125% allowance also considers motors running above their FLA and starting amps. I've seen many a motor running over 115% FLA. I'm not so sure one could determine via intent whether it's a matter of term. temp, insulation temp. or both,most likely both. In any case (and I really havent had to use the book in a long time) art 430 mandates a 125% increase in FLA., Surely that's the intended starting point. I dont see any exceptions there. As for derating for ambient and no. of conductors, these are an insulation thing. I cant see any exceptions there either.
Maybe I"m not thinking this part thru, but that's why I brought up the 20a ckt - 16a max on a 20a ckt-12ga wire. Isnt that similiar- the starting point is 20a- 16X125%. The code has already figured the 125% in for these ckts.That's the starting point, derate from there- for ambient and no. of conductors.

 

[automate0042]

Kwired, I went back and read your first post and saw how you calculated the value. 430.22 states that a motor conductors in cont. duty service SHALL have an ampacity of not less than 125% FLA. Note that the tables in 430 make no corrections for service factors etc. The 125% takes into account service factors, starting amps, continuous running, etc. A class 20 overload should knock a motor off w/ 600% fla in 20 sec. How long for it to clear a 120% ol? A lot longer. If you go to 310 and look up a conductor for 96X1.25, you'll find a 1 awg. That's the min. Looking at 310, it says you must derate your selected conductor (1 awg)by the percentage listed. Further to that, you must also derate for the conductors in the conduit. If I did my calculations right and was looking in the right column, that comes out to be a 2/0 after derating.

 

[david luchini]

That is fine, I do some things also without knowing why, but most of the time I want to know why, if anyone has any reason as to why your input is appreciated. Maybe what is printed is a mistake as to what the actual intent was - it does happen.

I couldn't say for sure, but I'd suspect that it has to do with the fact for 210.19 branch circuits with the min conductor sized at 125% of continuous and the actual conductor ampacity being large enough for the load, you would still have to protect the conductor at its ampacity or at the next standard size up. For a motor load, this is not the case.

For example, (let me change to OP's 96A load to 95A just to demonstrate) a 95A continuous non-motor load would require a minimum conductor size of 95*1.25% = 119A. #1AWG would be the minimum conductor size. From 210.20, you would need an OCPD size that is not less than 125% of the continuous load, so you would need at least a 125A OCPD. Now to correct for 6 ccc's and 46degC, you would need a conductor with a starting ampacity of 95/0.8/0.82 = 145, for which #1 THHN would fit the bill. However, the #1 THHN for the conditions would not be properly protected by a 125A OCPD. You would need a #1/0 with a final ampacity of 112 even though the actual load is only 95.

For a 95A motor load, you could have an OCPD of 250A and an overload of 118A protecting the circuit. If you had the #1 THHN with a final ampacity of 95, how would the conductors be protected? This together with the fact that undervoltage or overvoltage from the motor nominal would cause in increase in motor running amps would to me seem to be the reason for requiring the conductors to have an ampacity at least 125% of FLA.

 

[kwired]

I was thumbing thru pretty fast but- I admit that I have been skimming over the posts but I see a few things. I thought that the code specified any continuous load of 3 hrs or must have conductors rated at 125% of full load amps- no matter the load? For motors, this 125% allowance also considers motors running above their FLA and starting amps. I've seen many a motor running over 115% FLA. I'm not so sure one could determine via intent whether it's a matter of term. temp, insulation temp. or both,most likely both. In any case (and I really havent had to use the book in a long time) art 430 mandates a 125% increase in FLA., Surely that's the intended starting point. I dont see any exceptions there. As for derating for ambient and no. of conductors, these are an insulation thing. I cant see any exceptions there either.
Maybe I"m not thinking this part thru, but that's why I brought up the 20a ckt - 16a max on a 20a ckt-12ga wire. Isnt that similiar- the starting point is 20a- 16X125%. The code has already figured the 125% in for these ckts.That's the starting point, derate from there- for ambient and no. of conductors.

I am at the point of conceding that the starting point for insulation rating is 125% of full load current, there is no language to support my line of thinking previously and I am not really sure why I was thinking that was the way to do it. The fact that motor overload settings generally will allow up to 125% of full load current does give some credibility to the reason for it.


I still say you are going the wrong direction with the 20a ckt - 16a max rule. This can be kind of considered a general rule in a way, but is not always the case. 240.4(D) limits overcurrent protection to 20 amps on a 12 AWG conductor, but does not limit the load to anything less. Rules relating to continuous loads create the limitations.

A circuit with non continuous load is permitted to be loaded to 100% of the non continuous load.

 

[automate0042]

Actually, I was just trying to find an analogy. By way of limiting the load on a gen. purpose 20a ckt to 16 amps, "they" have already built into the ckt a 125% cont. use factor. Also of note, limiting the current of 12 awg in these ckts to 20a vs the 25 in 310-16, there's another 125% derating. But as a "starting point" in this case, you wouldn't use the 16a to derate for temp like using the 96 fla, you'd use the 20a value, like using the 183 amp value. Maybe a bad analogy.

 

[Larry Bohn]

Thanks for all the replies, gentlemen. It's my opinion that the motor circuit in the original post, (or circuits), can operate up to the point the overloads will trip. In this case, they are set at 125% of the nameplate FLA. This leads me to think the circuit MAY operate without tripping at a current that will approach 120A.
The conductors should be sized to carry that load for an undetermined period of time. Applying the derates to the conductor, I'm going with the choice of 2/0 for the THHN conductor size.