Load calcs for a 100A panelboard with 60A main

[Cow]

I've always treated pretty much all motors as continuous loads. Sure, the average air compressor may cycle on and off and only run for a few minutes at a time but I still consider it continuous. My thoughts are, unless there is an interlock preventing it from running for 3 hours or more, it's continuous.

 

[kwired]

The NEC may treat motors as continuous loads... but all the treatment is done through and in Article 430 only and nowhere else. We need to be clear on this so as not to confuse others.

I can agree with that.

 

[Smart $]

I've always treated pretty much all motors as continuous loads. Sure, the average air compressor may cycle on and off and only run for a few minutes at a time but I still consider it continuous. My thoughts are, unless there is an interlock preventing it from running for 3 hours or more, it's continuous.

It is statements like yours that make applying the NEC requirements confusing for those just learning.

By definition a continuous load operates at full current for more than three hours... period.

In most cases, even motors operating for more than three hours continuously do not do so at full current.

When factoring continuous loads by 125% under any Article other than 430, motor loads are NOT included.

NOTE: Special considerations are made for fire pump motors and their supplies.

 

[iwire]

By definition a continuous load operates at full current for more than three hours... period.

In most cases, even motors operating for more than three hours continuously do not do so at full current.

But if they do they are continuos loads correct?

Is there anyplace in the NEC that tells us motors are not continuos loads?

It seems to me many pumps and fans have the same mechanical load for hours on end.

 

[Smart $]

But if they do they are continuos loads correct?

By definition, yes.

Is there anyplace in the NEC that tells us motors are not continuos loads?

You have to apply deductive reasoning to 430.24, Article 430 Parts IV & V, and how they relate to other article's use of the continuous load concept. Beyond that, trying to explain exactly how Code is incongruent in this regard is a bit much. The short of it is, 125% factoring is seldom compounded (PV circuits are the only place I'm aware of at present).

It seems to me many pumps and fans have the same mechanical load for hours on end.

That's a fact. Please don't ask for documentation.

 

[iwire]

The short of it is, 125% factoring is seldom compounded

I have never compounded it and I am not saying we should. I am just looking for the tidbit in the NEC that allows us to ignore the typical rules for continuous loads found in chapter 2.

That's a fact. Please don't ask for documentation.

:lol:

I will not.

 

[Smart $]

I have never compounded it and I am not saying we should. I am just looking for the tidbit in the NEC that allows us to ignore the typical rules for continuous loads found in chapter 2.

430.24 is as close as you'll get in simple words.

Note 430.24 is referenced by 220.14(C).

See 210.19(A) Informational Note No. 2.

Note 215.2(A)(1) states the load is as calculated in Article 220.

Note these concepts are not duplicated in the following respective sections for overcurrent protection (210.20, 215.3).

 

[iwire]

430.24 is as close as you'll get in simple words.

Note 430.24 is referenced by 220.14(C).

See 210.19(A) Informational Note No. 2.

Note 215.2(A)(1) states the load is as calculated in Article 220.

Note these concepts are not duplicated in the following respective sections for overcurrent protection (210.20, 215.3).

Thanks

 

[GoldDigger]

On the DC side of PV we do have two factors of 1.25. But I would not call it compounding since the two factors, although numerically equal have very different reasons behind them.

 

[kwired]

Is there anyplace in the NEC that tells us motors are not continuos loads?

If all motors are continuous there would be no need for section 430.22(E), 430.23(B), 430.33, possibly other sections that specifically apply to intermittent duty motors.




Something I just found that more directly answers the OP question is in 430.26. Doesn't exactly tell us how to make said calculations but gives us permission to use other characteristics then just FLA of the motors involved.

 

[iwire]

If all motors are continuous there would be no need for section 430.22(E), 430.23(B), 430.33, possibly other sections that specifically apply to intermittent duty motors.




Something I just found that more directly answers the OP question is in 430.26. Doesn't exactly tell us how to make said calculations but gives us permission to use other characteristics then just FLA of the motors involved.

If you are going to quote me at least try to answer the question I asked in the quote.

 

[kwired]

If you are going to quote me at least try to answer the question I asked in the quote.

I thought I did. That last sentence maybe should have been in it's own post though.

 

[Smart $]

If all motors are continuous there would be no need for section 430.22(E), 430.23(B), 430.33, possibly other sections that specifically apply to intermittent duty motors.

Something I just found that more directly answers the OP question is in 430.26. Doesn't exactly tell us how to make said calculations but gives us permission to use other characteristics then just FLA of the motors involved.

I think you are mixing terms.

I never view anything about a continuous duty motor as relating to a continuous load... other than if it is a by definition continuous load, the motor must be a continuous duty motor.

 

[kwired]

I think you are mixing terms.

I never view anything about a continuous duty motor as relating to a continuous load... other than if it is a by definition continuous load, the motor must be a continuous duty motor.

Maybe. I understand continuous duty means it is rated for using continuously at nameplate rating. Reality is motors often are only supplying something less then nameplate rating when it is for continuous durations. Then there is the continuous load factor so to speak that applies to it running three hours or more no matter what load level is in relation to full load rating of the motor. But we already size conductors and overload protection for continuous duty motors as though they were continuously fully loaded, so no need to further derate conductors and overcurrent protection just because they will run continuously.

I don't think there is a problem with derating a conductor or overcurrent protection for non motor loads either, even if it is not a continuous load - but would you ever derate it twice in that situation?

 

[Smart $]

...we already size conductors and overload protection for continuous duty motors as though they were continuously fully loaded, so no need to further derate conductors and overcurrent protection just because they will run continuously.

Exactly. Plainly evident at the branch circuit level [210.2]. Not so much at the feeder/service level.