# Question on Table 430.22(E)



## Finhead (Nov 28, 2007)

Can someone explain how to read Table 430.22(E), or at least tell me where my thought process has run amuck? For example:

Suppose a motor must be selected to serve an intermittent duty freight elevator. If a 5-minute motor is selected, the nameplate FLC can be reduced by a factor of 0.85. If a 30-minute motor is selected, which presumably is a better motor, the FLC can only be reduced by a factor of 0.9. If a continuous duty motor is selected for the same application, its circuit conductors must be sized at 140% of the FLC. Makes no sense!

Looking at it another way, conductors for a continuous duty motor expected to run for extended periods shall be sized at 125% of FLC, but if the same motor is employed for intermittent duty or periodic duty, the conductors must be sized at 140%. If that's not bad enough, the percentage goes to 200% for varying duty applications.

Any ideas will be greatly appreciated
Brian


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## goose134 (Nov 12, 2007)

Maybe something to do with inrush current? When the motor slams on and off repeatedly, the amperage is spiking and dropping constantly. Not so much on a motor that starts three or four times a day.


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## brian john (Mar 11, 2007)

I have not grabbed the NEC prior to responding which is always a bad idea for me...But I do believe goose has a good handle on this. The starting and stopping results in additional heating of all components.


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## old man? (Feb 8, 2008)

I'll take a stab at this, MAYBE I'll be right.

If it's "intermittent duty", the size of the conductors may possibly be smaller than if it is rated "continous duty".

IE: If the nameplate says "100 amps", a 5 minute rated motor has to have wire that is rated at least 85 amps.
If it is a 30 minute rated motor, the wire has to be rated at least 90 amps.
If it as a continous duty rated motor, the wire has to be rated at least 140 amps. ( If it is 1200 RPM or less)
If it is continous rated more than 1200 RPM, use 430.6(A)(1).


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## RePhase277 (Feb 5, 2008)

I'll quote the NEC Handbook in that section:

"Branch circuit conductors for a motor with a rated horsepower used for 5-minute short-time duty service are permitted to be sized smaller than for the same motor with a 60-minute rating, due to the cooling intervals between operating periods."

It specifically says "short-time", but I think it applies to intermittent as well. It is probably just that in a 5 minute application the conductors won't heat to the point of insulation damage. But in varying or continuous duty, that motor may run for long enough, or be started numerous times in a short period (before the conductors can cool), to cause significant heating of the conductors.

A continuous-duty motor has circuit conductors rated for 125% because the inrush happens once in a great while, whereas a varying duty motor may start and stop frequently enough that the heating from the inrush current alone may be great enough to warrant a conductor sized to 140%.

InPhase277


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## Finhead (Nov 28, 2007)

I can't disagree with what you are saying. I think it is a matter of perspective. Let's start with specific load -- an intermittent duty freight elevator. 
If I select a 5-minute moter for this application, the table says I may reduce the nameplate rating to 85%
If I select a continuous duty motor for the same application, I must increase the nameplate rating to 140%

Keep in mind that both motors will do the same work during the same time periods.

It sure seems to me that a motor designed to run indefinitely would have an easier time on a freight elevator, since the windings have a chance to cool off, as opposed to the 5-minute motor which requires a long off period in order to cool off.

According to the 2008 NEC Handbook (page 583), a 5-minute motor will run for five minutes and be off for 55 minutes. If a continuous duty motor is subjected to the same regimen, does it make sense to increase the conductor size?

Thanks for all the comments.
Brian


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## RePhase277 (Feb 5, 2008)

Finhead said:


> I can't disagree with what you are saying. I think it is a matter of perspective. Let's start with specific load -- an intermittent duty freight elevator.
> If I select a 5-minute moter for this application, the table says I may reduce the nameplate rating to 85%
> If I select a continuous duty motor for the same application, I must increase the nameplate rating to 140%
> 
> ...


Well, if a continuous duty motor is used, it is likely that it will be used more often than a 5 min motor, just by virtue of the fact that it can withstand the longer run time. I think it is just a precaution against said use.

Now, the AHJ may let you get away with reduced size conductors if there were some sort of interlock circuit that would allow a continuous duty motor to run for 5 min only, and be off for 55 min. 

InPhase277


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## Finhead (Nov 28, 2007)

In Phase277

Keep in mind that the example I sited was an intermittent duty freight elevator. Both motors will be doing the same work for the same time periods. 

Finhead


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