Motor duty cycle- 430.22(E)

[greenspark1]

I typically size motors at 125% FLA, end of discussion. Then I came across this table which has values up to 200% for a continuous duty motor on a varying duty load. A cheaper 5 min rated motor with the same load would only require 110% conductors. This table seems to be 100% backwards and required larger conductors for smaller/worse motors. Can someone explain the thinking and if they ever use this table?

I have a couple of intermittent auger motors and can't understand why I would size them at 140% instead of the standard 125%. I've read the commentary and all I can find online and it hasn't helped. Thanks!

 

[david luchini]

I have a couple of intermittent auger motors and can't understand why I would size them at 140% instead of the standard 125%. I've read the commentary and all I can find online and it hasn't helped. Thanks!

The 140% for the intermittent would be based on the motor nameplate current rather than the Table 430.250 (or other table) current.

The 140% may require a smaller ampacity than 125% of the Table current.

 

[greenspark1]

The 140% for the intermittent would be based on the motor nameplate current rather than the Table 430.250 (or other table) current.

The 140% may require a smaller ampacity than 125% of the Table current.

OK that's a good point that you may get smaller wires using 140% of the 430.22(E) versus 125% of the 430.250 table value.

But per 430.22(e), why would a continuous rated motor need larger wires than a 5 minute motor?

 

[david luchini]

OK that's a good point that you may get smaller wires using 140% of the 430.22(E) versus 125% of the 430.250 table value.

But per 430.22(e), why would a continuous rated motor need larger wires than a 5 minute motor?

A 5 minute motor can only be run for 5 minute then it must be allowed to cool. As such, there is less time for heat to build up.

 

[tom baker]

Nearly all motors are Design B, and its intended to start and run for long periods of time. When you start and stop any more frequently, there is heating due to inrush, and your conductor size goes from 125% to 140%.

 

[greenspark1]

Nearly all motors are Design B, and its intended to start and run for long periods of time. When you start and stop any more frequently, there is heating due to inrush, and your conductor size goes from 125% to 140%.

Good explanation, thanks. This makes complete sense if you only look at the continuous column.

But for the same load- say an intermittent pump. Why would a 5 minute motor requires smaller conductors than a continuous motor (85 vs 140%)? The 5 minute is only meant to be started and used for 5 minutes out of 60. The continuous is designed for starting and then continuous use. You can use either type of motor for your intermittent pump, but why the huge wires for continuous.

I'm not trying to be obstinate, I just don't understand and hate just following the Code without understanding it.

 

[GoldDigger]

It is not so much that the continuous motor requires bigger wires (for a particular amperage) than any other continuous load. It is that the 5 minute motor can use smaller conductors because the conductors, like the motor, will not build up as high a temperature in only 5 minutes and will then get a chance to cool back down.
A conductor overloaded by only 50% will not reach maximum insulation temperature instantly.
But that is only logic.

 

[The Steig]

Temperature de-rating for "Other Than Continous Duty"

Temperature de-rating for "Other Than Continous Duty"

Table 430.22(e) is clear on the ampacity. If this table is applied for a non-continuous pump (such as a septic or sump pump), is it still necessary to derate for temperature?

 

[RB1]

An intermittent duty motor can be worked much harder than a continuous duty motor. So a smaller motor can do more work. Think about the starter on a car. The same motor used continuously will burn out, but for short term it is OK because heat build up is not a concern.