Multiple motor calculations 430.24 and 409.20 Motor control centers

[dm9289]

In reviewing conductor sizing for multiple motors 430.24 Says 125% largest motor plus 100% other motors.

I have seen many processes with Motor control centers MCC that have for example 10 motors of different sizes and for the process to run all motors will run continuously. So in calculating feeder size for the MCC is it saying to ignore that all 10 motors are continuous? Example biggest motor FLC x1.25 plus the sum of the other 9?
From 409.10 it refers to their duty cycle is that referring to Table 430.22E or utilizing continuous as duty cycle?

 

[xformer]

See 430.24 for conductors
See 430.62 for sizing the OCPD.

 

[dm9289]

See 430.24 for conductors
See 430.62 for sizing the OCPD.

the 430.24 is where im confused. Do i ignore that all 10 motors are continuous duty

 

[don_resqcapt19]

the 430.24 is where im confused. Do i ignore that all 10 motors are continuous duty

430.24(2) applies to all but the largest motor.

 

[xformer]

430.24 - Conductors 125% largest motor + 100% all others.

 

[dm9289]

430.24(2) applies to all but the largest motor.

So you are saying ignore that all 10 motors are continuous duty and just 125% for largest

 

[dm9289]

430.24 - Conductors 125% largest motor + 100% all others.

So ignore all are continuous?

 

[xformer]

So ignore all are continuous?

430.24 - Conductors 125% largest motor + 100% all others.

 

[xformer]

So ignore all are continuous?

When you size the conductors to the individual motors, 430.22 will apply and they will be sized at 125%.

 

[don_resqcapt19]

So you are saying ignore that all 10 motors are continuous duty and just 125% for largest

That is what the code tells you to do.

Also continuous duty and continuous load are not the same thing. "Continuous Duty" is a rating that you find on the motor nameplate, and continuous load is a load that operates at its maximum current for at least 3 hours.

 

[dm9289]

That is what the code tells you to do.

Also continuous duty and continuous load are not the same thing. "Continuous Duty" is a rating that you find on the motor nameplate, and continuous load is a load that operates at its maximum current for at least 3 hours.

Thank you i meant continuous load

 

[paulengr]

That is what the code tells you to do.

Also continuous duty and continuous load are not the same thing. "Continuous Duty" is a rating that you find on the motor nameplate, and continuous load is a load that operates at its maximum current for at least 3 hours.

Where does it say maximum current? Very few loads will meet that criteria, even motors. I get your point about the load running at/near maximum rating but we size conservatively…as if that could happen, not if it does.

The “125%” thing is based on some very questionable math but it seems to work in practice. The 125%/100% rule just makes it so you are not double counting the extra capacity…it’s a diversity rule.

 

[don_resqcapt19]

Where does it say maximum current? Very few loads will meet that criteria, even motors. I get your point about the load running at/near maximum rating but we size conservatively…as if that could happen, not if it does.

The “125%” thing is based on some very questionable math but it seems to work in practice. The 125%/100% rule just makes it so you are not double counting the extra capacity…it’s a diversity rule.

Continuous Load.
A load where the maximum current is expected to continue for 3 hours or more. (CMP-2)

I agree that using the code language, very, very few loads are actually continuous loads.

 

[dm9289]

I agree that using the code language, very, very few loads are actually continuous loads.

I hear you that in many applications there are not many true continuous loads. In my little world of continuous flow manufacturing almost all loads are continuous. We have vent fans that are never off, high bay lighting never off, combustion blowers never off unless line breaks down, all line motors never off unless line breaks down. Hahahaha you get the point in my little end of the world 95% of equipment continuous. So thats why some off this seemed a little odd to me being as we have several MCCs all of which are mostly continuous loads.

 

[don_resqcapt19]

I hear you that in many applications there are not many true continuous loads. In my little world of continuous flow manufacturing almost all loads are continuous. We have vent fans that are never off, high bay lighting never off, combustion blowers never off unless line breaks down, all line motors never off unless line breaks down. Hahahaha you get the point in my little end of the world 95% of equipment continuous. So thats why some off this seemed a little odd to me being as we have several MCCs all of which are mostly continuous loads.

But the intake air temperature and humidity actually changes the load, so still may not be continuous based on the code definition.

I did a project where a 75 hp intake fan started fine in the summer with the warmer air, but in the fall when the temperature dropped, the increased density of the air was enough that the fan motor could not start within the trip time of the standard Class 20 overloads. The motor was inside so the extra required starting time was not caused by cold bearings and lubricants. The motor was changed out to a 100 hp in lieu of installing Class 30 overloads.

 

[paulengr]

But the intake air temperature and humidity actually changes the load, so still may not be continuous based on the code definition.

I did a project where a 75 hp intake fan started fine in the summer with the warmer air, but in the fall when the temperature dropped, the increased density of the air was enough that the fan motor could not start within the trip time of the standard Class 20 overloads. The motor was inside so the extra required starting time was not caused by cold bearings and lubricants. The motor was changed out to a 100 hp in lieu of installing Class 30 overloads.

Code only says the load must run 3 hours or more. Current is not an issue. It is not part of the criteria unlike CEC.

 

[petersonra]

Code only says the load must run 3 hours or more. Current is not an issue. It is not part of the criteria unlike CEC.

Continuous load. A load where the maximum current is expected to continue for 3 hours or more.

 

[kwired]

I hear you that in many applications there are not many true continuous loads. In my little world of continuous flow manufacturing almost all loads are continuous. We have vent fans that are never off, high bay lighting never off, combustion blowers never off unless line breaks down, all line motors never off unless line breaks down. Hahahaha you get the point in my little end of the world 95% of equipment continuous. So thats why some off this seemed a little odd to me being as we have several MCCs all of which are mostly continuous loads.

Some those may change speed, reverse, cycle on/off as part of their normal usage even if the production line runs 24/7.

Others may run continuously at a fairly fixed load but is not necessarily the maximum rated load of the motor.

 

[don_resqcapt19]

Code only says the load must run 3 hours or more. Current is not an issue. It is not part of the criteria unlike CEC.

Did you not bother to read the code definition I posted in post #13. The current is very clearly part of the criteria in the NEC per the Article 100 definition.