1.25% of the largest motor

[CONDUIT]

I get asked this question over and over in my code class. When you are sizing conductors for multiple motors why do you only figure 1.25% of the largest motor and not all of them. I know all about sizing the fuse and breakers for inrush. I was wondering where this 1.25% for the largest motor comes from. I also know about the continous load requirements. Does any one know with out guessing? I thought that maybe it had something to do with the service factor. But that doesn't make sense either because all of the motors will have a service factor or temperature rise!

 

[Dennis Alwon]

If you size 125% for the largest motor than that is also 125% + of protection for all the motors. Why would you want to add 125% on all the motors. The conductor gives full protection on all the motors buy applying it to the largest motor.

 

[texie]

I'm not sure of the how and why of this rule, but it has been this way for as long as I can remember. I might add that this rule can catch you up if, say, you have a 5 HP single phase motor and a 7.5 HP 3 phase motor the largest motor is the 5 HP for this purpose.

 

[kwired]

If you size 125% for the largest motor than that is also 125% + of protection for all the motors. Why would you want to add 125% on all the motors. The conductor gives full protection on all the motors buy applying it to the largest motor.

I don't quite follow you on this one. First 125% is for conductor ampacity selection and has nothing to do with overcurrent protection.

Short circuit and ground fault protection can be much higher than minimum ampacity. If you consider motor overload protection, depending on what level of protection is allowed, you can potentially have up to 125% of full load current on all the motors without tripping anything.

 

[Flapjack]

125% of the motor FLA deals with the long time portion for protection from overloads. so for overload protection, you would size your ocpd at 125% or less for branch circuits (short circuit and ground fault won't matter because that is in the short time...). The conductors would also be sized at 125% to be protected by the ocpd.

As Dennis said, I don't see any reason to size the feeder at 125% for all of the motors, but you can if you want. At a minimum the feeder should be sized at 125% of the largest motor and 100% of all other motors. No matter how many motors you have, the extra 25% of the largest motor will cover the largest motor given that all of the other motors are running. Say the largest motor wasn't running, then the feeder would still cover any of the smaller motors as well. If you had a lot of motors running simultaneously, then you might want to size the feeder beyond the recommended minimum.

Any objections to my understanding?

 

[RichB]

125% of the motor FLA deals with the long time portion for protection from overloads. so for overload protection, you would size your ocpd at 125% or less for branch circuits (short circuit and ground fault won't matter because that is in the short time...). The conductors would also be sized at 125% to be protected by the ocpd.

As Dennis said, I don't see any reason to size the feeder at 125% for all of the motors, but you can if you want. At a minimum the feeder should be sized at 125% of the largest motor and 100% of all other motors. No matter how many motors you have, the extra 25% of the largest motor will cover the largest motor given that all of the other motors are running. Say the largest motor wasn't running, then the feeder would still cover any of the smaller motors as well. If you had a lot of motors running simultaneously, then you might want to size the feeder beyond the recommended minimum.

Any objections to my understanding?

I object to your understanding!:angel:The NEC is NOT meant to be understood!:happyno:

Seriously tho--Your explanation is how I have always done the math