Reducing Motor Circuit Conductors Size with Capacitors

[MrJLH]

Looking for some guidance here:

Can a motor circuit conductors be reduced if using a capacitor on the line side of a motor starter? Looking for where this is clearly laid out.

NEC 460.9 states not to consider the the conductor size when the capacitors are attached to a motor overload, but I'm asking if the capacitor is attached to the load side of a breaker or line side of a starter in a combination starter.

Thanks

 

[don_resqcapt19]

My understanding is that the reduced current on the circuit is only on the line side of the capacitors. However, as you stated, 460.9 does not permit reducing the motor circuit conductor size.

 

[texie]

There is allowance in 430 or 460 to size motor conductors smaller than what 430.22 requires. I.E. 125% of NEC table value in most cases.
The reason for 460.9 is to prevent the overloads from being to large when connected on the load side of the overload protector.

 

[MrJLH]

There is allowance in 430 or 460 to size motor conductors smaller than what 430.22 requires. I.E. 125% of NEC table value in most cases.
The reason for 460.9 is to prevent the overloads from being to large when connected on the load side of the overload protector.

Sorry, Looking at 430.22 right in from of me and I gotta admit I dont see it!

 

[texie]

Sorry, Looking at 430.22 right in from of me and I gotta admit I dont see it!

I just noticed that I missed the word "no" in my first sentence. I should have said there is NO allowance. But I see you got my point.

 

[LarryFine]

My understanding is that the reduced current on the circuit is only on the line side of the capacitors. However, as you stated, 460.9 does not permit reducing the motor circuit conductor size.

If you're asking what I think you're asking, conductors and equipment between motors and their PF correction must be sized for real and reactive power, while the supply conductors only have to carry real power.

 

[Jraef]

Having the capacitors on the LINE side of the controller means they are on-line all the time, which is often worse than not having them at all. You only want the caps on when the motor is connected, so you connect them down stream of the contactor. Ahead or below the OL relay makes a difference in the setting of the OL relay, but not the conductor size.

 

[paulengr]

Having the capacitors on the LINE side of the controller means they are on-line all the time, which is often worse than not having them at all. You only want the caps on when the motor is connected, so you connect them down stream of the contactor. Ahead or below the OL relay makes a difference in the setting of the OL relay, but not the conductor size.

As an example had to deal with a 1500 HP, 20 pole motor. Normally it ran under loaded so power factor was around 0.55. They wanted to run an additional 100 HP load but including controls on the existing 4160 V, 300 MCM SHD-GC we were already at ampacity limits on the feeder. This is not a controller load side argument.

Keep in mind here that’s a NEMA cable designation in an MSHA jurisdiction. NEC scopes it out. NEC does not apply except that it is used as a design standard, not a legal requirement.

Adding enough KVars to counteract the flux to bring the power factor up to 0.85 under normal cases kept it under 1.0 even if it was fully loaded (0.97 if I recall). This freed up enough ampacity in the feeder to allow the additional 100 HP load. Upstream relaying was set to cable ampacity which is important because if the cap bank is disconnected or fails we have to protect the cable.