Control Panel Breaker Sizing For UL Approval

[Shmantz]

Does anyone know what the procedure is for calculating the Main Breaker size for a Control Panel that requires UL certification? A panel shop we are using says that UL requires the Breaker to be sized at 80% of the total FLA of device ratings in the panel. I am questioning this because we have situations where they ended up with a 250Amp Breaker when the actual running load is measured at 20Amps maximum! If anybody can help I would appreciate it. Thanks.

 

[DRWeig]

Hi Schmantz,

There are several procedures in UL 508 and 508A, depending on the loads to be controlled and powered by your panel. However, the procedures result in the maximum breaker size. Smaller trip ratings can be used if appropriate for the load.

Can you give us a whole lot more information? The more you tell, the better the quality of the answers you'll get.

Thanks!

Good on ya,

Goober Dave

 

[Shmantz]

Thanks for the response DRWeig,
I can tell you that in the example I gave there are 5 servos
3 of which run intermiitently (2 sec. intervals before an index)Other loads are chain conveyors that are loaded every 2min and belt conveyors which have a load cycle of about 10sec......

Just got a reply back from UL:

I believe you are referring to section 30.2.1 in UL 508A.

Section 30.2.1 states that "An inverse-time or instantaneous-trip circuit breaker shall not carry a full-load current of more than 80 percent of its nominal ampere rating. "The key is CARRY A FULL-LOAD CURENT."

If the loads do not come on at the same time, then they do not contribute to full load current at a given time.

I hope the above is clear.

I assume a FLA reading of a running machine should be enough to provide to UL!

 

[dpc]

I assume a FLA reading of a running machine should be enough to provide to UL!

Actually, I would assume you would need to use the FLA on the nameplate of the motor.

That is the NEC requirement - but I'm no expert on UL 508.

David Castor

http://www.cvoes.com

 

[DRWeig]

Hi again Shmantz,

I think you'll actually be getting outside of UL and into the NEC for the main disconnect sizing.

Your five motor loads can be handled as multi-motor equipment -- the feeder will have to be sized based on nameplate FLA, not measured FLA. Basically, the feeder has to be able to handle all the loads that can run at once, plus an allowance for the starting current of the largest one (125%). Once the feeder is sized, its overcurrent and short circuit protection can be chosen.

I don't have my NEC handy -- but it would be worth your while to look at section 430 under multi-motor equipment.

To satisfy UL, you just have to make your feeder protection 125% of calculated, with an upper limit of 80% of the feeder ampacity.

That may all sound confusing -- but that's because it's a bit of complex calculation to do.

Let's see if anyone else responds to simplify it. You might consider flagging this post for deletion, and re-posting your question in Electric Motors, Generators, and Controls Engineering forum. Some really good minds in there who have dealt with this before.

Good on ya,

Goober Dave

 

[Shmantz]

Thanks guys...I will do that.