NEC 517 equipment loads on critical branch

[jimrettberg]

517.33(9) allows for 'selected power circuits needed for effective hospital operation' to be put on the critical branch of the essential electrical system. Does anyone agree this notation allows putting major air handling systems and chillers on the critical branch? The hospital prefers these loads on the critical branch. I dissagree with the concept but can see where there may be a loop hole in the code. The hospital essential electrical system is over 150Kva.

 

[steve66]

If you read the end of that paragraph, it says "single phase motors may be connected to the critical branch." That implies that larger, 3 phase motors are not allowed on the critical branch.

I really don't understand why they wouldn't want to put these on the equipment branch? Large air handlers and chillers can cause power quality issues including surges and voltage drops. It would be much better to have these on a separate branch without all the sensitive electronic equipment you would find in an OR.

And there is a very good chance that a chiller or large air handler can take out the entire branch. I've seen a small condensing unit do this, and it wasn't good. Selective coordination is supposed to reduce this possiblility, but circuit breakers don't always function the way they are supposed to.

 

[jimrettberg]

If you read the end of that paragraph, it says "single phase motors may be connected to the critical branch." That implies that larger, 3 phase motors are not allowed on the critical branch.

I really don't understand why they wouldn't want to put these on the equipment branch? Large air handlers and chillers can cause power quality issues including surges and voltage drops. It would be much better to have these on a separate branch without all the sensitive electronic equipment you would find in an OR.

And there is a very good chance that a chiller or large air handler can take out the entire branch. I've seen a small condensing unit do this, and it wasn't good. Selective coordination is supposed to reduce this possiblility, but circuit breakers don't always function the way they are supposed to.

I agree with your comments with the exception of noting what the code 'implies'. The code doesn't say three phase motor loads are not to be on the critical branch only that fractional hp single phase motors can be on the critical branch. What I'm looking for is anyone that has used the wording of the code in black and white to an extreme as this would be. Thanks for your comments though as it pointed out how the motor loads could effect sensative critical loads.

 

[david luchini]

The emergency system is limited to circuits essential to life safety and critical patient care (517.30(B)(2)). I don't see how air handlers or chillers would be essential to life safety or critical patient care.

 

[jimrettberg]

The air handler serves the operating rooms and support rooms around it. The chiller maintains the design temperatures required by ASHREA. Both 'could' be considered as relative to the life safety of a patient.

 

[steve66]

The air handler serves the operating rooms and support rooms around it. The chiller maintains the design temperatures required by ASHREA. Both 'could' be considered as relative to the life safety of a patient.

No. 517.34 describes what should be connected to the equipment branch. This clearly includes ventilation for the OR's(517.34(A)(7)). Cooling for OR's could also be added to the equipment system per 517.34(B)(8). But these can definately not be added to the critical branch.

I don't see a loop hole in the code. It seems very clear to me. Even if there were a "loop hole", I would not even consider doing this.

IMO, the fact that the hospital is requesting this shows a total lack of knowledge or understanding about the difference between the equipment system and the critical system.