Several motors or loads on one branch circuit NEC430.53

[PaulBr]

I am looking for some help.
For HVAC unit I have three Copeland compressors units, which are connected (piped) in tandem. Each compressor rated three phase 460V, 60Hz,load current is 18.6 amp. Each compressor will operate from its own contactor rated for 34 amp equipped with electronic overload relay adjustable range 15-45.
Am I allowed by NEC have only one branch circuit protection circuit breaker for all three compressors? If yes, how to size it? I was reading NEC paragraph 430.53,and got lost.
Thank you,

PaulBr.

 

[waross]

Will the compressors start together or will a staggered start be used? Are there condenser fans loads in addition to the compressors?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PaulBr]

This is mobile HVAC unit. It has three compressors connected in tandem (piped in parallel in electrical terms). The way system works (how it was described to me) Initially only one compressor starts. If cooling from one unit is not enough to fulfill demand, then second compressor start, and if cooling from two units is not enough then third compressor starts.
Condenser fans are separate load and on separate branch circuit.

Paulbr

 

[rbulsara]

If the HVAC unit has a factory label or data sheet showing single point of connection, follow that.

If not, NEC does permit multiple motors on a circuit. Basic rule for the conductors is sum of FLA of all motors plus 25%.

The breaker size typically would be rated for 250% of FLA of the largest motor plus[/1] sum of FLA of the rest of the motors.

Rafiq Bulsara

http://www.srengineersct.com

 

[PaulBr]

This mobile HVAC unit is new design. So there are no factory label or data sheet. Straight forward approach is make each compressor unit on separate branch circuit which will consist of circuit breaker, contactor, and overload relay. However NEC permit multiple motors on one branch circuit. Therefore instead of 3 circuit breakers only 1 can be used (cost saving).
rbulsara, can you point to paragraph in NEC which is saying:
"The breaker size typically would be rated for 250% of FLA of the largest motor plus[/1] sum of FLA of the rest of the motors. "

Thank you.
Paulbr.

 

[waross]

In the case of catastrophic motor failure the breaker often opens on the fault current before the overloads can respond.
A normal control scheme would let a failed compressor trip off and the system would continue to function on the remaining units.
If you are pushing a penny pinching design out the door and to the end user the consequences, one breaker is fine.
If you are building a quality machine you may want to reconsider the one breaker choice.
An interesting exercise may be to write up a step by step trouble shooting chart with one breaker and again with three breakers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Paulbr:

In NEC 430 there are separate articles for motor circuit conductors, overload protection and short circuit protection.
Each of them has a sub-article on "Several motors" and also "Multiple motor equipment". The breaker would be in the short circuit part.
Please read each one of them. I know they are there, I would leave up to you to browse through the narrow part of the book for a thorough understanding. You still need individual overload protection.




Rafiq Bulsara

http://www.srengineersct.com