Sizing conductors and OCP for chiller

[rginiewski]

I have always been taught to size wiring and overcurrent protection for mechanical equipment based on MCA and MOCP. In one case, the printed info is: 496 MCA, 800 MOCP. I sized this as 2 sets of 3-250 Kcmil (510A capacity), and an 800A circuit breaker. But I can't get past NEC 240.4(C), stating the wire ampacity must meet the overcurrent device. Is my design not Code compliant?

 

[bob]

Re: Sizing conductors and OCP for chiller

Look at 240.3 and then check table 430.52. You can use an inverse time breaker up to 250 % of the FLA.

 

[steve66]

Re: Sizing conductors and OCP for chiller

But 240.3 covers equipment. The branch circuit conductors also have to be protected aginst short circuits, ground faults, and overloads. I think 240.4(G) applies more than 240.3 It refers to Article 440. Look at 440.22 B1 which finally refers to 440.22 A. Correct me if I'm wrong, but I think this says the breaker can't be sized more than 175% of the rated load current. It also has an exception to allow 225%.

 

[bob]

Re: Sizing conductors and OCP for chiller

If this is A/C equipment you are correct. The post did not mention A/C equipment. However the equipment was tested with the 496 MCA & 800 MOCP.
If this is listed on the nameplate then it should be as listed per the mfg.
The conductors are protected against fault currents by the breaker and overloads by the motor/compressor overloads.

 

[jhines]

Re: Sizing conductors and OCP for chiller

Have talked with ALCAN concerning use of aluminum conductors. The UL on HVAC units seem to be for copper only.