Contactor Sizing

[fifty60]

I am trying to size a contactor to apply/remove power from the entire equipment. The equipment consists of heaters, a transformer, three-phase refrigeration compressors, and a single phase air circulator motor.

The FLA for the machine if 38A, which would land me with a 50A branch circuit OCPD. Should I make sure that my contactor has at least a 38A inductive rating, and a 50A resistive rating? Or, should the contactor have a 50A inductive rating?

 

[fifty60]

Basically the question is when I have a mixed inductive and resistive load, when sizing a contactor should I base the entire inductive rating off of the full FLA of the machine, or only the inductive portion. For example, if my contactor is rated for 30A inductive, and 45A resistive, and I have a machine that has 25A of inductive loads but also another 15A of resistive loads so that the overall FLA is 40A, would I be safe using the 30A inductive/45A resistive contactor?

Or, would I need to to make sure that the contactor is at least 40A inductive-rated?

 

[Jraef]

The safe bet would be to use the 40A inductive rating. The difference in size and price is so minimal that you have already used up any savings in the time it took you to ask this question. That is a main concept behind what is called "NEMA" sizing; spend a little more on hardware, a lot less on engineering time.

 

[SceneryDriver]

What jraef said. Be aware that NEMA style contactors are built more ruggedly than their IEC counterparts. NEMA units can take abuse and hold up fairly well. IEC units will fail in short order if not precisely and generously sized.


SceneryDriver