RPC Contactor

RonRock · Feb 14, 2018

The plan that I am following is for a 10 HP RPC. I am building a 7.5HP. The plan has 2 contactors one on the line in side and one on the 3 phase side controlling the start caps. The plan calls for the line in contactor to be rated @ 60A inductive load, and the second rated @ 40A inductive load. I should add that they are both 3 pole with 24V coil. The first also has 1 aux contact that goes to the remote switch terminals.

So since my RPC is 7.5HP am I correct that the contactors should be sized for my motor that draws 21.6A? Such as a 25A contactor. Do I need a larger one on the first that also controls the Aux contact and potential relay?

Toms Wheels · Feb 14, 2018

The contactor needs to be sized for the RPC size. To do this you only need ONE contactor. Two wires from main panel, call them BLK and WH, go to input side of contactor, then to the RPC, also on output side is B,W to each 3 ph load. From the RPC comes the third leg call this OR, this goes to the INPUT side of the contactor, then output to the load.

If using a fused manual disconnect as you should. The OR wire from the RPC goes first to the input side of the fuses, then through that to the contactor.

This way all 3 wires are covered with fuses, and disconnected when they should be.

Don Kinzer · Feb 14, 2018

Toms Wheels said:
From the RPC comes the third leg call this OR [...]

The more conventional color coding for 208/220 3-phase is red, black and blue with green (or bare) used for ground. If present (as it may be in an RPC), neutral would be white.

RonRock · Feb 14, 2018

Thanks guys. That seems pretty straight forward. I was correct in my thinking that the contactors are sized for the RPC idler.

Not sure why the plan calls for one to be oversized.

As an aside bit of info. The potential relay that you suggest Don is on it's way. Thanks for the help. We'll see how this works out.

SAF · Feb 14, 2018

Don Kinzer said:
The more conventional color coding for 208/220 3-phase is red, black and blue with green (or bare) used for ground. If present (as it may be in an RPC), neutral would be white.

For a 208V Wye system red blk blu is acceptable. However a typical 240V RPC output is equivalent to a center tapped delta with a 208V hi leg. To follow code, the hi leg wiring should be orange, the other phase lines can be any color other than wht or grn.

110.15 High-Leg Marking. On a 4-wire, delta-connected
system where the midpoint of one phase winding is
grounded, only the conductor or busbar having the higher
phase voltage to ground shall be durably and permanently
marked by an outer finish that is orange in color or by other
effective means. Such identification shall be placed at each
point on the system where a connection is made if the
grounded conductor is also present.

SAF Ω

TDegenhart · Feb 14, 2018

You could go with a NEMA size 2, 45 amps enclosed, instead of a size 3.

Tom

Don Kinzer · Feb 14, 2018

RonRock said:
Not sure why the plan calls for one to be oversized.

The idler motor horsepower dictates the *minimum* rating of the contactor. Except for the fact that they usually cost more and take up more space, there is no problem with having an "oversized" contactor.

RonRock · Feb 14, 2018

Don Kinzer said:
The idler motor horsepower dictates the *minimum* rating of the contactor. Except for the fact that they usually cost more and take up more space, there is no problem with having an "oversized" contactor.

Thanks again, I was wondering about that. That opens up my search a bit.

I figured that a contactor is basically just a relay, so it shouldn't matter the size as long as it's large enough to carry the load. And has a coil of appropriate voltage to be controlled with what is switching it on and off. I wasn't so sure if the potential relay that is connected to the first contactor in my plan has an affect on the needed contactor amperage.

vc3006 · Feb 15, 2018

RonRock said:
The plan that I am following is for a 10 HP RPC. I am building a 7.5HP. The plan has 2 contactors one on the line in side and one on the 3 phase side controlling the start caps. The plan calls for the line in contactor to be rated @ 60A inductive load, and the second rated @ 40A inductive load. I should add that they are both 3 pole with 24V coil. The first also has 1 aux contact that goes to the remote switch terminals.

So since my RPC is 7.5HP am I correct that the contactors should be sized for my motor that draws 21.6A? Such as a 25A contactor. Do I need a larger one on the first that also controls the Aux contact and potential relay?

DISCLAIMER: I am not an electrical professional! But I did build and wire my own house, and it passed on first inspection!

The best videos, with step by step instructions, and explanations, on building a 7.5hp RPC can be found at Building a Phase Converter - Part 1 - YouTube. It is a 3 video series that starts with a prototype layout, and ends with final installation. I am at the point of testing in my RPC build. Hope this helps.

RonRock · Feb 15, 2018

Good pointer vc3006. He does a good job of explaining things.

You Tube is one of the best things on the internet.

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