# 208v motor, 120v starter coil



## LogicDB (Feb 12, 2010)

Sort of a custom combo starter, basic three wire control and pilot light...

Any reason for a control transformer when all I have to do is pull a neutral (with the three phase power leads) and fuse the control hot (picked up off line side of starter tab)?
Any articles that pertain to this?

Thanks


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## Hippie (May 12, 2011)

As long as your neutral originates from the same source as your hot you're fine


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## LogicDB (Feb 12, 2010)

Right, nothing about it seems wrong, just haven't seen it done before. Naturally because my motor is 7.5 hp, my neutral is #12 (minimum size wire in conduit per job spec) and my line side 208v leads are #10 (from the panel to the combo starter). The control hot is getting fused down to 2a or something @ the tap, and the neutral will only be #14 in the can. 
The disco will still kill everything...


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## millelec (Nov 20, 2010)

just my personal preference, but I like that if you lose either of the 2 phases that power the control power trans, you open contactor instantly, instead of whatever delay you incur waiting for fuses/overloads to kick in. still a one in three chance you can lose the one phase that doesn't power your control circuit & then the fuses/overloads do the job they're supposed to, but hate stressing the motor/circuit any more than I have to.


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## LogicDB (Feb 12, 2010)

Makes sense, thought about that, but its also quite common to fuse down a branch circuit and pull starters with a completely foreign source. Also, as you mentioned, the 1/3 chance that you don't have the faulty phase feeding the control transformer.


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## John Valdes (May 17, 2007)

I would install a coil for the voltage present. I guess you are using a NEMA starter? Everything about a NEMA starter is expensive.
I am guessing you have a combination starter? If you do, make sure you make the change (document) on the drawing inside the door if you do use a 120 volt coil.


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## LogicDB (Feb 12, 2010)

In the scope there is no control voltage allowed over 120v to ground. If the wires never left the can, I could see an exception, but I'm looping through limit switches, etc in the field.


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## Big John (May 23, 2010)

In a 208 system you still never have voltages higher than 120 to ground.

Personally, I'd either put in a 208 volt coil so you get full phase-loss protection, or I'd pull the neutral. Either way I'd sleep fine.

-John


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## LogicDB (Feb 12, 2010)

Big John said:


> In a 208 system you still never have voltages higher than 120 to ground.
> 
> Personally, I'd either put in a 208 volt coil so you get full phase-loss protection, or I'd pull the neutral. Either way I'd sleep fine.
> 
> -John


You're right, I don't know why I said that. I think that I misquoted the spec just now. I read it months ago and ruled out the use of a 208v coil based on phase to phase I think. Anyways, the pilot lights and everything are all 120v. I'd rather pull the neutral, just looking out for red flags. I have a bender monitoring relay that will shunt trip the main if there are any power source issues. (It's an engineered provision for a potential generator hook up and utility failure).


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## Hippie (May 12, 2011)

millelec said:


> just my personal preference, but I like that if you lose either of the 2 phases that power the control power trans, you open contactor instantly, instead of whatever delay you incur waiting for fuses/overloads to kick in. still a one in three chance you can lose the one phase that doesn't power your control circuit & then the fuses/overloads do the job they're supposed to, but hate stressing the motor/circuit any more than I have to.


In my experience, most every time I've had a motor lose a phase its been after the starter, having an xformer wouldn't have helped that


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## Jabberwoky (Sep 2, 2012)

I was taught to always have a control transformer when you pull off the same line as a motor because of line notching the motor causes. I have seen it with a VFD creating the notching but I don't recall seeing it with just a starter to the line.


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## JRaef (Mar 23, 2009)

Nothing wrong with it, done all the time. But as was said, the Neutral has to originate from the same source as the 208V.


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## LogicDB (Feb 12, 2010)

JRaef said:


> Nothing wrong with it, done all the time. But as was said, the Neutral has to originate from the same source as the 208V.


Sounds like a consensus. Thanks guys. Just wanted to make sure.


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