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240 volt 3 phase w/high leg #11049 06/26/02 02:40 PM
Joined: Dec 2001
Posts: 50
Joe Carpenter Offline OP
I have a machine to connect to 240 volt 3 phase no neutral(from phase converter). The machine has 120 volt controls. To get 120 volts the manufacturer of the machine used 1 leg of 3phase to ground. I did not like to see this. my preference would have been to use a control tranformer and use 2 legs of the line and drop it down to 120 volts. Have you run into this and what would you reccomend?

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Re: 240 volt 3 phase w/high leg #11050 06/26/02 03:41 PM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline
If the machine is receptacle-/cord-connected, there is no NEMA-standard 5-wire devices of that voltage rating. A "neutral conductor" is needed, and sized based on the control-circuit fuse, if any. Agreed it is a grey area.

Nota Bene: This is not a code-defined mutliwire branch circuit, for that needs equal ø-n voltages.

[This message has been edited by Bjarney (edited 06-26-2002).]

Re: 240 volt 3 phase w/high leg #11051 06/26/02 03:47 PM
Joined: Aug 2001
Posts: 7,520
pauluk Offline

Take my comments with a pinch of salt if you wish, as we have nothing like the high-leg delta arrangement in this country so I have no practical experience with it.

That said, I can't say I like the sound of just using the grounding conductor as the return for the 120V circuits. I'd be much happier using a step-down xfmr with a 240V primary.

One other point, is the phase terminal used to feed the 120V circuits clearly marked as such? If not, I would think it would be comparatively easy for somebody to one day rewire the unit with the phases switched around and accidentally apply 208V to the control circuitry.

Re: 240 volt 3 phase w/high leg #11052 06/26/02 04:09 PM
Joined: Dec 2001
Posts: 50
Joe Carpenter Offline OP
Getting the phases switched is clearly a possibility. I do however mark the high leg with orange as required and anybody with experiance will not change things to use that leg for the control circuit. As far as fusing the control circuit, the machine has 5 motors and motor starters they do not have auxillary relays and therefore the 120 volts for each one does not originate from a commom point. It is derived from each starter for each respective coil. This would make it difficult to fuse unless a fuse was put in for each starter coil

Re: 240 volt 3 phase w/high leg #11053 06/26/02 04:44 PM
Joined: May 2002
Posts: 132
elektrikguy Offline
I guess my only question would be why didn't the manufacturer just make the control circuit comparable to the line voltage and the 120 volt grounding question would disappear? Just making an observation. Do you think it is a cost question on the coils?

Re: 240 volt 3 phase w/high leg #11054 06/26/02 08:01 PM
Joined: Apr 2002
Posts: 2,527
Bjarney Offline
Joe C—If I understand the situation correctly, the only sure bet would be serving the grounded-circuit conductor/neutral [sic] at the same size as the ungrounded conductors.

Being multi-motor, does this machine have any electrical NRTL listing on the entire assembly, like UL508?

What size circuit is needed to supply the machine?

Re: 240 volt 3 phase w/high leg #11055 06/26/02 08:45 PM
Joined: Oct 2000
Posts: 5,389
sparky Offline
2 Q's here..

1) Is it the phase conversion that apperas to have a high leg? If so, is this of a correctable type/style re; taps vs. capacitors?

2)Can the 120V control circuits be fed via a 120 source, and possibly de-energized with a n axillary contact in the serving disco?

Re: 240 volt 3 phase w/high leg #11056 06/26/02 10:24 PM
Joined: Apr 2002
Posts: 7,238
HotLine1 Offline
Isn't "gypping the ground" a NEC violation??
Think about this: the ground wire (should be green) should only carry any voltage/current during a fault situation, and someone gets between it and ground and gets a shock, or killed. Experience teaches many sparkys not to assume that a "neutral" cannot shock you. We also watch for "gypped" grounds. I have seen instances where EMT was used for the return path (neutral), and a fire started by loose setscrew EMT fittings. Also, got rapped a few times from the "gypped" grounds.
By the way, the "gypped" term (maybe it's "gipped), came from an "old timer".

John (HotLine1)

Re: 240 volt 3 phase w/high leg #11057 06/27/02 12:39 AM
Joined: Jan 2002
Posts: 34
menegt Offline
I agree, control xfmr is what's needed needed here.

Re: 240 volt 3 phase w/high leg #11058 06/27/02 05:55 AM
Joined: Dec 2001
Posts: 50
Joe Carpenter Offline OP
Sparky , Your idea seems as feasible as any since the owner is trying to pinch pennies. I may be able to get the 120volt control voltage from another source like you said.
and Hotline, That was my greatest concern,The last thing they need is a machine with 120volt potential on the frame.


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