Combining different gauge wires

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Barbqranch

Barbqranch

Senior Member
Location
Arcata, CA
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Plant maintenance electrician Semi-retired
I may be installing a panel that will feed only two machines. One is 400 volts three phase, the other is 230 volts single phase. These are unique voltages for our plant, and so there will never be more load on it.
Lets say both loads are 35 amps. Can I feed the panel with two #8 for 2 of the 3 phases, and one #3 for the third phase (which will also carry the single phase load). And then there would be a #8 neutral for the single phase load.
I seems ok to me, but also seems sort of crazy.
 
I don’t think code will disallow it as long as it isn’t paralleled, but why would you want to do this? The resistances will be different, and a very small voltage imbalance will cause a much larger current imbalance.
 
OK, now I am further confused. I was hoping to use the two breakers as the disconnect. Does it have to be a single handle? If so, could I simply include a 70 amp backfed breaker in the panel as the disconnect for the transformer? In which case I would need to change the ampacity of all the to 70 amps. That is fine if that is what it takes.
 
It would be best to size the feeder all to the largest load for safety and confusion down the road. Cost will not be that much of a difference in the size feeder we are talking about. To do it the way you want you would need a fused disconnect with different sized fuses. Cost would probably be a wash unless your talking about a HUGE run, but than your #8 wouldn't allow for voltage drop. And then someone would have to worry about remembering not to load the other two phases, etc during repairs. Seems crazy.
 
Presumably this is being fed from a Delta y transformer so you also have to worry about protecting both the primary and the secondary of the transformer. I don't know what you would gain by trying to make the wire size a little bit smaller on one phase.

I'm not sure how I feel about the number eight neutral. My inclination is that wire is relatively cheap and electrician time to redo it down the road when someone decides to add something is expensive.

I might even run four number threes. If you have to go out and buy a spool of wire and you have some leftover you might as well use it up versus having 20 ft of it sitting in a corner of a stockroom collecting dust.
 
Thank you, you have convinced me to abandon the way I was going and take your advice. That is why I come here, to deal w/ people who know more and have more experience than I do.
 
Barbqranch said:
I may be installing a panel that will feed only two machines. One is 400 volts three phase, the other is 230 volts single phase. These are unique voltages for our plant, and so there will never be more load on it.
Lets say both loads are 35 amps. Can I feed the panel with two #8 for 2 of the 3 phases, and one #3 for the third phase (which will also carry the single phase load). And then there would be a #8 neutral for the single phase load.
I seems ok to me, but also seems sort of crazy.
Click to expand...
400V 3-ph, 230V single phase kinda suggests 50Hz.
 
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