I bought an oven that could be 3 phase but is hard wired from factory for 120/208/1. The spec sheet reads for this configuration L1 pulls 22 amps and L2 37 amps. A warning on the oven states not to energize the unit with more than 150 volts in any circuit to L1 or L3 (L3 is not used as wired) It has a cord with 4 legged 50 amp plug. A hand drawn diagram on the back of the oven, shows a nuetral (w) , ground (g) , black (x) and red (y) for the source receptical.

I have an outlet in my commercial space that I want to use for this oven. It has phase A & phase B (ungrounded) each sourced by a 8 AWG wire and one 10 awg Nuetral (grounded) as well as one 10 awg grounded wire.

I have a few questions:

I thought that in single phase 208V, current flowed back and forth between phase A & B. How can L1 (A) pull 22 amps and L2 (B) pull 37 amps? Does the neutral provide the path for the extra amps?

Do I need to gang two different breakers one 30 amp (for phase A "L1") & another 40 amp (for phase B "L2") in the power panel? If two breakers do their switches need to be tied together?

 

John, what voltage do you have in your Commercial space?

I think the question here is really the difference between 120/240 and 120/208, forget the 3 phase end of things its not important since you dont need it.

First up 120/240, this comes off the transformer from a center tapped 240V secondary, so it simply has 2 120v circuits connected together (neutral) therefore its always single phase and can only supply 120V and 240V loads. There is a 240V 3 phase that has a 208V leg to neutral but it cant be used.

Next 120/208, this can only be supplied by a 3 phase transfomer, it has 3 windings 120V each all connected together at one end (neutral), the voltage phase to phase is 120 X 1.73 (square root of 3) or you guesed it 208, 120/208 is normally used for building lighting and recepticals.

You can search these 2 and find diagrams that might make things easier to see.

I hope this isn't too confusing, it took me a while to learn, Rick

 

John,
If I'm following you correctly, the oven is wired for 120/208V 1ph. It apparently uses 208 v for the heating load and 120 v for the controls and probably fans since the one leg carries considerably more current. So you would be correct that the neutral is carrying some current. If you are in the states and follow the NFPA70 the 8 awg wire should be on no more than a 40 amp breaker. Local codes often overule NFPA. It should be a two pole breaker. The breaker protects the wire. The oven should have internal protection for each leg. If the power source in your commercial space is 120/208 then you just install one 2 pole 40 amp breaker for the 2 #8 wires. You said the white was grounded. It should be isolated from ground everywhere after the first distribution panel after the transformer. It will read grounded because it is tied to ground at it's first point of distribution.

 

Vetterick,
In your reply to John you stated:
"First up 120/240, this comes off the transformer from a center tapped 240V secondary, so it simply has 2 120v circuits connected together (neutral) therefore its always single phase and can only supply 120V and 240V loads. There is a 240V 3 phase that has a 208V leg to neutral but it cant be used."

Not trying to be picky but I believe in a 240V 3ph, one leg to neutral reads somewhere around 140V and you are correct, it should not be used. It is there only to bond the neutral to ground for short circuit protection.

 

Vetterick,

Not trying to be picky but I believe in a 240V 3ph, one leg to neutral reads somewhere around 140V and you are correct, it should not be used. It is there only to bond the neutral to ground for short circuit protection.

I don't think you really understand whats going on here, if you connected the wild leg of a 240 3 phase system to ground it would cause a ground fault!!!, trust me, this is what I do, the wild leg mathmaticly works out to 208V, I've personally measured them at around 185 to 210, the varience is due to unbalenced loads, 140V to ground/Neutral is very low, I'd suspect a problem.

Again I'd like to know what voltage John has? If he has 120/240 he's out of luck.

Rick

 

You can still get your 240 single phase from a 240 3 phase system . The phases should be Right-to-Left or Top-to-Bottom... phase "A" (Black) "B" (Red) "C" (Blue). If there is a high leg it should be phase "B" and may be marked (Orange). Just use the other two legs....NOT the high leg. measure from neutral or ground from each hot leg and use the two that are (voltage-of-the-day) closest to 120. the high leg comes from the center tap of one and one half of the windings. 208 does not come from a 240 v transformer. Your oven has heat elements that, if rated 240 v will work just fine on 208 v. They will just produce less heat and draw a little less power. Use a 40A 2 poll breaker , yes with handles tied to trip simutaniously incase of a short so that one leg isn't left hot. You need to run a #8 neutral conductor as well for the (white) unbalenced load; and a green chasis ground conductor that can be reduced to #10. The neutral needs to be full sized for possible short circuit to tripp the breaker. Hope any or all of this helps any of you. Please correct me if I made a mistake on any of this. Roger (Electrical Contractor / Electrician 31 yrs.)

 

Your oven elements are for 208 volts single phase, which is what you get from 2 wires of a 120/208 three phase service. They would overheat and probabaly burn out running them on 230-240 volts. Either change the elements or use a very large and expensive buck/boost transformer as a buck transformer to get the 208 required from the assumed 120/240 standard home service. If you have 3 phase comming in you are probably OK. Some places have both 120/240 and 120/208 services (2 meters). I guess its possible that you could rewire that oven for 3 phase delta if it has 3 elements that come on at the same time? That would cut the current draw in half for the resistive heating load. If you have 120/208 service!