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Old 08-31-2008, 12:55 PM   #1
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120/208/1 phase wiring questions


I have an oven that can be 3 phase wired but was and is hard wired from factory for 120/208/1. A plate on the oven reads 208 1 phase 28 amps. The spec sheet for the oven says for 120/208/1 configuration L1 pulls 22 amps and L2 pulls 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 configured) It has a cord attached with a Nema 4 legged 50 amp plug. A hand drawn diagram on the back of the oven, shows a receptical & plug wiring nuetral (w) , ground (g) , black L1 (x) and red L2 (y).

My space has a 3 phase power panel and I have an outlet that I want to use. It has phase A & phase B each wired by a 8 AWG THHN wire along with one 10 awg Nuetral (grounded at panel) and one 10 awg grounded wire. All wires are in conduit.

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? Is a 10 awg nuetral sufficient? Why is a single amperage stamped on the oven 28 amps while the spec sheet shows differing amperages for each leg.

How is the circuit protected. Do I need to gang to different breakers one 30 amp (for phase A "L1") & another 40 amp (for phase B "L2") in the power panel? If two breakers are used, do their switches need to be tied so they would trip together?

Last edited by JohnJon; 08-31-2008 at 12:57 PM. Reason: typo
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Old 08-31-2008, 01:13 PM   #2
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120/208/1 phase wiring questions


Where did you get those amperage numbers? What is the amperage on the label on the whole unit?

This is a 120/208v appliance, meaning it has both 120v and 208v loads. 208v would be things like heating elements. 120v would be things like small motors, lighting, timers, etc. THAT is what gives you your amperage imbalance and why the neutral is required.

If either of the amperage is over 32 then you will need a 50a circuit. You DO NOT use two different breakers. The breakers protect the circuit conductors, not the unit.

I have to say though, if this is a commercial space, and especially if you are not the property owner, you should NOT be doing ANY electrical work there unless you are licensed and insured to do so.
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Old 08-31-2008, 01:22 PM   #3
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120/208/1 phase wiring questions


Quote:
How can L1 (A) pull 22 amps and L2 (B) pull 37 amps?
I am thinking that the heating element is drawing 22 amps on L1 and L2. The convection motor and lights are 120 volts and drawing the additional 15 amps.



Quote:
A warning on the oven states not to energize the unit with more than 150 volts in any circuit to L1 or L3
A delta 120/240 3 phase system has a high leg of appx 200 volts. If you have this type of system they want you to make sure to put the high leg on L2

Quote:
Do I need to gang to different breakers
No. Your existing #8's on a 2 pole 50 amp breaker will be good.
#10 ground is also good.
#10 neutral is good for the load but is can be argued that it must be fuul sized to match the 8's. I'd use it.
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Old 08-31-2008, 01:37 PM   #4
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120/208/1 phase wiring questions


A oven decal lists the electrical as 120/208/1 28 amps.

The manufacturer spec sheet for this model shows different wiring options and further shows when the oven is wired as it is labeled 120/208/1 that L1 pulls 22 amps & L2 pulls 37 amps.

Why a 50 amp for the A & B and not 40 amp?

I find it useful to understand what I may need to pay for so I've identified the possible outlet, the wires that are in it (two 8 awgs & two 10 awgs)

I want to know if it I understand how this works and if it can be made to work by having an electrician wire a new receptical & breaker or if a whole new circuit will be need to be wired. Concerned also if the #10 neutral will need to or can carry the amperage.
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Old 08-31-2008, 03:06 PM   #5
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120/208/1 phase wiring questions


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A oven decal lists the electrical as 120/208/1 28 amps.

The manufacturer spec sheet for this model shows different wiring options and further shows when the oven is wired as it is labeled 120/208/1 that L1 pulls 22 amps & L2 pulls 37 amps.
There is no way for us to tell without seeing those specs.

Post the Mfg and model number. I bet there is documentation on the web we can look up.
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Old 08-31-2008, 04:29 PM   #6
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120/208/1 phase wiring questions


Quote:
My space has a 3 phase power panel and I have an outlet that I want to use. It has phase A & phase B each wired by a 8 AWG THHN wire along with one 10 awg Nuetral (grounded at panel) and one 10 awg grounded wire. All wires are in conduit.
as another mentioned, you may have more than the max of 150 volts (to ground) on one of the legs in a 3 phase system. You need to know what your service supplies you with to be able to make this determination.

On top of that, it is possible that you do not have the 208 voltage required at all. NEMA requirments are that an an appliance be capable of operating on +/- 10 percent but 240 volts is beyond the acceptable tolerance reuqired for a 208 volt rated appliance.

If you are not familiar with the different services available or how to determine what you have, you need to leave the entire thing to an electrician that can be sure the euqipment is proper for your service.
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Old 08-31-2008, 06:17 PM   #7
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120/208/1 phase wiring questions


Its a 3 phase wye. 208 volts AC from any of 3 phases to another and approx 119 volts from any of 3 phases to ground or neutral.

I've included a pdf from the manufacturer (page 2 has electrical spec) and as I mentioned it is wired with a Nema 14-50 plug for 120/208/1. I confirmed with a continuity tester that ground is wired to the G lug, Neutral to W, X & Y to L1 & L2 respectively and L3 not wired.

thanks for the help
Attached Files
File Type: pdf OP-3FM spec.pdf (71.1 KB, 194 views)

Last edited by JohnJon; 08-31-2008 at 06:19 PM. Reason: typo
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Old 08-31-2008, 06:23 PM   #8
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120/208/1 phase wiring questions


At 120/208/1 the load is:
L1- 22a
L2- 37a

This is regardless of what it says on the oven.

37A = 50A circuit
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Old 08-31-2008, 06:27 PM   #9
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120/208/1 phase wiring questions


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Originally Posted by Speedy Petey View Post
There is no way for us to tell without seeing those specs.

Post the Mfg and model number. I bet there is documentation on the web we can look up.
Again the specs are 120/208/1 L1 22 amps L2 37 amps. I provided the manufacturers spec sheet right from their web site in my last post.
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Old 08-31-2008, 06:35 PM   #10
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120/208/1 phase wiring questions


Ok then a 50 amp double pole breaker would be required.

Back to one of my original questions. This is a 3 phase wye service so these phases are all 120 deg separated right? So even if the load was balanced between L1 & L2 wouldn't there be a neutral current expected? Perhaps more since L1 & L2 are rated for different loads from the get go. If so is the 10 awg thhn wire sufficient?
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Old 08-31-2008, 07:09 PM   #11
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120/208/1 phase wiring questions


Yes, there will be current on the neutral. That is the reason it is required.
Yes, #10 will be enough.
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Old 08-31-2008, 11:28 PM   #12
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120/208/1 phase wiring questions


Thanks, all for the help
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