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Thread: Seeking electricians advice... wiring a double convection oven

  1. #1
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    Seeking electricians advice... wiring a double convection oven

    I am finally replacing our single electrical convection oven with NOS double unit that we purchased as a clearance floor model a couple of years ago. I have a run of 35 feet and the badge on the unit says it is 7.2 KV at 240/120 v. or 5.4KV at 208/110. The installation guide says NEC requires four conductor connection on new construction, but the next paragraph says I "must use a three-wire, single phase A.C. 208Y/120 04 240/120 Volt 60 hertz electrical system." I'm interpreting the latter is for use in an existing system. I intend to use new wire, and am thinking 3 wire 6 gauge to the junction box where the ovens whip will connect. Is this wire adequate or overkill and would a 45A breaker be appropriate? Or is this something I shouldn't be attempting myself? I have a little electrical experience, enough to have added a sub panel to my wood shop with several 110 and 220 circuits, but am hesitant to proceed without some pro help. Also,does the 208/110 on the badge describe a 3 phase circuit?

  2. #2
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    Yes, the 208/110 refers to a 3 phase circuit. Assuming you are installing this in a residence, it's the 240/120 rating you are concerned with. 7.2 KW (not KV) is 30 amps at 240 volts. 6 gauge is way overkill.
    8/3 NMB (romex) has ampacity of 40 amps and is what I would run, even though you could get by with 10/3. Your cable will have 3 current carrying conductors, red, white and black, and a bare ground. If you wired a subpanel, this is not much different. I would use a 40 amp breaker (2 pole). But if you have any doubts, just hire an electrician.
    --I had my patience tested. I'm negative--

  3. #3
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    Quote Originally Posted by Paul F Franklin View Post
    Yes, the 208/110 refers to a 3 phase circuit. Assuming you are installing this in a residence, it's the 240/120 rating you are concerned with. 7.2 KW (not KV) is 30 amps at 240 volts. 6 gauge is way overkill.
    8/3 NMB (romex) has ampacity of 40 amps and is what I would run, even though you could get by with 10/3. Your cable will have 3 current carrying conductors, red, white and black, and a bare ground. If you wired a subpanel, this is not much different. I would use a 40 amp breaker (2 pole). But if you have any doubts, just hire an electrician.
    Thank you Paul. I had to do gymnastics to see the badge under the bottom door so mistakenly saw the KW as a KV. I did double check and it is 7.2 KW. So a run of 35 feet does not merit 6/3 to account for voltage loss?

  4. #4
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    You're welcome. No, with 8/3 you'll have less than 1% voltage drop at 30A full load. That's no issue at all.
    --I had my patience tested. I'm negative--

  5. #5
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    Paul has it right

  6. #6
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    You probably need a neutral to supply 120 volts for lights and controls. Older dryers and, maybe stoves?, used two hots and the ground wire was also used as a neutral to supply 240. A separate ground is required now. If the appliance needs 120 then a separate neutral wire must be pulled as well as a ground.
    Bil lD

  7. #7
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    Quote Originally Posted by Paul F Franklin View Post
    Yes, the 208/110 refers to a 3 phase circuit.
    No, it refers to a single phase circuit that is derived from a 3 phase service. Large condo/apartment buildings frequently have a 3 phase main service. From there, each suite is fed with a 120/208V single phase sub-service. That's why there are ratings given on the nameplate. But if it did indeed say 208/110V, it was a misprint because there is no such thing as 110V utility voltage in US or Canada and you can't get 110V from a 208V service in any case. It would be 120/208V, or 208/120V if you prefer.

  8. #8
    People keep getting confused by three phase. I used to sell electrical generators (or try to). The big generators that hook to steam and gas turbines that utilities buy. I am a mechanical engineer by education. Many things about electricity are pretty arbitrary. Most of the world uses 50 hz (50 cycles per second) electricity while in the USA, we use 60 HZ. That is arbitrary but it changes the speed of the generator from 3,000 rpm (50 hz) to 3600 rpm (60 hz). Anyway, the rotor of the generator is wired to have three magnetic poles. There may be some sort of efficiency advantage of 3 versus 2 or 4 but as far as I know it is another of the arbitrary choices made early that everything is now standardized upon. The three sweeping electromagnets induce three phases of current in the stator which are 120 degrees apart. That current gets boosted in voltage to reduce losses in transmission and then reduced in voltage at our homes for safety. But we don't get all three signals, we get two. Maybe some big buildings get three, I don't know.

    A three phase motor has three electromagnets on it's rotor like the generator discussed above. So it needs all three phases. They can reportedly be made to operate with a capacator (shifting the voltage signal to kind of mimic the third leg), or a phase converter. The best way to do it involves an inverter to go to DC and then back out to three actual signals 120 degrees apart.

    I think of the power in my house as all one phase but I guess you could also view the 240V loads as two phase. The load is connected line to line and thus from one phase of the generation to another. A neutral is also run and a ground. If one of the leads is connected to the neutral (or ground) you get 120V. Most breakers in your panel tap one bar to give you 120V. The 240V breakers hook to both bars. In terms of voltage the neutral and ground should both be about zero. But they are not exactly the same thing. The neutral is insulated and meant to carry current. The ground is not insulated and meant only to conduct if there is a fault.

    I've sold most of the equipment used to generate and distribute power but not recently. I did not design it and do not claim to be an expert. But I am fairly familar with it.

  9. #9
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    Thanks all for the info,and yes Frank after doing some contortions I see that it does indeed say 120/240. Tiny printing, awkward positioning to see the badge and old eyes, sigh.

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    Quote Originally Posted by Jim Dwight View Post
    But we don't get all three signals, we get two. Maybe some big buildings get three, I don't know.

    I think of the power in my house as all one phase but I guess you could also view the 240V loads as two phase.
    Virtually all commercial buildings, large & small, have a 3 phase service. Single & small multi-family residential is single phase, large residential buildings are mostly 3 phase to the building.

    A 120/240V service is single phase, not 2 phase. That is a different thing.

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