Oven Wire Size Question

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SteveO28

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Installing a double oven for a new kitchen. LG WDEP9427. Install manual says it requires a 50amp breaker with 10g wire. Oven draws 42.5 amps. I planned on running a 6awg based on 42.5x1.25= 53.125a. Anyone want to fill me in why the manual calls for 10s?
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SteveO28 said:
Installing a double oven for a new kitchen. LG WDEP9427. Install manual says it requires a 50amp breaker with 10g wire. Oven draws 42.5 amps. I planned on running a 6awg based on 42.5x1.25= 53.125a. Anyone want to fill me in why the manual calls for 10s?
www.manualslib.com

Electrical Requirements; Making Electrical Connections - LG PrintProof WDES9428 Series Installation Manual [Page 12]

LG PrintProof WDES9428 Series Manual Online: electrical requirements, Making Electrical Connections. Ensure That Dedicated Circuit Protection Is Prepared As Recommended And That The Appliance Is Grounded Properly. • This Wall Oven Must Be Electrically Grounded In Accordance With Local...
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Likely, the writer who wrote that manual, followed the 90C column of the ampacity table, without connecting the dots that it needs to interact with 110.14(C), and applicable parts of articles 240 and 310. Most wire made today, has a 90C rating, but it is uncommon that you can directly take credit for it. It requires a rare situation that equipment on both sides of the wire, fulfills the burden of proof to use 90C terminations. Not just lugs marked AL9CU, but the entire manufactured product. 110.14(C) spells out when you can take credit for a 90C rating, despite 60C or 75C terminals, which is the primary value in a 90C rating.

Field-installed wiring has to comply with the NEC, even if a manual recommends less. Except in cases where the NEC endorses an exception, like HVAC equipment that has an Min OCPD & Min Circuit Amps (MCA) rating on datasheets. This particular exception has to do with the nature of motor loads. Product manuals may require a larger size than the NEC, and you comply with the greater of the two.

Factory-installed wiring inside equipment may not necessarily match the NEC, but that isn't your concern as an installer. This is all accounted-for, in the listing of the product.
 
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What intern came up with that, and why?
I figure calling it continuous is safe, but would it actually be? The breaker isn't even sized for continuous.
My logic is: even if the oven runs for 3hr+, the element won't be drawing full amps the entire time. If it's anything like my gas oven, it switches on for 1-2min then goes dormant a few minutes.
If the 42.5A is absolute full load with every burner and oven element on, what are the odds that all that runs continuously without stopping at full blast for 3 hours? Even on thanksgiving, I'm doubtful, but I'm not sure what code or the instructions for this say regarding that, since locally we only do gas. Someone enlighten me on best practice in a non-gas area. (same question for clothes dryer, I doubt the element runs continuously, plus it stops every hour usually).
 
NoahsArc said:
What intern came up with that, and why?
I figure calling it continuous is safe, but would it actually be? The breaker isn't even sized for continuous.
My logic is: even if the oven runs for 3hr+, the element won't be drawing full amps the entire time. If it's anything like my gas oven, it switches on for 1-2min then goes dormant a few minutes.
Click to expand...

You're correct. Thermostat-controlled ovens are not continuous loads, even if they run for over 3 hours, since they cycle on and off. You'd have to run them with the broiler for 3 hours, and if so, the breaker would trip before the wire overheats. An industrial oven might do that, but few (if any) people would use a residential oven that way.

This would explain why it is OK for a 42A oven to have a 50A breaker, instead of a 60A breaker as the 125% rule would imply. Still, the #10 wire is insufficient in the OP's example, and needs to at least be #8 Cu.
 
Electric cooking equipment is about the most intermittent kind of load you can get.

I recently had a customer we needed to add some load to test the operation of a backup inverter/battery system. Other than plugs and lights, the oven was the only large load. He cranked it up to 500, it ran steady 3 kw for a few minutes and then went to cycling, I would estimate it heated for 1 minute every 5 minutes.

Electric stovetop burners same deal, unless you put them all 4 on full blast they will be cycled on just a small percent of the time. It is rare that anybody has more than 2 or 3 even engaged at any one time.
 
Alright, I wasn't crazy.
So #8 with a 50A breaker. I think you can do a #10 on the neutral though, per 210.19A3ex2 [70% x ungrounded]
 
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NoahsArc said:
Alright, I wasn't crazy.
So #8 with a 50A breaker. I think you can do a #10 on the neutral though, per 210.19A3ex2 [70% x ungrounded] (this still seems overkill)
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You likely can. The neutral only needs to be sized for either A) matching the equipment ground size, or B) be sufficient for the actual current the appliance may put on the neutral, whichever is larger.

Ovens use the 240V or 208V circuit to power the Watt-intensive heating elements, and the 120V circuit to power the fan and controls. This means the amount of current on the neutral is small compared to the amount on the live wires.
 
SteveO28 said:
Installing a double oven for a new kitchen. LG WDEP9427. Install manual says it requires a 50amp breaker with 10g wire. Oven draws 42.5 amps. I planned on running a 6awg based on 42.5x1.25= 53.125a. Anyone want to fill me in why the manual calls for 10s?
www.manualslib.com

Electrical Requirements; Making Electrical Connections - LG PrintProof WDES9428 Series Installation Manual [Page 12]

LG PrintProof WDES9428 Series Manual Online: electrical requirements, Making Electrical Connections. Ensure That Dedicated Circuit Protection Is Prepared As Recommended And That The Appliance Is Grounded Properly. • This Wall Oven Must Be Electrically Grounded In Accordance With Local...
www.manualslib.com www.manualslib.com
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#6 nm is fine

The only times we ran #10 for an oven is when it was a single oven. So unless these are 2 separate ovens, which I doubt it is, then it would require one branch circuit not 2.
 
That chart in the instructions says the double ovens are 10700 W so I'd say #8 NM on a 40A breaker is fine, per 220.55
 
tortuga said:
That chart in the instructions says the double ovens are 10700 W so I'd say #8 NM on a 40A breaker is fine, per 220.55
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He did not state the KW rating but for the largest on the list: 10700/240=45 amps
 
I suppose its all in how you interpret 'one wall-mounted oven. To me its a double oven but it is a single wall mounted appliance so yeah I'd say to be safe your right 50A circuit.
 
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