is this correct? (table 220.55)

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crtemp

Senior Member
I installed a 10 gauge wire for a cooktop. I went to install the cooktop and it is rated at 7.8kw. I know that 7.8kw is 32.5 amps at 240 volts. I see in table 220.55 column b it says I can apply a demand factor of 80% to it. Does this mean I only have to install a wire that is good for 26 amps (80% of 7800 watts)? Is household cooking equipment considered a continuous load?

If not does this mean a 10 gauge wire is good for cooking equipment up to 9kw?

GoldDigger

Moderator
Staff member
7.8kw is 3.25kw at 240V? What do you mean by that?

crtemp

Senior Member
7.8kw is 3.25kw at 240V? What do you mean by that?

I just meant that 7800 watts is 32.5 amps which is over the maximum for a 10 gauge wire

crtemp

Senior Member
Here is another question. More of a hypothetical question though. I am looking at this double oven from Home Depot.

http://www.homedepot.com/p/Whirlpoo...lid=CJ3i1tPSw78CFUZbfgodLmgAsw#specifications

It says it needs a 40 amp circuit yet the upper and lower wattage are 2800 watts for a total of 5600 watts (23 amps). Now since 5600 watts falls under the column b of 220.55, could I take 5600 watts by 80% (4480 watts) and just run a 12/3 to the unit?

GoldDigger

Moderator
Staff member
For whatever reason, if the instructions call for a minimum 40A circuit, you have no choice under the NEC but to provide a 40A breaker.
Now you need to consider the wires. What wire size do you need for a 40A breaker with a dedicated 5600W load?
Are the heating elements all line to line 240V?

crtemp

Senior Member
For whatever reason, if the instructions call for a minimum 40A circuit, you have no choice under the NEC but to provide a 40A breaker.
Now you need to consider the wires. What wire size do you need for a 40A breaker with a dedicated 5600W load?
Are the heating elements all line to line 240V?

I don't understand what the point of table 220.55 is if every cooking unit comes with directions that call for more power than the units even draw.

GoldDigger

Moderator
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I don't understand what the point of table 220.55 is if every cooking unit comes with directions that call for more power than the units even draw.
Well, it affects load calculations upstream even if it does not override the manufacturer's instructions on circuit size.

Little Bill

Moderator
Staff member
Here is another question. More of a hypothetical question though. I am looking at this double oven from Home Depot.

http://www.homedepot.com/p/Whirlpoo...lid=CJ3i1tPSw78CFUZbfgodLmgAsw#specifications

It says it needs a 40 amp circuit yet the upper and lower wattage are 2800 watts for a total of 5600 watts (23 amps). Now since 5600 watts falls under the column b of 220.55, could I take 5600 watts by 80% (4480 watts) and just run a 12/3 to the unit?

The table 220.55 is for demand factors for calculating feeder/service loads. Note #4 is for branch circuit load. Since it's only a single appliance and the oven has a nameplate (spec sheet probably matches) rating of 40A you have to use that rating for your branch circuit. So if it says 40A, that's what you have to go by.
I would also speculate that since it is a self cleaning oven, they took the diversity factors out. Meaning if it was just a regular oven the thermostat would not let it run at full power long enough to be considered a continuous load. But because of the self cleaning the temperature gets extremely high and would pull more power, maybe for a period that would be considered continuous. So they rated the the power demand based on that.

crtemp

Senior Member
The table 220.55 is for demand factors for calculating feeder/service loads. Note #4 is for branch circuit load. Since it's only a single appliance and the oven has a nameplate (spec sheet probably matches) rating of 40A you have to use that rating for your branch circuit. So if it says 40A, that's what you have to go by.
I would also speculate that since it is a self cleaning oven, they took the diversity factors out. Meaning if it was just a regular oven the thermostat would not let it run at full power long enough to be considered a continuous load. But because of the self cleaning the temperature gets extremely high and would pull more power, maybe for a period that would be considered continuous. So they rated the the power demand based on that.

so how do you know which code to go by? Note #4 in 220.55 says the branch circuit rating will be the nameplate rating of the appliance whereas 110.3 (b) says it must be installed per instructions.

Little Bill

Moderator
Staff member
so how do you know which code to go by? Note #4 in 220.55 says the branch circuit rating will be the nameplate rating of the appliance whereas 110.3 (b) says it must be installed per instructions.

The nameplate should match the instructions. You may just be looking at the element ratings. The instructions, if tested/listed, should be what you go by. The listing would not violate the NEC if it is a recognized listing agency.

Also as I said, they probably require enough power for the self cleaning feature, not what they would use just for the element power without the cleaning feature.

crtemp

Senior Member
The nameplate should match the instructions. You may just be looking at the element ratings. The instructions, if tested/listed, should be what you go by. The listing would not violate the NEC if it is a recognized listing agency.

Also as I said, they probably require enough power for the self cleaning feature, not what they would use just for the element power without the cleaning feature.

So I'll just always go by the nameplate rating then. But am I correct in saying that I can size my circuit at 80% of the nameplate rating as long as it is under 8.75 kw?

Smart \$

Esteemed Member
So I'll just always go by the nameplate rating then. But am I correct in saying that I can size my circuit at 80% of the nameplate rating as long as it is under 8.75 kw?
Instructions for branch circuit rating override sizing per load value determined under 220.55 via 220.18(C). If the branch circuit rating is not specified on any label, on the packaging, or in the instructions, then yes, you can size based on calculated load.

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Dennis Alwon

Moderator
Staff member
Look at art. 210.19(A)(3)-- there are exceptions but they wont affect you. IMO unless the unit itself states the minimum circuit then you can use 10 awg.

210.19(A)(3) Household Ranges and Cooking Appliances. Branchcircuit
conductors supplying household ranges, wall mounted
ovens, counter-mounted cooking units, and other
household cooking appliances shall have an ampacity not
less than the rating of the branch circuit and not less than
the maximum load to be served. For ranges of 83?4 kW or
more rating, the minimum branch-circuit rating shall be
40 amperes.

david luchini

Moderator
Staff member
For whatever reason, if the instructions call for a minimum 40A circuit, you have no choice under the NEC but to provide a 40A breaker.
Now you need to consider the wires. What wire size do you need for a 40A breaker with a dedicated 5600W load?
Are the heating elements all line to line 240V?

The instruction manual say the unit is rate 7370w or 7400w, depending on convection or not.

Don't trust the "product overview" from the HD website.

So I'll just always go by the nameplate rating then. But am I correct in saying that I can size my circuit at 80% of the nameplate rating as long as it is under 8.75 kw?

No, 220.55 says the branch circuit load for a wall mounted oven shall be the nameplate rating of the appliance.

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Smart \$

Esteemed Member
Look at art. 210.19(A)(3)-- there are exceptions but they wont affect you. IMO unless the unit itself states the minimum circuit then you can use 10 awg.
If the instructions say a 40A circuit, you cannot use a lesser-rated circuit unless there is a minimum circuit rating* on the nameplate which is less (conflicting info allows user discretion?).

*Not just an ampere rating.

Smart \$

Esteemed Member
...
No, 220.55 says the branch circuit load for a wall mounted oven shall be the nameplate rating of the appliance.
If you are referring to Note 4, that's just specifying that you use the nameplate rating (or sum of units' nameplate ratings) as the base value for applying demand... not that you have to use the nameplate rating to size the circuit.

david luchini

Moderator
Staff member
If you are referring to Note 4, that's just specifying that you use the nameplate rating (or sum of units' nameplate ratings) as the base value for applying demand... not that you have to use the nameplate rating to size the circuit.

That doesn't make sense :?

If 220.55 says the branch circuit load shall be the nameplate rating of the appliance, and 210.19 says that the branch circuit conductors shall have an ampacity not less than the load to be served, then you have to use the nameplate rating to size the circuit.

Little Bill

Moderator
Staff member
If you are referring to Note 4, that's just specifying that you use the nameplate rating (or sum of units' nameplate ratings) as the base value for applying demand... not that you have to use the nameplate rating to size the circuit.

I disagree.

4. Branch-Circuit Load. It shall be permissible to calculate the branch-circuit load for one range in accordance with Table 220.55. The branch circuit
load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance.
for a counter-mounted cooking unit and not more than two wall-mounted ovens, all supplied from a single branch circuit and located in the same
room, shall be calculated by adding the nameplate rating of the individual appliances and treating this total as equivalent to one range.

Only a single range or combination of wall & counter top units are treated as one range according to note #4. The part in red says the nameplate is the branch circuit load. There is no demand factor for a branch circuit for this, only a feeder/service load demand factor. That's the way I read it anyway.

david luchini

Moderator
Staff member
Only a single range or combination of wall & counter top units are treated as one range according to note #4. The part in red says the nameplate is the branch circuit load. There is no demand factor for a branch circuit for this, only a feeder/service load demand factor. That's the way I read it anyway.

:thumbsup:

mwm1752

Senior Member
So I'll just always go by the nameplate rating then. But am I correct in saying that I can size my circuit at 80% of the nameplate rating as long as it is under 8.75 kw?

As long as you include all household cooking app. with in the 1.1/4 - 8.3/4 rating.I am not sure if you go with note 3 that column A must also be part of the calcs.
Talking load calc -- Are you predicting there will never be a larger range in this residence -- If you use a 40a breaker certainly you your calc will reflect it.

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