Ask Me Help Desk - Reduced Neutral for Subpanel Feeder

Hello,

I am adding a sub-panel for a new cooktop and double wall oven. They are rated 9KW and 7.5KW.

I already have 1" EMT run from my main panel to a good location under the kitchen in the basement (about 40').

I have some wire and a load center on hand that I would like to use if I can, instead of buying more at today's copper prices. I have about 100' of #4 THHN, which is enough for my 2 hots. I also have some #6 THHN and a Siemens 125A, main lug, 4-position load center (E0408ML1125SU).

I'm thinking that per 220.61(B)(1), I can use the #6 wire for my neutral since it will only be serving cooking appliances. Am I correct in applying this code section?

I would be using an 80A breaker for the feeder and a 40A for each appliance in the sub.

Assuming this is a standard 120/240 volt single phase service, then a reduced neutral can be used, as stated, rated 70% of the load of the ungrounded conductors.

But this is not necessary for you.

According to the demand table 220.19, and Note #4, your combination of cooktop and double oven is allowed by special exception, to be begin calculations at totalling the nameplate rating , and be considered as one range.

Special calculations are allowed for your situation, I will be brief with the answer, but not explain the calcs, as they are shown in a NEC handbook, and they are a bit too much to explain here.

9KW and 7.5KW=16500 total watts,

9.8 KW is the Max Demand allowed to size a single branch circuit for this combination of household appliances.

9800 watts /240 volts = 40.8 amps
Branch circuit to be rated min of 125% of 40.8 amps =51 amps

#6 THHN at max allowed center column of 75 Deg C is 65 amps, 90DC column not allowed.

All the conductors can be #6, on a 70 Amp circuit breaker, with a min of a #8 equipment ground.

See, the code can be your friend. Spread the word!

Are you planning to split into two circuits one for each unit just below the appliances? Or is the sub near the main? If more load can be added to the panel, the feeder will need to be larger to feed the kitchen equipment and the added load.

Thanks for your response tkrussell!

Quote:

Originally Posted by tkrussell

Assuming this is a standard 120/240 volt single phase service, then a reduced neutral can be used, as stated, rated 70% of the load of the ungrounded conductors.

Yes it is a standard 120/240 volt 200-amp service.

Quote:

Originally Posted by tkrussell

But this is not necessary for you.

According to the demand table 220.19, and Note #4, your combination of cooktop and double oven is allowed by special exception, to be begin calculations at totalling the nameplate rating , and be considered as one range.

Special calculations are allowed for your situation, I will be brief with the answer, but not explain the calcs, as they are shown in a NEC handbook, and they are a bit too much to explain here.

9KW and 7.5KW=16500 total watts,

9.8 KW is the Max Demand allowed to size a single branch circuit for this combination of household appliances.

9800 watts /240 volts = 40.8 amps
Branch circuit to be rated min of 125% of 40.8 amps =51 amps

#6 THHN at max allowed center column of 75 Deg C is 65 amps, 90DC column not allowed.

All the conductors can be #6, on a 70 Amp circuit breaker, with a min of a #8 equipment ground.

I hadn't considered using only one branch circuit.
I did look at the Demand Factor tables. It seems to me that if this was only part of the load calculations for a main panel or larger sub panel then it would be more applicable. My sub is going to only supply these two appliances. It is only a four-slot box so will be full with the two 2-pole breakers. The reason I don't think that I should use the Demand Factors allowed in Table 220.55 is that I am certain that when my wife is cooking for the holidays, that there will be a time when she has both ovens and all 5 burners of the cooktop going at one time:eek: I have seen her do this with our current range, and the reason she wants the double oven is so she can do something like a roast a turkey and bake cookies at the same time. So there is the possibility that the two appliances could be drawing a total of over 68A at one time.

Quote:

Originally Posted by tkrussell

Are you planning to split into two circuits one for each unit just below the appliances? Or is the sub near the main? If more load can be added to the panel, the feeder will need to be larger to feed the kitchen equipment and the added load.

My sub panel would be located in the basement, just below the kitchen. I don't need the sub for anything other than the cooking appliances, which is why I planned on using the 4-slot panel.

Quote:

Originally Posted by tkrussell

See, the code can be your friend. Spread the word!

LOL