Residential Range kW Calculations

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bcorbin

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This is probably old hat for you guys, but one of my first forays into residential design.

10-story apartment building...I jumped right in.

I need to know the wattage of an electric range/oven for my load calcs.

The range has multiple separate elements, but ranges don't seem to be marketed on the total kW output.

The product literature gives the rating of each element, but they are very misleading, IMHO.

The G.E. booklet, for example, says you need a 50A cord and circuit for a 12-18kW range at 208V.

A 50A cord can only hold 10.4kW at 208V, and the 50A breaker feeding it will only hold 8.32kW continuously.

Obviously, there is some sort of thermostat-caused demand factor built into the range itself.

So which number am I supposed to be using for my NEC range demand factor calcs?

a) the sum of the elements
b) the max for the cord and breaker
c) the continuous rating of the breaker

I really like commercial construction. :lol:

Any education here would be greatly appreciated.
 
david luchini

david luchini

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Take a look at Table 220.55. If you have a range rated at 18kW, Note 1 says to increase the maximum demand in Column C buy 5% for each kW over 12kW. That would be an increase of 30% over the 8kW demand listed in Col C.

8kW * 1.30 = 10.4kW. You're 50A circuit is good for 10.4kW, so I don't see any problem. There is no need to figure the load as "continuous."
 
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bcorbin

Senior Member
The range hasn't been selected yet.

The good thing is, there will be 46 of them, so if I make a mistake, I'll make it 46 times. :roll:

The architect told me nothing more specific than "the kind they sell at Home Depot." :roll::roll:

So getting ahead of the situation, I am trying to understand the methodology so I can argue more effectively for more information. I don't want to promise them there is enough power available.

My point is, the elements on some range add to 12 kW, and yet they seem to defy physics and work just fine with a 50A cord and circuit. Even though it's obviously not drawing 12 kW, does the NEC want us to use that number anyway?
 
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bcorbin

Senior Member
david luchini said:
8kW * 1.30 = 10.4kW. You're 50A circuit is good for 10.4kW, so I don't see any problem. There is no need to figure the load as "continuous."
Click to expand...

I'm just cringing thinking of Thanksgiving, when there will be 30-40 turkeys baking for 5 hours. I'll somehow be the stupidest engineer on Earth when that happens.
 
david luchini

david luchini

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bcorbin said:
I'm just cringing thinking of Thanksgiving, when there will be 30-40 turkeys baking for 5 hours. I'll somehow be the stupidest engineer on Earth when that happens.
Click to expand...

It doesn't matter if you have a turkey in the oven for 5 hours. Firstly, I've never seen a turkey roasted at 500 or 550 degrees (or whatever the maximum setting is) and secondly there is thermostatic control in the oven, such that it will switch its elements on and off to maintain a set temperature. Therefore the load is non-continuous.
 
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suemarkp

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Kent, WA
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Retired Engineer
The range will have a nameplate somewhere. If you can find the model at Home Depot, try opening the bottom drawer. Many times you can see a nameplate inside that area where the storage drawer is. Not sure if they have the nameplate information on the manufacturers website. Typical nameplates are 10 to 13 KW at 240V. A 40A breaker is sufficient for most, as the KW shrinks when used at 208V.

Designing circuits and panel loads is hard for residential if they don't have the exact device in hand. Many things are misleading (like the 6HP compressor which really has a 3 HP motor).
 
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bcorbin

Senior Member
Hv&Lv said:
Aren't all electric ranges rated for 208-230V?
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It would seem likely.

At least I know this one is.

It's rated 11,600 kW, but I suspect that is at 240V.

I was thinking since I'm doing 208V, I could use 10,053W instead, but it looks like with ranges this big (Table 220.55), you're pretty much using a flat kW rating per range anyway.

Good learning experience.
 
Little Bill

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
bcorbin said:
It would seem likely.

At least I know this one is.

It's rated 11,600 kW, but I suspect that is at 240V.

I was thinking since I'm doing 208V, I could use 10,053W instead, but it looks like with ranges this big (Table 220.55), you're pretty much using a flat kW rating per range anyway.

Good learning experience.
Click to expand...


Are you sizing each apartment as a separate unit or the building as a whole?
 
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Hv&Lv

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-
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Engineer/Technician
bcorbin said:
It would seem likely.

At least I know this one is.

It's rated 11,600 kW, but I suspect that is at 240V.

I was thinking since I'm doing 208V, I could use 10,053W instead, but it looks like with ranges this big (Table 220.55), you're pretty much using a flat kW rating per range anyway.

Good learning experience.
Click to expand...

Wouldn't this be important information for calculations on service size? Is each apt going to have a seperate service?
 
kbsparky

kbsparky

Senior Member
Location
Delmarva, USA
Is this going to be a high-end type of apartment building? Or a cheap one?

You could go to HomeD and look at both high-end units, and cheap ones, and get the nameplate info for basic calculations.

Remember that any nameplate ratings at 240 Volts will be reduced by 25% when connected to a 208 Volt source. Example: A range rated at 12 kW @240 volts will only be a 9 kW unit on your 208 Volt supply. Use the 9 kW number when figuring your demand calculations.
 
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