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#93943 - 06/23/05 07:03 PM electric cooktop rating
gserve Offline
Member

Registered: 03/11/03
Posts: 92
Ran into this in a double wide home. 7.7 KW rated cooktop fed with 10/2romex and a 20A DP breaker.Did calculation came up with 32.83A 7700W divided by 240V=32.8A also did it like this 7000.7W divided by 240V=29.16A. What is the correct way to do this calculation?

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#93944 - 06/24/05 03:27 AM Re: electric cooktop rating
JoeRossi Offline
Member

Registered: 01/31/03
Posts: 26
Loc: Washington, NJ
The basic formular is
Power = Current X Voltage (PIE)
Therfore ; 7700watts / 240 votls = 32.08 amps
If this is in a dwelling you can go to table 210.24 and use #8 wire. Article 240.6(A) allows a 35A circuit breaker. I would then price that out vs. a 30A circuit breaker and my time if I have to go back and replace it.
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#93945 - 06/24/05 05:40 AM Re: electric cooktop rating
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
The premanufactured housing industry has it's own requrements and they are different than the stick built requirements. Stick built construction would have us sizing this circuit per 220.19>Table 220.19> Col. B.
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#93946 - 06/24/05 07:24 AM Re: electric cooktop rating
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
In dwelling units, I believe table 220-19, Column C would apply (same answer in this case as column B posted above):
7,700W x 80% = 6160
6160 / 240V = 26 Amps <-- a little too much for #10's.

NEC Chapter 5 deals with special occupancies, article 550 deals with mobile & manufactured homes specifically. I did a quick skim thru my old NEC here at work and did not see anything that appears to change the way dwelling unit cooking appliance branch circuits are calculated. However, please do your own homework here.

Radar
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#93947 - 06/24/05 10:20 AM Re: electric cooktop rating
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
I thinking #10 wire is adaquate for the range in this problem. #10 is worth 30a. and so it could be protected with a 30a. overcurrent protection device. Radar- Why would you think otherwise?
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#93948 - 06/24/05 11:05 AM Re: electric cooktop rating
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
The 125% rule for branch circuits feeding loads that are considered to be continuous (one that can be on for 3 hours or more). NEC 210-22(c) & 220-3(a), probably other sections as well.

26 amps calculated above x 125% = 33 amps of required branch circuit ampacity.

One way to try to argue your point is to point out that #10 THHN has an actual ampacity of 40 amps, but must not have overcurrent protection of greater than 30 amps. So 80% of 40 amps is 32, which is adequate for a 26 amp load. 80% is just the 125% rule applied backwards. Nevertheless, I believe applying the 125% rule to the 26 amps required will put you into a 40 amp circuit with #8's.

Radar
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#93949 - 06/24/05 11:44 AM Re: electric cooktop rating
gserve Offline
Member

Registered: 03/11/03
Posts: 92
That is the question I am asking what is the min branch circuit size 30 or 40 amp? The manufacturer ran #10/2 romex and I can easily change the 20A to a 30A if only a 30A is required.

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#93950 - 06/24/05 12:20 PM Re: electric cooktop rating
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
There is nothing that I can fnd that would have us adding in 25% for a range as dscribed n a residential settng. The numbers in Table 220.19 Column B apply to this 7.7 KW range and continous load does not apply. AWG #10 is the proper wire and it can be protected by a 30a. overcurrent protective device. Might check the examples in the Annex and see that there is no additional 25% needed.

[This message has been edited by George Little (edited 06-24-2005).]
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#93951 - 06/24/05 01:16 PM Re: electric cooktop rating
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
I agree George. As a sometime cook I can not think of a scenario where you would have all the burners on high for 3 hours. (although it could be argued they are all on "high" for a short period whenever you have them on since the control is basically, long term, pulse width, modulation)
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#93952 - 06/24/05 01:56 PM Re: electric cooktop rating
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
The simple answer to Gserve's question is that since 10-2 wiring is installed by the unit's manufacturer, NEC 310-16 states the maximum overcurrent protection for #10 wiring is 30 amps. So, presuming the NEC applies in a mobile home, a 30 amp CB is the max size permissable to protect the wiring, a 20 amp would be OK (no violation) but may produce neusance tripping during operation of the cooktop. Installing a 40 amp CB would be a violation.

As far as the cooktop being a continuous load goes, I believe it would be considered continuous because it could potentially be on for a period of 3 hours or more. No one would really do that, of course, but there is nothing inherant to prevent it, like a timeout interlock or some such device. But regardless, all this is beside the point of the question asked, which was what size CB to use.

Sorry to have gotten off track,
Radar
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