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#93943 06/23/05 10:03 PM
Joined: Mar 2003
Posts: 92
G
gserve Offline OP
Member
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?

2017 / 2014 NEC & Related Books and Study Guides
#93944 06/24/05 06:27 AM
Joined: Jan 2003
Posts: 26
J
Member
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.


Joe Rossi
#93945 06/24/05 08:40 AM
Joined: Jan 2004
Posts: 1,507
G
Member
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.


George Little
#93946 06/24/05 10:24 AM
Joined: Apr 2004
Posts: 349
Member
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


There are 10 types of people. Those who know binary, and those who don't.
#93947 06/24/05 01:20 PM
Joined: Jan 2004
Posts: 1,507
G
Member
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?


George Little
#93948 06/24/05 02:05 PM
Joined: Apr 2004
Posts: 349
Member
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


There are 10 types of people. Those who know binary, and those who don't.
#93949 06/24/05 02:44 PM
Joined: Mar 2003
Posts: 92
G
gserve Offline OP
Member
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.

#93950 06/24/05 03:20 PM
Joined: Jan 2004
Posts: 1,507
G
Member
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).]


George Little
#93951 06/24/05 04:16 PM
Joined: Jul 2004
Posts: 9,653
Likes: 2
G
Member
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)


Greg Fretwell
#93952 06/24/05 04:56 PM
Joined: Apr 2004
Posts: 349
Member
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


There are 10 types of people. Those who know binary, and those who don't.
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