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?
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.
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.
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
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?
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
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.
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).]
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)
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
Even if you have all burners on for over 3 hrs it still would not be a continuous load. The elements in electric cooking appliance turn on an off even if they are on high. That is the reason for the demand factors in table 220.55. An appliance rated at 12kw (50 amps @ 240 volts) can be connected to a 40 amp circuit.
Curt
Just to clarify I am talking about the branch circuit feeding the cooktop not the wiring within the cooktop.Just want to know what the min feeder wire size and OCP is for 7.7KW and whether the demand factor of 80% can be applied to use the already installed 10/2 romex and use 30A breaker.
Gserve, A 30 amp branch circuit would be fine for this installation. I would make sure that the #10 wire installed by the factory is continuous and there wasn’t a good reason a 20 breaker was installed.
334.80 NM cable ampacity refer to 310.15 at 60degree C. You start with romex derating at 30 amps. 422.10A allows the use of table220.19. That table allows derating of load serve. GREAT. But, fpn #4 to table. The branch circuit load shall be nameplate rating.
If the ampacity at 100% is 32.8,then the 10 wire with no room for derating is not big enough. Unfortunatly you need to either disconnect a burner or rerun #8 nm on 35 cb.
Also, Has anyone noticed that Trane brand AC condensors for homes are coming at maximum fuse 25,35,or 45. Whe make life more difficult?
Where does my liability stop? I would not have known this had it not been hooked up to a 20A breaker by mistake. There is also a single built in oven where the load is only 3.3KW and 20a breaker would be enough and that 20A breaker was for that oven ,not the cooktop.Part of my job when running feeders to the manufactured home is to test things out and thats how I found out because cooktop was hooked up to 20A breaker by mistake and it tripped after a few minuets.
I'm suprised that the cable is a two-conductor NM cable. The requirement for neutral-grounded (i.e., 3-pronged-plug) appliances has always (well, not always) been to use either service cable or an insulated-neutral cable, especially with mobile homes.
As Curt noted above, try to determine that the wiring is 10-2 all they way to the panel, meaning there isn't some 12-2 spliced in there somewhere, if you can. Once you determine the run is 10-2 (or greater), you can install the 30 amp CB. Remember, the CB's job is to protect the wiring, not the appliance. A 30 amp CB will provide proper overcurrent protection for #10 wiring.
As to the Larry's post, we're all assuming the cable is 12-2G, meaning there is an uninsulated ground wire in there also.
Radar
I have varified that 10/2 romex was run from the panel to a crossover point (junction box)and then from that JB to a JB under the cabinet where the cooktop is.The 20A breaker was for the built in oven(3.3KW)They had them wired to the wrong breaker in the panel.ie:30A to oven and 20A to cooktop when the oven was supposed to be wired to the 20A and cooktop to 30A.
I meant that the cooking appliances should have been fed with either a 12-3 or 10-3 plus-ground cable, or an SE cable, even before the separate-neutral-and-ground requirement.
If they need a neutral they need a white insulated wire. It could also be the grounding conductor before 96 but it was still called a neutral and was supposed to be insulated. The exception allowed SE but I still saw inspectors say that had to be SER as long ago as 71. The normal installation I saw was the white went to the 3d prong of the plug and the ground landed on the backstrap of a bakelite receptrace or the box.
According to the instructions that came with the cooktop only straight 240V is needed, no neutral needed.