Electric cooktop calculation

[Mustwin351]

Just wanted to get second opinions on wire and overcurrent protection sizes on this unit.

It has a nameplate rating of 30.8 amps @ 240V.

I was thinking 240.4(B) that allows the next standard overcurrent size to be use would apply and I could 10 awg copper thhn and a 30 amp breaker but 240.4(D) "Small Conductors" limits the overcurrent protection size for 10 awg to 30 amps so it would appear I would need 8 awg.

Interesting note artlcle 210.19(A)(3) states that ranges 8.75 kW or more require a minimum branch circuit ratiting of 40 amps... so does that mean my cooktop I mentioned at the top which is rated at 7.4 kW can in fact be on a 30 amp circuit with number 10 copper?

 

[kwired]

Just wanted to get second opinions on wire and overcurrent protection sizes on this unit.

It has a nameplate rating of 30.8 amps @ 240V.

I was thinking 240.4(B) that allows the next standard overcurrent size to be use would apply and I could 10 awg copper thhn and a 30 amp breaker but 240.4(D) "Small Conductors" limits the overcurrent protection size for 10 awg to 30 amps so it would appear I would need 8 awg.

Interesting note artlcle 210.19(A)(3) states that ranges 8.75 kW or more require a minimum branch circuit ratiting of 40 amps... so does that mean my cooktop I mentioned at the top which is rated at 7.4 kW can in fact be on a 30 amp circuit with number 10 copper?

You need at least a 30.8 amp conductor. 10 AWG is only good for 30 (after factoring in 240.4(D). Next size up rule would apply to overcurrent protection which could be 35 in this case but still going to need 8 AWG conductor regardless.

7.4 kW is less then 8.75 kW so I don't know where you are going with that line of thinking. 7.4kW @ 240 volts is 30.8 amps just like you said your nameplate says. If the unit were 7.2 kW or less then you are at or below 30 amps and a 30 amp conductor and 30 amp overcurrent device is acceptable.

 

[Mustwin351]

7.4 kW is less then 8.75 kW so I don't know where you are going with that line of thinking. 7.4kW @ 240 volts is 30.8 amps just like you said your nameplate says. If the unit were 7.2 kW or less then you are at or below 30 amps and a 30 amp conductor and 30 amp overcurrent device is acceptable.[/QUOTE]


Just thought it strange that they chose 8.75 kW for their example and not the lowest kW rating that could utilize a 40 amp overcurrent device.

Also of interest is this particular unit in the link to follow which is rated at 30.8 amps but the manufacturer states that it requires a minimum 30 amp circuit.
http://www.frigidaire.com/Kitchen-Appliances/Cooktops/Gas-Cooktop/FFEC3024LB/

 

[Little Bill]

I think you will be fine with a 30A circuit since the spec sheet calls for a minimum of that. Having said that, if you haven't run the wire yet, I would probably run a #8 just in case they go larger now or down the road.

 

[JFletcher]

30.8A would likely be all 4 burners on high, on startup, on a 240V circuit. I fail to see how the mfg can spec a min 30A circuit for 240V use. I'd pull 8/3 if it's going in a house, 10/3 if the service is 208V.

 

[infinity]

At 7.4kW wouldn't T220.55 allow a demand factor of 80%?

 

[Smart $]

At 7.4kW wouldn't T220.55 allow a demand factor of 80%?

No. Table 220.55 is for service and feeder load calculation. But see 220.18 and also Table 220.55 Note 4. What it amounts to, you can for a range but not a cooktop.

 

[Mustwin351]

Currently there is a bad cooktop in an existing house that needs replacing that is on a 30A breaker...I have not checked the wire size out yet...I will likely find out more info today and will update. it has been difficult to find units that dont require a 40 amp breaker.

I thought it was odd that the manufacturer from that link I posted earlier specified a 30 amp circuit when it draws 30.8.

 

[topgone]

Currently there is a bad cooktop in an existing house that needs replacing that is on a 30A breaker...I have not checked the wire size out yet...I will likely find out more info today and will update. it has been difficult to find units that dont require a 40 amp breaker.

I thought it was odd that the manufacturer from that link I posted earlier specified a 30 amp circuit when it draws 30.8.

I think it all comes down to the demand factor of 80% applied for ranges 8 3/4 kW and below. At 7.4 kW, your demand amps will be 0.8 x 7.4 /240 = 24.67A. A 30 A circuit breaker would suffice.

 

[Smart $]

I think it all comes down to the demand factor of 80% applied for ranges 8 3/4 kW and below. At 7.4 kW, your demand amps will be 0.8 x 7.4 /240 = 24.67A. A 30 A circuit breaker would suffice.

While that applies to a range, we're talking about a cooktop... and as I mentioned earlier you cannot apply T220.55 demand to a cooktop (except under Note 4 conditions).