Understanding 220.55 and OCPD choice

[sw_ross]

If I have an appliance (Range) that is a double oven (electric) rated a total of 6.5kw with a gas cooktop that is not built in, I use the nameplate rating and table 220.55 to determine the branch circuit rating (i.e. 6500watts/240v=27amps x 80% = 22 amps Branch Circuit rating). I used note 4 and column B.

Then I get a little confused about choosing the ocpd. Would I protect this at 22amps (next standard size up)? My boss provided me with 8/3 for this circuit, I'm just trying to decide what size breaker I need for the panel makeup.

Thanks,
Sky

 

[hillbilly]

6.5KW @ 240V....I would use a 30A breaker.

Since your boss has already bought #8 (NM?).....and if the installation instructions don't limit the circuit amperage.....you could use a 40 Amp breaker if you want.

Just my opinion

steve

 

[barbeer]

6.5KW @ 240V....I would use a 30A breaker.

Since your boss has already bought #8 (NM?).....and if the installation instructions don't limit the circuit amperage.....you could use a 40 Amp breaker if you want.

Just my opinion

steve

What he said! ^^

 

[haskindm]

If you have a single appliance with two ovens and a separate gas cooktop, the installation does not meet the requirement of note 4 exactly. If I understand what you have correctly; you have a single appliance (consisting of two ovens) rated at 6.5kw. This would make it eligible to use column "B" for one appliance in 220.55 table. So your 80% figure is correct and the 22 amps is correct. You could wire this with #10 and either a 25 or 30-amp breaker. The breaker is sized to protect the conductor, protecting the appliance is secondary. As others have said, since the boss has supplied #8 wire (I assume copper) you could install a 40-amp breaker. Again the breaker is there to protect the conductor at its ampacity, not to protect the appliance.

 

[sw_ross]

Thanks for the responses. I guess what threw me off was that my boss provided me 8/3 when I had thought it was going to be a 30amp circuit w/ 10/3 (at least my understanding of using T220.55). I didn't have a chance to talk with him about it and wasn't sure if I was making a mistake.

I understand that the ocpd is sized to protect the conductor primarily, but got thrown off a bit here. I also understand that the code is a minimum and it's not wrong to oversize your conductors (I guess if my boss provided me with 6/3 for this range I would put a 60amp breaker on it, even though it's only pulling 27amps!).

Thanks again,
Sky

 

[suemarkp]

If you have a single appliance with two ovens and a separate gas cooktop, the installation does not meet the requirement of note 4 exactly. If I understand what you have correctly; you have a single appliance (consisting of two ovens) rated at 6.5kw. This would make it eligible to use column "B" for one appliance in 220.55 table. So your 80% figure is correct and the 22 amps is correct. You could wire this with #10 and either a 25 or 30-amp breaker. The breaker is sized to protect the conductor, protecting the appliance is secondary. As others have said, since the boss has supplied #8 wire (I assume copper) you could install a 40-amp breaker. Again the breaker is there to protect the conductor at its ampacity, not to protect the appliance.

Two things: Note 4 says "The branch circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance". Since this isn't a range, doesn't the circuit need to be rated for 100% of the nameplate value but a service load calculation can use 80% of that?

Next, there is some implicit protecting of the appliance by the branch circuit. In 422.11(E), the overcurrent device on an individual branch circuit shall:
(1) Not exceed that marked on the appliance.
(2) Not exceed 20 amperes if the overcurrent protection rating is not marked and the appliance is rated 13.3 amperes or less; or
(3) Not exceed 150 percent of the appliance rated current if the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes. Where 150 percent of the appliance rating does not correspond to a standard overcurrent device ampere rating, the next higher standard rating shall be permitted.

So a 40A circuit could be used for this oven, but I believe a 50A circuit may be too large (well, 150% of 6500VA is 40.6, so maybe you could round up to 50).

 

[sw_ross]

Ahh yes, I need to keep that 150% rule in mind. My previous comment about putting a 60a breaker on the circuit if 6/3 was provided would not work then.

Thanks