Range calculation

david luchini

david luchini

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Staff member
Location
Connecticut
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Engineer
wwhitney said:
What's confusing is that the OP says "Nameplate is 208v, single phase, 91amps, 17.3kw." And 17,300/208 = 83A. Which is not 91A.

Given the discrepancy, obviously best to go with the higher number. Possible reasons for the discrepancy that occur to me are non-unit power factor (but on an electric range?), or perhaps it's European and "17.3 kW" is referring to the heat output, not the electrical input.

Cheers, Wayne
Click to expand...
Not very confusing. Convection ovens have fans.
 
G

Greg1707

Senior Member
Location
Alexandria, VA
Occupation
Business owner Electrical contractor
retnoc said:
Hello everyone,

Long time follower, first time posting.
I've been in the field for over 20 years, but mostly industrial/commercial. I'm getting a lot of contradictory replies from license holders regarding the proper breaker for an electric range, and can use some assistance.

Nameplate is 208v, single phase, 91amps, 17.3kw

Using table 220.55, column C I'm seeing a demand factor of 10,260kw.

When I calculate that by 208v, I'm getting 48amps.

Would I be safe to use a 60amp breaker?

The replies I've gotten have ranged from
60-->80->125amps.

Thanks in advance.
Click to expand...
220.55 deals with sizing feeders and services? The OP is sizing a branch circuit?
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
david luchini said:
Not very confusing. Convection ovens have fans.
Click to expand...
What's the power factor of a fan, typically?

If 91A vs 83A is due to power factor, then the "out of phase" amps (not sure what the usual terminology is) would be sqrt(912 - 832) = 37A. So if the power factor of the fans is 0.5, that would mean the fans account for 37A/sqrt(3) = 21A of the real power usage of 83A. Is the real power usage of a convection oven really 25% fans?

Cheers, Wayne
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
david luchini said:
The 91A vs 83A is due to having a fan vs not having a fan.
Click to expand...
You're suggesting that 91A is "MCA" including the fan, and 17.3 kW is just the rating of the heater? That would suggest a 1.5 HP motor (208V FLC is 6.6A) for the fan in a 17.3 kW heating convection oven. Not clear if a fan of that size is plausible, but that's the best explanation so far. Thanks.

Would have been nice if the OP included a picture of the name plate.

Cheers, Wayne
 
T

TX+ MASTER#4544

Member
Location
Texas
Occupation
electrical Code instructor and mentor
to all interested posters,
Re: the 17.3 kW commercial cooking oven

Yes, Ohm's law reveals 83 amps for the oven.
T.310.16 and in the 75 degrees celcius column, you will find listed 85 amps for a THHN 4 AWG size copper conductor.
Because the oven is not a continuous load then there's no need for any additional 125 % calculation.

Size the conductors OCPD using Section 240.6(A) Standard Ampere Ratings. It refers to use T.240.6(A) "Standard Ampere Ratings.....".
An 83 amp load requires a 4 AWG copper conductor at 85 amps.

Selecting an OCPD from T.240.6(A) you will find that there's no standard 85 amp device listed.

So, we have a choice of a standard 80 amp or 90 amp OCPD. The 4 AWG conductor rates at 85 amps. I chose the 90 amp OCPD.

Section 240.4(B) Overcurrent Devices Rated 800 Amperes or Less. "The next higher standard OCPD rating(above the ampacity of the conductors being protected) shall be permitted to be used provided all........."

Following that table you will find (1) (2) (3) conditions that allow you to round up. We met all three of them.
I chose the 90 amp OCPD. Remember ,the conductors are rated 85 amps.

Now, back to the T.220.56 Demand Factors....... We only had one kitchen equipment so there's no increase or decrease it's at 100%.
All 17.3 3kW load and no other additional calculation.

Someone mentioned a 91 amp load for something? And a violation of section 220.18, there's none such.

Final note: I selected a 4 AWG THHN conductor from the 90 degree column at 95 amps but the equipment terminals are rated for a 75 degree conductor, hence the 85 amp rating that we used.

Thanks for reading,
Comments accepted
TX+MASTER#4544
 
david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
TX+ MASTER#4544 said:
to all interested posters,
Re: the 17.3 kW commercial cooking oven

Yes, Ohm's law reveals 83 amps for the oven.
T.310.16 and in the 75 degrees celcius column, you will find listed 85 amps for a THHN 4 AWG size copper conductor.
Because the oven is not a continuous load then there's no need for any additional 125 % calculation.

Size the conductors OCPD using Section 240.6(A) Standard Ampere Ratings. It refers to use T.240.6(A) "Standard Ampere Ratings.....".
An 83 amp load requires a 4 AWG copper conductor at 85 amps.

Selecting an OCPD from T.240.6(A) you will find that there's no standard 85 amp device listed.

So, we have a choice of a standard 80 amp or 90 amp OCPD. The 4 AWG conductor rates at 85 amps. I chose the 90 amp OCPD.

Section 240.4(B) Overcurrent Devices Rated 800 Amperes or Less. "The next higher standard OCPD rating(above the ampacity of the conductors being protected) shall be permitted to be used provided all........."

Following that table you will find (1) (2) (3) conditions that allow you to round up. We met all three of them.
I chose the 90 amp OCPD. Remember ,the conductors are rated 85 amps.

Now, back to the T.220.56 Demand Factors....... We only had one kitchen equipment so there's no increase or decrease it's at 100%.
All 17.3 3kW load and no other additional calculation.

Someone mentioned a 91 amp load for something? And a violation of section 220.18, there's none such.

Final note: I selected a 4 AWG THHN conductor from the 90 degree column at 95 amps but the equipment terminals are rated for a 75 degree conductor, hence the 85 amp rating that we used.

Thanks for reading,
Comments accepted
TX+MASTER#4544
Click to expand...
The oven has a load of 91A. A #4awg conductor and a 90A ocpd is a violation of the Code.
 
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