Wall oven breaker size

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tonype

Senior Member
Location
New Jersey
I am having a disagreement with someone on sizing of wall over OPD (no spec sheet available). Unit has 18.7 A rating at 208-V (3.9 kw). Other person says 20-amp breaker is OK. My position is that it is too low - should be 25 or 30. What is proper method to calculate this?
 

iwire

Moderator
Staff member
Location
Massachusetts
12 AWG and a 20 amp breaker is fine. An oven is not a continuous load and furthermore Table 220.55 allows it to be calculated at just 80% of its KW.
 

hillbilly1

Senior Member
Location
North Georgia mountains
Occupation
Owner/electrical contractor
Don't forget, you must follow the manufacturers instructions.
No spec sheet - internet search or a phone call to the manufacturer should get you your answer.
Gentlemans bet, minimum it calls for is #10 on a 30amp OCP

I would'nt bet on it, I had one where the calculation came out to a 50 amp breaker, but the manufacture called for a 40 amp.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I am having a disagreement with someone on sizing of wall over OPD (no spec sheet available). Unit has 18.7 A rating at 208-V (3.9 kw). Other person says 20-amp breaker is OK. My position is that it is too low - should be 25 or 30. What is proper method to calculate this?

Look at table 220.55. This is being used on a 208 volt system?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Lets go to something I have trouble getting my brain adjusted with.
Let's make it a 5 kw 208 volt unit. Looking at it directly, 5 kw @ 208 would be 24 amps. I understand it's not continuous so
at this point a 25 amp breaker with a #10 NM would suffice.
BUT
As Bob pointed out, Table 220.55 would allow us to use a 0.8 multiplier and now it's 4 kw at 208 and a 20 amp breaker with a #12 NM would suffice.
BUT
210.19(A)(3) states that branch circuit conductors shall have an ampacity not less than the maximum load.

Seems contradictory....
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
I think that is because I brought service / feeder calculations to a branch circuit question. :ashamed1:

Agreed.... and 220.55 is in Section III which is "feeder and service load" but Art 220 is "Branch-Circuit, Feeder and Service calculations"

I am under the assumption, since Part II does not have "branch circuits" in it's heading, Table 220.55 should not be used for branch circuits.

...... but then you did so and you are sharp :D
 

jumper

Senior Member
Okay I think I found the deal killer for a single oven.

T220.55 Note 4. Branch-Circuit Load. It shall be permissible to calculate the branch-circuit load for one range in accordance with Table 220.55. The branch circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating of the appliance.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Well it appears to me that T. 220.55 may just be in the wrong section but IMO it applies. Many of us have argued for years that the Table is only for calculations(demand loads) for feeders and services and not to be used for branch circuits. The status quo suggests otherwise so I will go with the status quo even tho I am a tsimrofnocnon.
 
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