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# how does derating work?

## how does derating work?

#1
11-09-18, 09:06 AM
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Join Date: Apr 2014
Posts: 134
how does derating work?

Hello, my in-laws got a cooktop (240V, 30A) and a wall oven (240V, 30A) to replace their old electric range (240V, 40A). The only 240V line to the kitchen is on a 40A breaker.

Obviously, the line is not capable of providing the max load of 60A.

The question is if I can use derating here for two kitchen appliances on one same line: (30+30)*0.65=39A, which is just below the breaker rating.

Will the 40A breaker be easily tripped with the two appliances on the same line?

Thanks and best wishes! H.

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#2
11-09-18, 10:48 AM
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Derating is reducing the current that is carried thru a wire based on airflow and ambient temperature.
With a continuous load.... you can only connect up to 80% on that circuit.
So a continuous load on a 40A breaker would mean a maximum of a 32A.

A range or oven is not considered a continuous use device but you still need two circuits there,

#3
11-09-18, 11:24 AM
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Join Date: Apr 2014
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Originally Posted by PJmax
...

A range or oven is not considered a continuous use device but you still need two circuits there,
Thank you, Pjmax! Probably, "derating" is not the right term to use here, "demand factor" is? Not clear though why 80% for two appliances. For two, it is 65% according to this table below. It is said that it is permissible for appliances up to 8.75kW to be on one line and to use demand factors.

Or is it assumed that, irrespective of the ratings, each appliance has to have its own breaker?

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#4
11-09-18, 11:31 AM
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Depending on how many burners are on for the cooktop and their settings and what the oven setting is, the 40 amp breaker may or may not trip with both appliances on at the same time.

But ......... If (and it probably is true that) the cooktop is rated for a branch circuit of no more than 30 amps. Same for the oven. Good practice, if not code in some cities, states that a hard wired appliance or tool or device that is likely to use more than half the circuit amperage rating should have the circuit to itself. So you need to downgrade the breaker on that 40 amp circuit to be 30 amps and thus you need to string another (30 amp) circuit for the other appliance.

#5
11-09-18, 11:39 AM
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Demand factor is different but a problem.... that AllanJ also mentioned.... is that an appliance has a maximum circuit protection rating. In other words..... you need to protect each appliance based on the label rating. Usually 30A each.

You'd need a sub panel at the appliances with two 30A breakers. That on a 40A circuit is not too effective. You would have to carefully balance what you were using on each appliance so as to not trip the 40A breaker.

#6
11-09-18, 11:57 AM
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Join Date: Apr 2014
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Thank you, PJmax and AllanJ! Appreciate the feedback. Yes, I haven't thought about circuit protection rating for an appliance.

The old electric range has a dedicated sub-panel with two 240V breakers, 40A each. Only one breaker is being used. Will see if a second line could be pulled from the same sub-panel.

#7
11-10-18, 07:25 PM
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Article 220 deals.with sizing the service, not the circuits to the appliances.