Why do my appliances show two sets of power consumption ratings and which one applies to my house?

Question

Aptos, CA, USA

I'm trying to determine how much power my electric oven is using (as part of a larger project to possibly upgrade our electrical service (with help from a licensed electrician of course).

When I check the sticker on the side, it has the following info:

VOLT           KW             HERTZ          
----           ---            ------
120/208        2.60           60
120/240        3.40           

The circuit going to it is dedicated from the breaker box and the breaker itself is rated for 60 amps (two 30 amps connected together).

How do I know which line item applies to me?

Answer 1

Assuming you have the split-phase power supply that is normal for residential settings in the United States, you should use the line item for 120/240V.

The 120/208V line is for 3-phase supplies, which you typically find in commercial or industrial settings (each leg of a 3-phase supply is 120V relative to the neutral, and is at a 120° phase difference to each of the other legs, so the voltage between them is 120V * sqrt(3) = 208V).

Answer 2

If the wires going to the main panel connect to dedicated breakers, a 2-pole ( a single 2-pole breaker), two single-pole ganged (1-wire to each breaker), then the voltage is 240. Please measure, using a voltmeter,between the 2 wires.

I will add that the 3.4 kW at 240 VAC, given the 120/240 choice, means that the current draw will be 3.4 kW = 240 VAC x XX Amps,,, or XX Amps = 14.17 A. This assumes the power factor = 1 (electric resistance loads, instead of motors, which for an oven is true).

Power = VI (x power factor which = 1 for electric resistance heaters), or votage multiplied by current = 240 x 14.17 = 3.4 kW, the name plate rating. That means that when the oven is fully loaded, all burners on, oven on, etc, this is what the manufacturer claims the MAXIMUM power draw will be. It will usually be less than this because all burners will not be on at the same time and the oven may not be used with the top burners.

The main panel breaker will reflect the maximum power draw for this appliance. The diversity (all breaker loads will never be active at the same time) of all the applicances in the home means that the main panel does not need to add up to the sum of the breakers installed.