Re: Amps vs watts.

Posted by MarkB_MI on September 21, 2022 at 23:24:57 from (174.240.147.69):

In Reply to: Amps vs watts. posted by DeltaRed on September 21, 2022 at 19:08:24:

For purely resistive loads, you divide the watts by the voltage. HOWEVER, you need to understand that the rated power (watts) ASSUMES a nominal voltage, typically 120 volts. If your voltage is higher or lower, the actual current will be higher or lower, respectively, than this calculated value.

Here's an example: A 1200 watt resistive device, rated at 120V, will draw 10 amps at its rated voltage. But what if your house voltage is 125 volts? The actual current draw will be 10.4 amps. (Ohm's law tells us the load resistance is 120 volts ÷ 10 amps = 12 ohms; at 125 volts the current is 125 ÷ 12 ohms = 10.4 amps.) Probably close enough for your purposes.

BUT, when you start dealing with inductive loads, particularly motors, it's a whole 'nother ball game. First, although motors are often rated by power (either in horsepower or watts), they draw VOLT-AMPS, aka apparent power. Without going too deep, just understand that a AC motor's volt-amps will be significantly higher than its power. For example, a 1200 watt motor might actually draw around 1700 volt-amps.

The other thing about motors is they tend to draw however many amps they need. This means that as their supplied voltage drops, they tend to draw MORE AMPS, unlike resistive loads which behave in the opposite way. That's why a motor that works just fine plugged into the wall will blow the circuit breaker when you put it on an extension cord.

I'm going to assume the appliances you're dealing with are purely resistive, but if they're not tell us what you have and we'll go from there.

Replies:

• Re: Amps vs watts. miangus 08:11:14 09/22/22 (0)

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