Calculating current draw

[kevin509]

Wondering how to calculate current draw when you use a 240V element on 120V? Using a 240V 5500W element on 120V. I know the wattage is approximately quartered but do then use normal calculation for current draw?

1375/120=11.433333?

 

[l3asturd]

exactly right. For ACTUAL current draw use a voltmeter on your outlet and substitute 120V with actual volts.

To geek it up a bit more:

P= Vsquared / R so

5500 =240sq/ R so R= 10.47 ohms

P = 240sq/ 10.47 = 5500 Watts
P = 120sq/ 10.47 = 1375 Watts


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[passedpawn]

exactly right. For ACTUAL current draw use a voltmeter on your outlet and substitute 120V with actual volts.

But then you need ACTUAL resistance of the element. It is going to be about 10 ohms when hot (like a incandescent lamp element, the resistance might change dramatically once it gets hot). Most ohm meters do not measure low ohms very accurately.

You might think to just measure the current directly, but most voltmeters will not measure the current directly since they are fused for 10A, and you will likely exceed that and blow the fuse.

For 120V, nothing's easier than using a kill-a-watt. They measure up to 120V/15A and will tell you instantaneous power (Watts) and total energy used (kWh). Home depot and Lowes have them for ~$25 (they were on sale last year).

 

[l3asturd]

But then you need ACTUAL resistance of the element. It is going to be about 10 ohms when hot (like a incandescent lamp element, the resistance might change dramatically once it gets hot). Most ohm meters do not measure low ohms very accurately.

Thanks, didnt think I geeked up enough! I was just pointing out most people don't actually have 120VAC at their outlet. In fact I've seen anywhere from 110VAC to 128VAC here locally. My home outlets are 122VAC

 

[passedpawn]

Thanks, didnt think I geeked up enough! I was just pointing out most people don't actually have 120VAC at their outlet. In fact I've seen anywhere from 110VAC to 128VAC here locally. My home outlets are 122VAC

Hah! Can never be enough geekage. Nerds unite!