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Old 07-19-2009, 08:20 PM   #16
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Originally Posted by darren View Post
Okay what are you talking about, i have never heard of this before.
That's 'cause it's original, sort of.
It's a more refined version of putting a 100w incand. lamp across an open breaker to check new work for shorts. The problem with incand. lamps is that the filament resistance depends on the current through it.
I carry a 100W, 100Ω rheostat that I got at a Hamfest to do these kinds of checks.

Series circuit consisting of 120v source, hair dryer, unknown load.

120/10A = 12Ω hair dryer ohms

120v/15A = 8Ω. load resistance for 15A
120 [8/(8+12)] = 48v across the load.
120-48 = 72v across the dryer. More than 72v is more than a 15A load.

Similarly for a 120v/20A = 6 Ω load (80v across the dryer) and for a 120/200 = 0.6 Ω load (114v across the dryer).

If the hair dryer fan actually runs at these low voltages you can leave it in the circuit for more than one or two seconds.

Last edited by Yoyizit; 07-19-2009 at 09:01 PM.
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Old 07-19-2009, 09:15 PM   #17
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Wow way to complicated for me, i will stick with turning the breaker on and seeing if it pops.
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Old 07-19-2009, 10:09 PM   #18
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Wow way to complicated for me, i will stick with turning the breaker on and seeing if it pops.
That's my current method too lol. Though normally I'm 99% confident everything is ok. If I'm doing something more complicated I will find myself going back and forth often during the work to turn it on/off while testing wires that are not yet connected. I also often put two wires together (with no power) then test voltage at another end to see if it reads close to 0 then test for closed circuit using the feature on my multimeter, then I know where those two wires go. I found myself tracing stuff this way for a few hours when I found some weird setup in a junction box. Turns out the circuit that powers the lights in my crawlspace also powers the fridge.
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