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Measuring 240V Current with Clamp Meter

89K views 63 replies 21 participants last post by  Tinstaafl 
#1 ·
I have a clamp meter which I've used on 120V but how would I measure current at the panel on a 240V circuit. Is the current whats on one leg or do I measure both legs and add together. I did a lot of searching and really couldn't find a clear answer...

Thanks!
 
#3 ·
You get 0 cause the sine waves cancel each other out since they're on different phases??? (I might have just butchered that idk) :laughing:

So I measure one leg and that reading is the true current reading no adding in the measurement for the other leg etc?
 
#6 ·
So to expound on the OP's question, lets say that I use the clamp meter to read current on each leg of my main electrical panel individually-

If phase A reads 26 amps while phase B reads 19 amps, how much current am I using? Am I using 26 amps (the highest phase), 45 amps (both phases added up), or 35 amps (the average of both phases), or is there a different amount that's calculated?
 
#21 · (Edited)
To size a wire or breaker you can use the amperage of the higher leg. But for power draw you cannot.

When you clamp one leg, you're seeing (say 10 amps) on that 120V leg. So that's 120V * 10 = 1200 watts (for a hot water heater, stove or oven.) Double that to account for the other leg and you get 2400 watts. That the reasoning of why you can get away with only clamping one leg and say 240V * 10 amps = 2400 watts.
 
#27 ·
So a 120v heat element uses 1 leg 120v & other goes to neutral.
A 240v element uses 2 120v legs for a total of 240v. The legs are 180deg out of phase from each other.

The controls, & small motors, typically are 120v, either by a 240 to 120v transformer, or 1 120v leg to neutral.
In the case of a 240v transformer, the amp draw should be equal on both legs. The other will be unbalanced, but not by a lot.
 
#37 ·
480, you need to think what is going on with the MAGNETIC part of the transformer action, and how the energy is pushed back and forth between Magnetic and Electrical energy, Simple Harmonic 'Motion'.

E max, M minimum....The ends of the winding are 180 degrees out of Phase ALWAYS on one phase transformer, thus the Voltage is maxxed out, the electrons travel twice as far(double Voltage as half leg voltage)

Finally, count the winding count across the 240 tap vs. each leg in ratio to the High Voltage side of the transformer
Say 7200Volt supply to 120, = 60/1 7200/240 = 30/1 or 60 windings highside vs 2 on low with 3 leads, one tap in center for neutral, the other two at each end.

The transformer is a electro-magnetic gearbox or coupling ....
Nicolai Tesla was one smart cookie.....
 
#40 · (Edited)
We can play word games till the cows come home. Call it what you want. As said, take a scope, connect ground to ground and 2 scope probes to the two legs (line 1 and line 2) in your panel. Here is what you'll see:

Rectangle Font Parallel Slope Handwriting


If you don't see how the 2 line voltages are 180 degrees out of phase, well, that's a different discussion.

Edit: A multiwire branch circuit is only possible because the 2 lines are out of phase. Think about it. If they were in phase, the current in the neutral would be the sum, not the difference.
 
#41 ·
Don't try to teach electricity to an electrician. Per your illustration, the voltage measured between L1 and L2 would be zero.

Pay attention to the reference points and their significance. And yes, semantic vagaries can confound the daylights out of any otherwise rational discussion unless you take the time and effort to drill down to the actual science behind the math. So to speak.
 
#43 ·
It's the same phase.

115/120 vac is derived from the center tap of the transformer.

On a scope the waveforms would each be in phase with each other, but would have different amplitude.

The phase comes into play when you have 3-phase which is coming from the generator. If you look at the three phases on a scope the waveforms would be identical but they would have different starting points. Approximately 0.333 second apart from each other.

Their relationship to each other is at the transformer the delta , primary side, and the secondary side Y has the center tap.

Delta / Wye transformer.

I can't explain it anymore than this without cracking a book. It's been over forty years since I truly studied it
 
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