# Measuring 240V Current with Clamp Meter



## DemRem

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!


----------



## 480sparky

If you measure both at the same time, you'll get either zero or a really low reading .

Measure one leg.


----------



## DemRem

480sparky said:


> If you measure both at the same time, you'll get either zero or a really low reading .
> 
> Measure one leg.


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?


----------



## 480sparky

DemRem said:


> 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?



You MAY get zero.... depends how balanced the loads are across the two lines to neutral. 

Measure both legs, and any difference should be what you read on the neutral.

For instance, an electric clothes dryer uses 240v for the heating element, but the motor and controls are all 120v. So one leg of the 240 will have a higher current than the other because the motor & controls will not draw current from the 'other' leg.

Straight-up 240v loads (air conditioner compressor, water heater...) measure just one leg as there's no neutral.


----------



## iggy

read both legs individually


----------



## tedanderson

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?


----------



## DemRem

480sparky said:


> You MAY get zero.... depends how balanced the loads are across the two lines to neutral.
> 
> Measure both legs, and any difference should be what you read on the neutral.
> 
> For instance, an electric clothes dryer uses 240v for the heating element, but the motor and controls are all 120v. So one leg of the 240 will have a higher current than the other because the motor & controls will not draw current from the 'other' leg.
> 
> Straight-up 240v loads (air conditioner compressor, water heater...) measure just one leg as there's no neutral.


OK sorry I'm having trouble with this...so your saying that for regular 240V 2 wire circuit (like water heater) I measure any one leg to get current.

But for 3 wire L1 - L2 = N and I need to *add the current on both legs* to get total current because one leg could be feeding the 120V parts of the appliance and the other is just used for the 240V? Or would I just take the *higher of the two leg readings*?

I ask because I think I might need to check my appliances for drawing too much amps but not enough to trip the breaker.


----------



## DemRem

tedanderson said:


> 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?


Yes this is a perfect example of what I'm asking :thumbup:



iggy said:


> read both legs individually


But then what do those numbers mean? The example above would help???


----------



## 480sparky

DemRem said:


> OK sorry I'm having trouble with this...so your saying that for regular 240V 2 wire circuit (like water heater) I measure any one leg to get current.
> 
> But for 3 wire L1 - L2 = N and I need to *add the current on both legs* to get total current because one leg could be feeding the 120V parts of the appliance and the other is just used for the 240V? Or would I just take the *higher of the two leg readings*?
> 
> I ask because I think I might need to check my appliances for drawing too much amps but not enough to trip the breaker.



If there's no neutral, both legs will read the same so you only need to measure one of them.

If there's a neutral, you need to measure both legs... the higher of the two is your total current.


----------



## iggy

on 220 circuit the highest leg current is the total draw for the circuit.
a=26A b=19A circuit draw is 26A 
a=18A b=18A circuit draw is 18A


----------



## DemRem

480sparky said:


> If there's no neutral, both legs will read the same so you only need to measure one of them.
> 
> If there's a neutral, you need to measure both legs... the higher of the two is your total current.


:thumbsup: Thanks thats what I needed to know!


----------



## Speedy Petey

iggy said:


> on 220 circuit the highest leg current is the total draw for the circuit.
> a=26A b=19A circuit draw is 26A
> a=18A b=18A circuit draw is 18A


It's not quite that easy. Besides, amperage means nothing without the voltage. 

In your example, the second figure would be 18A @ 240V.
The first is more confusing. It's 19A @ 240V, with "a" phase drawing an additional 7A @ 120V. Or some combination thereof.


----------



## iggy

Speedy Petey said:


> It's not quite that easy. Besides, amperage means nothing without the voltage.
> 
> In your example, the second figure would be 18A @ 240V.
> The first is more confusing. It's 19A @ 240V, with "a" phase drawing an additional 7A @ 120V. Or some combination thereof.


yes it is that easy. Amperage is what we were discussing try to keep up.


----------



## txgencon

If you have enough wire to flip a loop in one leg, you can measure both at once. That's how it's metered with a single window type current transformer.


----------



## 480sparky

txgencon said:


> If you have enough wire to flip a loop in one leg, you can measure both at once. That's how it's metered with a single window type current transformer.


Which will add the two together... and that number means nothing.


----------



## txgencon

480sparky said:


> Which will add the two together... and that number means nothing.


It means something if you're trying to determine load on a transformer.


----------



## Speedy Petey

iggy said:


> yes it is that easy. Amperage is what we were discussing try to keep up.


No, it's not that easy. Try not to be a douche. :whistling


----------



## 480sparky

txgencon said:


> It means something if you're trying to determine load on a transformer.


So 50a + 50a = 100a load on the transformer?


----------



## Natedogg1990

480sparky said:


> Which will add the two together... and that number means nothing.


That number is total amperage which then is multiplied by 120v to get power in watts


----------



## Seven-Delta-FortyOne

9 year old discussion. And you're wrong anyway.


----------



## supraspl

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.


----------



## supraspl

In the case of the 240V dryer with a 120V motor, one leg is pushing harder off the neutral so you would want to measure both hots and multiply each one by 120v (or whatever your voltage is while under load)


----------



## 480sparky




----------



## supraspl

Yeah I know it old but this thread came up 1st in a google search so might as well be thorough.


----------

