How To Measure Amperage Draw


Old 02-24-06, 06:55 PM
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How To Measure Amperage Draw

I have two flash.instant water heaters. They each take a 60AMP breaker to run and they are wired directly from the main panel (no subpanel).

Each unit uses 3 wires, black, white and ground. Obviously, using 240V here (residential single phase).

What I want to know is how to measure the amperage draw when each unit is on. I have a digital multi-meter and, from the markings and settings on the front of it, I am pretty sure it is capable of measuring amps.

I went on the internet and saw someone describe how to measure the draw of a headlight on a car, was pretty straight forward but didn't seem to work out no matter what wires I tested or what setting I used on the DVM. Maybe it had to do with me trying to test alternating current and the information I read was direct current...just don't know.

Can someone clear this up for me, too many unknowns for me to figure it out on my own.


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Old 02-24-06, 07:02 PM
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You CANNOT measure the amperage of those units with a multimeter. The most a multimeter can measure is 10-20 amps.
You would need a clamp-on amp meter to check this.

Why are you needing to measure the amperage. Is it to troubleshoot the units or to see how much they draw?

A clamp meter is a great trouble shooting tool. It is worthless at gauging usage or electrical cost.

The name plate rating will tell you pretty accurately what they are drawing when they are one. Heating elements are either on or off. They do not change with load like a motor will.
Old 02-24-06, 07:29 PM
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Thanks for the answer, that is kind of what I thought.

These units have an adjustment knob on them so I can adjust the heat of the water coming out. This affects the amperage drawn by the units. I also want to compare the draw of one to the other.

I have an infrared thermometer and when I put it near the copper pipe to get the water temp going in and then coming out, I don't get an accurate reading. The amp draw will tell me if they are working at full capacity or not.

Old 02-25-06, 07:28 AM
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If you just want a ballpark comparison without knowing the actual amperage, go clock the Kwh meter on the house. However much faster the disk is turning on one is the proportional amount more energy it is drawing than the other.
Of course, the trick here is to make sure the only thing running at the time are the heaters. If your AC or 'fridge cut on at the wrong time, it will skew the results.

If you want to use this method to actually calculate the amperage, look at the Kwh meter and find the constant. It will be labeled as "Kh" and is usually a value something like 7.2 (or 6, 12, etc. depending on how old the meter is). Clock the disk revolutions with a stopwatch (5 or 10 revolutions is usually sufficient) while the load you want to monitor (and ONLY that load) is running.
Use this formula;

(3600 X #rev. X Kh) divided by (time in seconds) = watts.

For example, say the meter Kh that you got from the nameplate is 7.2, and you've clocked 10 revolutions of the disk with your stopwatch and it was 43 seconds.

(3600 X 10 revolutions X 7.2) divided by (43 seconds) = 6028 watts.

Since the water heaters are a resistive load, you can divide the watts by the voltage (240 volts) to get amps (in this case 25.1 amps).

You can also use this method to calculate how much they will run up your bill.
First, convert the "watts" result to Kw by dividing by in this example it's now 6.028 Kw.
Calculate how often the device runs (say 10 minutes out of every hour)
So in a day, it will have run a total of 4 hours, and in a month around 120 hours.

120 hours times 6.028 watts = 723.36 Kwh

If you're paying 12 cents per Kwh........$.12 X 723.36) =$86.80 per month

Hope this helps!
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