How to use clamp meters to measure current?

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Old 02-22-18, 07:57 AM
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Question How to use clamp meters to measure current?

I have a DMM that has a clamp. I want to use it to measure current being used in a given circuit. This is how I use it:

- Disconnect the two probes
- Clamp it on to a NMB cableI want to measure where the clamp is closed and the cable is inside the closed clamp (so clamp is not latching on to the cable and stays open, clamp is closed around the cable).
- Use the 20A or 200A depending on the load

When i do this for my A/C condenser, its showing me 0.324 if I choose 20A and 0.06 if I use 200A setting. My condenser is at 40A breaker and as per my energy monitor it uses around 4000 watts.

The clamp meter shows bogus readings for other things like washing machine or compressor.
 
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Old 02-22-18, 08:03 AM
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The clamp only goes around 1 conductor at a time. For 240 volts
Geo
 
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Old 02-22-18, 08:47 AM
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Originally Posted by Geochurchi View Post
The clamp only goes around 1 conductor at a time. For 240 volts
Geo
Hmm thats what the problem was probably. Thanks
 
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Old 02-22-18, 02:49 PM
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Didn't your meter come with directions? If not, Google "How to use clamp on ammeter".
 
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Old 02-22-18, 04:59 PM
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The clamp only goes around 1 conductor at a time. For 240 volts
The same for both 120v or 240v. With 240v..... you can measure either hot leg. The measurement should be the same. With 120v the clamp goes around just the hot wire.
 
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Old 02-22-18, 06:28 PM
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With 120v the clamp goes around just the hot wire.
Even for 120V, it can be done on either wire. Same current flows in neutral wire as well.

It usually is a little harder to get to neutral inside load center though.
 
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Old 02-23-18, 03:22 AM
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But how can anyone clamp it on the hot wire only without taking a box apart, it doesnt sound practical. Is this a limitation of the type I have or they are all the same?

I did read the directions it came with, but as it is a $20 DMM, they werent very good.
 
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Old 02-23-18, 04:27 AM
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But how can anyone clamp it on the hot wire only without taking a box apart, it doesnt sound practical. Is this a limitation of the type I have or they are all the same?
This is just how the clamp meter works. It measures current in a wire by measuring magnetic field. (Same as how a transformer works)
When there are both wires in a clamp meter, equal current is flowing in a opposite direction canceling magnetic field out. This results in near 0 current measured.
If you wrap same wire twice, it will measure twice the current.

You could get something like below link for a 120V outlet.
https://www.amazon.com/Extech-480172...amp+meter+plug
 
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Old 02-23-18, 04:28 AM
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Some times it can be a challenge. just hope you have enough wire in box to pull out to clamp.
 
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Old 02-23-18, 04:49 AM
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Thanks for all the valuable info. I appreciate it. Line splitter is perfect for me, I bought one from amazon. Thanks again!
 
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Old 02-24-18, 05:46 AM
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But how can anyone clamp it on the hot wire only without taking a box apart
sdkskd
This is one of the reasons what code requires 6" of free conductor in a box. (3" past the face of the box)
 
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Old 04-01-19, 11:40 AM
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Before starting a new thread to ask a question, I searched and found this thread which I think answers my question but I wanted to confirm ---

I am trying to determine the total current my household is drawing at different times with different loads operating. My question is -- to determine the total current being drawn by household do I need to take the reading off of both hot feeders going from service entry to main-breaker and add the two readings together, or is the reading off of one feeder an accurate representation of current amps being drawn by household? Both readings are always near identical (with minor differences probably being accounted for in margin of error of amp clamp measurement). I just wanted to confirm I was doing this correctly. Thanks in advance!
 
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Old 04-01-19, 12:11 PM
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do I need to take the reading off of both hot feeders going from service entry to main-breaker and add the two readings together
Yes. That is how you should measure total current.
 
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Old 04-01-19, 12:42 PM
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so my next question is this - There have been numerous instances where I intentionally turn on a number of our larger appliances/loads (dryer, stove, AC window units..etc.) and take a reading on both of the hot feeders, and they both indicate an identical reading north of 65 amps. If I am supposed to add these together, that is over 130amps being drawn, but my main-breaker is only 100 amps and never trips. Is my main-breaker bad?

Also, if I take a reading on both feeders, then turn on another appliance in the household and take another reading, the reading on both feeders increases by the same amps.
 
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Old 04-01-19, 01:06 PM
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Quote:
do I need to take the reading off of both hot feeders going from service entry to main-breaker and add the two readings together
Yes. That is how you should measure total current.
That is not correct and contradicts what you said in post #6.

A 100 amp breaker can carry 100 amps on each leg and it does not add up to 200 amps.
 
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Old 04-01-19, 01:37 PM
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In a residential service, you have three current carrying conductors: Hot - Neutral - Hot, which power a variety of 240V and 120V loads. At any point in time, the current in the neutral is the absolute value of the difference of current in the hots.

Neutral = Abs(Hot1 - Hot2)

The 100A main breaker will trip if current in either of the hots exceeds 100A for a period of time. "Total current" does not really make sense as a concept, because you have to look at current in each leg plus the neutral. You could compute total power or total energy by doing math on the component currents.
 
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Old 04-01-19, 02:37 PM
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Well. My mistake. I have over simplified. It is bit more complicated when adding up current from multiple circuits of different phases.

Current will flow from L1 to L2 or L2 to L1 (in AC it is really just back and forth) and the difference between 2 will flow to neutral.

So, you cannot really measure total current on split bus. You can just say how much current is flowing in each legs.
On 100A 2 pole breaker, you can have 100A 120V on each bus or 100A 240V.

The closest total amperage you can measure is I_L1 or I_L2 which ever is greater.
 
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Old 04-01-19, 03:48 PM
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If you want to calculator how much POWER(watts) you are using, You need to add the current from both hots and multiply by the voltage(120 volts).
The current on each hot is what determines if your breaker will trip not the total of the two.
 
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Old 04-02-19, 07:56 AM
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Thank you to everyone who commented on my question it has been very helpful, and I am learning a lot.

The original purpose of trying to measure the total amps/current the household is pulling is because my wife and I recently purchased an EV, and our household charging options are either a 120v 15amp circuit (charger will only draw 12 amps continuous when charging on this circuit) or a 240v 50amp circuit (charger will draw a maximum of 32amps continuous), We live in an old house and only have a 100amp service. I know that with EVs the consensus is to upgrade your service to 200amps, but we will only be in our current home for another year or so, so I wanted to determine how much available "room" we had in terms of available amps our current 100amp service can safely supply to charging the EV in worst-case scenario where we are operating numerous other big loads (dryer, range, dishwasher...etc.). I say "worst-case scenario" because the vast majority of our charging happens at 12amps on the 120v between the hours of 2am - 6am. But in a pinch it would be nice to know what my existing 100amp service can safely handle if we need to charge during daytime hours on the 50amp circuit. I have the ability to reduce the amps the EV charger pulls, so if 32 amps is to excessive or taxing on the service I can dial it down in 1 amp increments all the way down 1amp.

If I am understanding all of the information I have been provided correctly, if either of the hot legs goes beyond 100amps it will trip the main-breaker. The main-breaker is rated for 100amps so should be able to safely handle 80amps continuous (correct?). So in the testing that I have done so far, when turning on as many appliances and loads in the house as I can think of (including charging the EV at full 32amps), the main breaker has never tripped, and I have never gotten amp readings on either hot legs above 70amps. I would never expect these scenarios to play out, I just wanted to see I would be running into issues with tripping the main breaker if we ran a lot appliances and charged the EV simultaneously.

So for the short amount of time we live in our current house would you consider it safe, with our existing service, to charge our EV (anywhere from 12-32amps) during hours where we are sleeping and most appliances are not running, but also safe to charge at up to 32amps during daytime hours in very infrequent scenarios where we would be mindful of running any other large appliance while car was charging?
 
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Old 04-02-19, 08:05 AM
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Keep in mind under overload of a few amps the main breaker won't trip for 15 or 20 min so any overload may not last long enough to trip the breaker. Loads such as oven and water heater may be intermittent.
 
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Old 04-02-19, 08:14 AM
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Your main breaker can handle 100 amps continuous.
 
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