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#46579 - 12/26/04 09:40 PM How to measure current
Chester Offline
Registered: 04/24/04
Posts: 11
Loc: Ayden, NC USA
I've gota bit of a dumb question, at least in the eyes of some of you pros - but remember that I am only a student.

I am curious as to how I can test current draw on a particular appliance, in this case, a microwave. Now, I have read that soldering can be done with the contacts, getting the test leads hooked up in series with the circuit wires etc. etc. etc....however, I am wondering what is to stop me from simply putting the test leads of my multimeter on the prongs as the current is being drawn and as the appliance is running. Now, this seems to raise a red flag, so I've never done it. Perhaps it's a safety issue. Would this, as least in theory, work?

If not, how do you guys go about actually testing current? Say you are wanting to find out what a motor is drawing and there's no longer a readable label (with wattage/voltage info) on it? I appreciate your answers, as I've always been curious about this.

Work Gear for Electricians and the Trades
#46580 - 12/26/04 10:02 PM Re: How to measure current
Norstarr Offline
Registered: 09/25/04
Posts: 86
Loc: Wi
Chester, go to and look for a tester called a clamp-on multimeter. Not sure but maybe you can rent one at home depot although they may not rent this type of tool to a novice. You can pick one up on ebay probably very reasonable. Amprobe can supply you with aan adapter that plugs into the wall receptacle to split the wiring, you plug the appliance into the adapter and then use the meter. Very simple. Small multimeters are not rated for appliance current so hooking one up in series will just give you something to fill your trashcan with.
#46581 - 12/26/04 11:54 PM Re: How to measure current
ComputerWizKid Offline
Registered: 09/20/03
Posts: 119
Loc: Connecticut, USA
If it just for Cord and plug appliances use a kill a watt or simulaor plug in meter link here Link I have one and it is farily accurate. one cavvet you have to plug it into an extension cord to read it from a plug behind something (Or to fit in some plugs)

[This message has been edited by ComputerWizKid (edited 12-27-2004).]
Theres always enough room in the junction box.You just need a bigger hammer
#46582 - 12/27/04 02:04 AM Re: How to measure current
marcspages Offline
Registered: 10/15/04
Posts: 48
Loc: London, UK
Hi Chester,

The safest way of doing this, as suggested, is to use a clamp-on current probe of some nature - either a clamp-on meter, or a clamp-on probe for a multimeter ( example here ).

If you're going to do this on a number of devices, then you may want to build yourself a handy coupler such as this one (the loop on the side made specifically for current clamps).

If microwave ovens are your thing, then you may wish to look here , as microwaves have a serious inrush current (and depending on the design which may use on/off power control, may have such an inrush each time the magnetron is switched on!).

Hope thgis helps.

#46583 - 12/27/04 04:27 AM Re: How to measure current
winnie Offline
Registered: 09/15/03
Posts: 649
Loc: boston, ma

As others have said, the best way to do this is with a clamp on ammeter and one of the units that you insert between the plug and receptacle that separates the hot and neutral and lets you run the clamp around a single conductor.

However your question itself indicates a basic misunderstanding of voltage and current. You seem to be suggesting that you could pull the plug out just a little bit, and place your meter leads against the bare blades of the plug. Is this what you are suggesting?

If so, then you will not be able to use this technique to measure current. You would be able to measure voltage this way; but if you tried this with your meter on the current setting then you would be short circuiting the supply through your meter and sparks would fly.

Voltage is measured between two locations, using a high resistance meter connected between those locations. Current is a measure of the rate of charge flow past a single location, and is measured using a very low resistance meter 'inserted in the path', or by measuring the magnetic field around a conductor.

#46584 - 12/27/04 01:01 PM Re: How to measure current
Trumpy Offline

Registered: 07/05/02
Posts: 8211
Loc: SI,New Zealand
I'd be rather careful working on innards of a microwave oven if I were you.
The reason being, not only the large currents that flow when it is operational, but also bear in mind that most m/wave ovens also generate in excess of 2-3000 Volts to run the Magnetron and no Multimeter (or body part, for that matter) will carry that amount of voltage without damage.
Also bear in mind that operating a microwave without it's cover in place could also expose you to large amounts of microwave radiation.
Be very careful, if you aren't sure what you're doing, take the thing to someone that does.
Let's face it, these days if you're not young, you're old - Red Green grin
#46585 - 12/27/04 02:22 PM Re: How to measure current
Chester Offline
Registered: 04/24/04
Posts: 11
Loc: Ayden, NC USA
Thanks for your replies guys - no need to worry, my question was about theory. I am not going to be doing anything with a microwave. I simply meant to use that for illustration purposes. Sorry about that. I should have been clearer.

Although I am a student/novice, I do understand the difference between voltage and current. However, I certainly have a long way to go at this point. My textbooks really only focus on wiring and code, not necessarily theory.

Although I can and have done any number of electrical jobs safely, I don't jump into anything if I'm not completely sure about what the job entails and that I am able to handle it. I appreciate all of your responses, and look forward to learning more from this great site!

#46586 - 12/27/04 02:42 PM Re: How to measure current
NJ Wireman Offline
Registered: 08/26/04
Posts: 186
Loc: New Jersey (South Jersey)
Why not just unscrew the outlet from its box and place your clamp on meter around the hot wire. Just make sure nothing else is drawing on that circuit.
#46587 - 01/02/05 04:44 AM Re: How to measure current
Spark Master Flash Offline
Registered: 07/28/03
Posts: 141
Loc: Las Vegas, Nevada, USA
Hey Chester,

Just thought I'd expand on something here. If you don't separate the wires and you try to put an amp clamp around both the hot and the neutral to check amperage, you'll get nothing on the meter. It only works when you can separate the wires and put the amp clamp around the hot one.
"When in doubt, short it out"
#46588 - 01/02/05 05:21 AM Re: How to measure current
iwire Offline
Registered: 01/05/03
Posts: 4391
Loc: North Attleboro, MA USA
Good morning Flash

It only works when you can separate the wires and put the amp clamp around the hot one.

You are certainly correct that you must separate the conductors and only put the amp clamp around one of the conductors at a time.

However it does not matter which conductor you put in the amp clamp, the ungrounded (hot) or the grounded (neutral) you will get a reading on both. The reading should be equal on both the ungrounded and grounded conductor.

If they are not exactly equal the appliance has a problem with leakage current and you should find the 'difference' carried on the grounding conductor.

Bob Badger
Construction & Maintenance Electrician
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