Safe way to measure AC current?

[timothydavidson]

Hello! This is my first post here. I've done a little research on this and haven't found a good answer.

I am curious about how to safely measure the amp draw from low amp devices (laptop, phone charger, small electronic stuff) plugged into a 120v wall socket. I have the Fluke 87v, i410 current clamp, AC line splitter for higher amp devices. I saw someone on Youtube take a power cord and split out the hot, neutral, and ground cables, and leave one cable disconnected as to connect to a meter, but is that safe?

Thanks for the help!

[IanB]

Don't mess with the wiring. There is one simple answer: the "Kill A Watt" device. It will tell you all you need to know.

[Nuzzler]

Sounds like you have all the necessary gear. Option 1 would be to plug the device you want to measure (laptop for example) into the AC line splitter and use your clamp. Option 2 would be to split out the wires of a cable (like a short extension cord) and use your clamp. A clamp is usually best IMHO.

As IanB noted...the Kill-A-Watt is another great option if you have one. Idiot-proof and safest overall.

- Nuzz

[Ian.M]

Kill-A-Watts and similar energy meters typically only have 10mA resolution, which isn't a whole lot of use when a true 1W 120V gadget will only draw about 8 1/2 mA.

That depends on exactly how you make the current measurement.  If you have proper shrouded plugs to fit the meter on your 'breakout' lead, you always check first that the current will be within the meter's range and you never connect or disconnect the meter with the circuit powered or power it with the meter plugs loose, its at least as safe for you and your test equipment  as any other bench work taking measurements on live circuits.   Use an isolating transformer for improved personal safety.

Personally, if I was doing a lot of that sort of stuff, I'd make a breakout box,  with a double insulated loop of the live wire for a current clamp,  two shrouded terminals for current , with a heavy duty switch to short them to bypass the meter (useful if you expect a switch-on surge that exceeds the current meter range), another shrouded terminal for voltage (between it and one of the current terminals), and a neon lamp across the line input as a warning the box is powered.   

[tautech]

Hello! This is my first post here. I've done a little research on this and haven't found a good answer.

I am curious about how to safely measure the amp draw from low amp devices (laptop, phone charger, small electronic stuff) plugged into a 120v wall socket. I have the Fluke 87v, i410 current clamp, AC line splitter for higher amp devices. I saw someone on Youtube take a power cord and split out the hot, neutral, and ground cables, and leave one cable disconnected as to connect to a meter, but is that safe?

Thanks for the help!

Welcome to the forum

As you have a current clamp it is quite safe to make up a short extension lead with the live conductor separated for easy access with your clamp. If your clamp has a large enough opening you might choose to have the Phase as a double insulated single conductor and N and Earth as a 2 core cable for all round better isolation/protection.

[timothydavidson]

Sounds like you have all the necessary gear. Option 1 would be to plug the device you want to measure (laptop for example) into the AC line splitter and use your clamp. Option 2 would be to split out the wires of a cable (like a short extension cord) and use your clamp. A clamp is usually best IMHO.

As IanB noted...the Kill-A-Watt is another great option if you have one. Idiot-proof and safest overall.

- Nuzz

Thanks for the reply! The current clamp is great for higher amp readings, but mine can't measure anything lower than 500mA

[Kilrah]

I have one of these things I got several years ago at a sale, has a handy solar-powered wireless display and resolves pretty accurately down to 0.1W, it will show the standby consumption of an unloaded charger.

Very good meter but the wireless on/off function is unusable though, after it falsely triggered shutdown twice while set up to measure my desktop PC system's consumption I got mad, opened it and shunted the relay 
 

[veedub565]

I'm not familiar with the kill a watt or Fluke 87, but a current probe would work. Most of  the ones I've come across have been the Tektronix  TCPA300 kind.

Usually though we have a mains breakout box at work and just use an ordinary bench multimeter.

[electr_peter]

Power usage of items is best measured with specialised equipment. For example, something like 0-01W-660W-Electric-Power-Energy-Monitor-Tester-Socket-Watt-Meter-Analyzer-9801 can measure low power levels accurately and is very easy to use.


Manual AC mains current measurement is not recommended, because:

• it is very dangerous. One mistake and you are not in this world any more. Or you house burns down
• measurement equipment (especially DIY type) and open wiring cannot be left unattended - it is just asking for a trouble.
• Measuring current is very messy in terms of wiring - you need break/make several wires, amount of wires quickly becomes a nightmare. Special adapters help a bit.
• you can not determine real power easily with DMM only. You need 2 simultaneous channels. Even with current clamps offset issue exists (which makes results inaccurate unless compensated).

I suggest to use a Kill-a-Watt type device or device in a link (if you need better accuracy).

[timothydavidson]

Kill-A-Watts and similar energy meters typically only have 10mA resolution, which isn't a whole lot of use when a true 1W 120V gadget will only draw about 8 1/2 mA.

That depends on exactly how you make the current measurement.  If you have proper shrouded plugs to fit the meter on your 'breakout' lead, you always check first that the current will be within the meter's range and you never connect or disconnect the meter with the circuit powered or power it with the meter plugs loose, its at least as safe for you and your test equipment  as any other bench work taking measurements on live circuits.   Use an isolating transformer for improved personal safety.

Personally, if I was doing a lot of that sort of stuff, I'd make a breakout box,  with a double insulated loop of the live wire for a current clamp,  two shrouded terminals for current , with a heavy duty switch to short them to bypass the meter (useful if you expect a switch-on surge that exceeds the current meter range), another shrouded terminal for voltage (between it and one of the current terminals), and a neon lamp across the line input as a warning the box is powered.

Hey Ian! This is the answer I was searching for. Thank you for the tips in the second paragraph about safety. Those are good things to keep in mind. Do you have a link to where I can buy shrouded plugs?

Also, I did a google search for breakout boxes and didn't see anything like what you were describing, but I think that's exactly what I'm looking for. Do you have any pictures or links to point me in the right direction of building one?

Thank you so much for the help!

[ez24]

... low amp devices (laptop, phone charger, small electronic stuff) plugged into a 120v wall socket.

I triple this suggestion
https://www.amazon.com/P3-P4400-Electricity-Usage-Monitor/dp/B00009MDBU/ref=sr_1_1?ie=UTF8&qid=1472523394&sr=8-1&keywords=p3+international+kill+a+watt

These are also sold at local hardware stores.

If you want accuracy you can leave something plugged in for months and find out how many kilowatts used and then look at your electric bill and calculate the cost.  Of course you do not have to leave in months, I usually go for 24 hours to 1 week.   I found out an old refer used too much so I got rid of it ($30 a month).   My favorite information is the number of kilowatts because that is what I am billed at.

Plus take a hint at the name "Kill A Watt" not "Kill Me"

[IanB]

Also, I did a google search for breakout boxes and didn't see anything like what you were describing, but I think that's exactly what I'm looking for. Do you have any pictures or links to point me in the right direction of building one?

This is the kind of thing that it might be hard to find instructions for. If someone has enough knowledge about electrical safety to safely break a mains circuit and measure the current, then they have enough knowledge to make the tools to do it and do not need instructions. If someone does need instructions, then they are probably entering dangerous territory by attempting it. It would therefore be unwise and irresponsible for someone to provide such instructions online due to liability issues. I can see why people would be reluctant to do so.

[timothydavidson]

That definitely makes sense. I think I'll pick up a Kill a watt and call it good for now. Thanks!

[IanB]

That definitely makes sense. I think I'll pick up a Kill a watt and call it good for now. Thanks!

Bear in mind that you may find things on eBay such as the device linked above by electr_peter that have more functions and better resolution than the actual Kill-A-Watt. On the other hand, the Kill-A-Watt can be found in almost any electrical/hardware/electronics store.

[Ian.M]

I'm not going to give detailed instructions on how to build a breakout box for current measurement as if you don't already know what's needed for safe mains wiring in a project, you probably shouldn't be doing it.

The search term you want for the connectors is shrouded 4mm banana  then plug or socket.  You'll also need some plug to plug test leads with the same connectors to go between the box and your meter.

Use a good quality plastic project box big enough to mount a single unswitched wall socket to (unless you can get a smaller chassis mount socket).  For the power in, I suggest a IEC 60320 C14 inlet connector, like a PC PSU, so you can easily isolate the box when changing the meter connections. Don't trust an ordinary switch to provide safety isolation. For the switch that shorts the current sockets, don't use one with any accessible metal parts, so no metal lever toggle switches.   For the current clamp loop, its probably easiest to use a short length of ordinary mains flex + cord grips to suit, to get the double insulation and sufficient strength, but only run the live through the loop, cutting the other wires short inside the box at the cord grips.

[IanB]

...to get the double insulation...

About the double insulation. Twin core figure-8 mains flex is common, if not ubiquitous, here in the USA on all sorts of consumer products. How does this satisfy the double insulation requirement without an outer cover?

[Brumby]

This is the kind of thing that it might be hard to find instructions for. If someone has enough knowledge about electrical safety to safely break a mains circuit and measure the current, then they have enough knowledge to make the tools to do it and do not need instructions. If someone does need instructions, then they are probably entering dangerous territory by attempting it. It would therefore be unwise and irresponsible for someone to provide such instructions online due to liability issues. I can see why people would be reluctant to do so.

I'm not going to give detailed instructions on how to build a breakout box for current measurement as if you don't already know what's needed for safe mains wiring in a project, you probably shouldn't be doing it.

While this doesn't solve your problem, I must agree with this philosophy.

It is one thing to give advice and instruction when you can watch over the shoulder of someone and correct and guide them as they do it - but it is quite another to do so remotely.  Some hints and tips have been given, together with some appropriate product suggestions.  If these help you sort out your own solution then well and good - but if you are still unsure, then I would strongly advise to think twice.

My own 2 cents worth is this:  Whether provision for a clamp meter or inline meter connection - make sure there is no exposed conductor anywhere when power is applied.

[ez24]

There is nothing wrong with AC
Very easy to make a guitar with it

[Brumby]

There is nothing wrong with AC
Very easy to make a guitar with it

Everything you see this guy do comes under the heading of DO NOT TRY THIS!

I get cold shivers watching his stuff.

[David Hess]

Thanks for the reply! The current clamp is great for higher amp readings, but mine can't measure anything lower than 500mA

AC line splitters usually have a x10 range to increase the sensitivity of the current clamp.

https://www.amazon.com/Extech-480172-AC-Line-Splitter/dp/B0000YHN9W

[Ian.M]

...to get the double insulation...

About the double insulation. Twin core figure-8 mains flex is common, if not ubiquitous, here in the USA on all sorts of consumer products. How does this satisfy the double insulation requirement without an outer cover?

If your figure 8 mains flex doesn't have two layer insulation with a different colour inner layer so any damage that breeches the outer insulation will be clearly visible, , its not fit for purpose, even if it may technically be permitted or grandfathered in under product safety regulations that may be laxer than is common in 1st world 220V countries.

I'd use round flex for this - it will stand up better to getting scratched by the current clamp jaws.

[djacobow]

The clamp meter with line splitter approach, of course, will NOT allow you to measure power factor, and that actually is quite important, particularly for something "below 1W in idle" like a switch mode iThing charger unloaded.

The Kill-A-Watt will do this "properly," though not with enough precision to be very useful for < 1W loads.

I actually own the Antai device. It does have a bit more resolution and sensitivity, and seems to be accurate, too. I have one. However, mine is not useful for long term energy measurements because it reliably locks up after a few minutes. Also, the manual is the impenetrable in English. Here is an old thread on my unit: https://www.eevblog.com/forum/testgear/teardown-antai-atx-9801-handheld-plug-power-meter/msg613966/#msg613966. Note that someone else has one and his does not lock up.

I went through this whole saga awhile back, trying to measure the power draw of some unloaded small PSUs. In the end, if you need accuracy, you need a decent instrument. Look at the Chroma 66201/66202 for a decent baseline single-phase instrument. For more coin, look at the Yokogawa WT210/WT230.  Hioki also has one PW3335. One useful keyword to search for is "IEC 62301" which is the standard for standby power for which many people who buy such meters are gunning.

I believe Dave also a nice unit, a bit cheaper, but in the same general class. Can't remember the name.

Oh, and there is the Gossen Metrahit Energy which Dave also likes. Plus: handheld and useful as a DMM.

One other thing: you can modify the shunt in a Kill-A-Watt to make it more sensitive. You basically will remove the shunt and replace it with a higher resistance. Since this is hard to do accurately, you will essentially get garbage current/power measurements, that you will need to scale appropriately. Of course, to determine how much to scale, you'll need some known load that was measured by some other means.

Finally, if you really want to do this right, you probably want a programmable AC source, as well as a power meter. Some of these have integrated power meters, too. BK Precision 9801 looks nice.

[Brutte]

Thanks for the reply! The current clamp is great for higher amp readings, but mine can't measure anything lower than 500mA

Just spit the cable and roll the hot and cold wires on a clamp, in opposite directions of course. 10x would do.

[R005T3r]

Actually the safest and easiest way is to measure the current is trough the neutral not the live wire. It's the easiest way: you don't need to have an amp clamp or other fancy instruments, just plug your multimeter and you are good to go, it's the safest: you are measuring it on the neutral, so, even if there's some voltage it's not enough to kill yourself...

Actually I don't understand why people are measuring it trough the live wire all the time. Current won't change if measure it trough the live or the neutral...

[Ian.M]

Actually the safest and easiest way is to measure the current is trough the neutral not the live wire. It's the easiest way: you don't need to have an amp clamp or other fancy instruments, just plug your multimeter and you are good to go, it's the safest: you are measuring it on the neutral, so, even if there's some voltage it's not enough to kill yourself...

Wrong.
Unless you measure the current as voltage across a permanently connected current shunt, all it needs is for there to be a bad connection resulting in an open-circuit neutral between your meter and the supply, (or, even with a permanently connected shunt,  for there to be a bad connection in the supply neutral) and your meter will be at full mains live potential via the load.   

For your own safety, *ALWAYS* treat the Neutral with as much respect as you do Live wires, as it can go live without any warning.

This is compounded by the plug and wall socket situation in the USA and much of Europe.  In the USA two pin plugs are supposed to be polarised by contact width.  With worn sockets and sub-standard plugs, this is far from certain.  In Europe, the three conductor Schuko plug is reversible!