Simple amp meter

Doug - Iowa · Mar 22, 2012

I'd like to get some amp readings off my power service to the shop without removing the cover and using the hand held clamp-meter. I recall making a simple amp meter years ago by wrapping a strand of wire around the line and hooking it to a voltmeter. It's just a crude transformer. But that was only a few amps, I'd like to make a larger one to measure 200A. Meter CT's for that size are expensive, anyone have a lead on how to make your own?

JMOR · Mar 22, 2012

(quoted from post at 22:02:09 03/22/12) I'd like to get some amp readings off my power service to the shop without removing the cover and using the hand held clamp-meter. I recall making a simple amp meter years ago by wrapping a strand of wire around the line and hooking it to a voltmeter. It's just a crude transformer. But that was only a few amps, I'd like to make a larger one to measure 200A. Meter CT's for that size are expensive, anyone have a lead on how to make your own?

oes your power meter have the flat aluminum disk with the black mark of it that can be seen spinning faster & faster with more load?
If yes, I can tell you how to use it to measure what you want, excepting pf?

wisbaker · Mar 22, 2012

I got an off brand clamp on analog about 12 years ago for $50.00, think I've still seen them under $100. I had to have one that went to 300A to measure draw when trouble shooting presses and centrifuges.

buickanddeere · Mar 22, 2012

If you manage to build a working CT , it won't function with a volteter. No offence but this has all the ingrediants for 911 report by Emergency Responders. An open CT can build high voltages. In particular. When high current exists on the primary.Why are you reluctant to clamp an ammeter around a insulated conductor.

David G · Mar 23, 2012

B&D is correct, an open CT can reach fatal voltages.

TGP · Mar 23, 2012

(quoted from post at 21:12:50 03/22/12)

(quoted from post at 22:02:09 03/22/12) I'd like to get some amp readings off my power service to the shop without removing the cover and using the hand held clamp-meter. I recall making a simple amp meter years ago by wrapping a strand of wire around the line and hooking it to a voltmeter. It's just a crude transformer. But that was only a few amps, I'd like to make a larger one to measure 200A. Meter CT's for that size are expensive, anyone have a lead on how to make your own?

Click to expand...

oes your power meter have the flat aluminum disk with the black mark of it that can be seen spinning faster & faster with more load?
If yes, I can tell you how to use it to measure what you want, excepting pf?

I would interested in how you can measure the draw,if not too much trouble?
Tom

buickanddeere · Mar 23, 2012

We already tried to warn Doug why do you want to attempt this
as well? You could just read the meter usage in KW or note
how many turns of the disk = a KW.

JMOR · Mar 23, 2012

(quoted from post at 09:09:25 03/23/12)

(quoted from post at 21:12:50 03/22/12)

(quoted from post at 22:02:09 03/22/12) I'd like to get some amp readings off my power service to the shop without removing the cover and using the hand held clamp-meter. I recall making a simple amp meter years ago by wrapping a strand of wire around the line and hooking it to a voltmeter. It's just a crude transformer. But that was only a few amps, I'd like to make a larger one to measure 200A. Meter CT's for that size are expensive, anyone have a lead on how to make your own?

Click to expand...

oes your power meter have the flat aluminum disk with the black mark of it that can be seen spinning faster & faster with more load?
If yes, I can tell you how to use it to measure what you want, excepting pf?

Click to expand...

I would interested in how you can measure the draw,if not too much trouble?
Tom

I suggest that you take a more direct approach and measure the Watts passing through your electric meter. Not only will you be able to see parasitic current draw (everything in house turned off & unplugged), but you will be able to measure the power consumption of any one electrically powered device in you home whether it be a light bulb (point of reference for your meter), hot water heater (both measure & see it cycling on/off), air conditioner/heat pump, or anything electric that you have in your home. You can compare your 'reading' with the stated name plate draw (Watts) of the device/appliance.

If your meter looks like the picture, you will see a spinning disk of aluminum partially protruding through the face plate below the dials. Circled in red is a number called the "K sub h" factor & units are watt-hours per revolution. Here it is 7.2. Your meter might be 3.6 or some other number. Use the number on your meter. This disk will turn one revolution in one hour if the power being used is 7.2 watts.
To obtain your power useage at any time, you use your watch with sweep second hand and count the number of disk revolutions in one minute & write that number down as 'revolutions per minute'. Multiply that number by 7.2 (or 3.6 or whatever is on your meter). Next multiply that result by 60. The final result is equal to the power in Watts being used by everything in your home at the moment of your measurement.

(rev/min) X (K sub h) X 60 = Watts

or

(rev/min) X (Watt-hr/rev) X (60min/hr) = Watts

Janicholson · Mar 23, 2012

Wikipedia Current Transformer. The turns ratio is pretty dramatic, so a not of caution is included from Wiki here: Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as the transformer secondary will attempt to continue driving current across the effectively infinite impedance. This will produce a high voltage across the open secondary (into the range of several kilovolts in some cases), which may cause arcing. The high voltage produced will compromise operator and equipment safety and permanently affect the accuracy of the transformer.

Jim

JMOR · Mar 23, 2012

I submit that it is no longer a CURRENT TRANSFORMER, when its calibrated load is not connected.

buickanddeere · Mar 23, 2012

Thankyou JMOR . Your detailed and accurate account
should reduce the number of Darwin Awards .if some folk had
been tempted but now educated not to try measuring current
as initially proposed.

TGP · Mar 24, 2012

(quoted from post at 09:19:57 03/23/12)

(quoted from post at 09:09:25 03/23/12)

(quoted from post at 21:12:50 03/22/12)

(quoted from post at 22:02:09 03/22/12) I'd like to get some amp readings off my power service to the shop without removing the cover and using the hand held clamp-meter. I recall making a simple amp meter years ago by wrapping a strand of wire around the line and hooking it to a voltmeter. It's just a crude transformer. But that was only a few amps, I'd like to make a larger one to measure 200A. Meter CT's for that size are expensive, anyone have a lead on how to make your own?

Click to expand...

oes your power meter have the flat aluminum disk with the black mark of it that can be seen spinning faster & faster with more load?
If yes, I can tell you how to use it to measure what you want, excepting pf?

Click to expand...

I would interested in how you can measure the draw,if not too much trouble?
Tom

Click to expand...

I suggest that you take a more direct approach and measure the Watts passing through your electric meter. Not only will you be able to see parasitic current draw (everything in house turned off & unplugged), but you will be able to measure the power consumption of any one electrically powered device in you home whether it be a light bulb (point of reference for your meter), hot water heater (both measure & see it cycling on/off), air conditioner/heat pump, or anything electric that you have in your home. You can compare your 'reading' with the stated name plate draw (Watts) of the device/appliance.

If your meter looks like the picture, you will see a spinning disk of aluminum partially protruding through the face plate below the dials. Circled in red is a number called the "K sub h" factor & units are watt-hours per revolution. Here it is 7.2. Your meter might be 3.6 or some other number. Use the number on your meter. This disk will turn one revolution in one hour if the power being used is 7.2 watts.
To obtain your power useage at any time, you use your watch with sweep second hand and count the number of disk revolutions in one minute & write that number down as 'revolutions per minute'. Multiply that number by 7.2 (or 3.6 or whatever is on your meter). Next multiply that result by 60. The final result is equal to the power in Watts being used by everything in your home at the moment of your measurement.

(rev/min) X (K sub h) X 60 = Watts

or

(rev/min) X (Watt-hr/rev) X (60min/hr) = Watts

Thank you!
That's what I was looking for. No I'm not about to make some home made instrument and stick it into a panel or meter base.
I since found this info. under a google search,but you explained it much better.
Thanks Again.
Tom

Doug - Iowa · Mar 24, 2012

Thanks guys. I'm fully aware of the dangers of a CT. What I am really looking for is monitoring the load on my big saw. I typically just clamp my Fluke meter on the line and leave the cover off the panel. If I set it just right I can see the readout from the saw table. When its just me in the shop that's no big deal, but with others coming by (grandkids) I'd like that panel closed up.

I can buy a cheapo clamp meter at HF and wire a loop of the saw circuit out of the panel to hang it on. No big deal. I just thought it would be interesting to make one that measured the whole shop. Thanks.

Simple amp meter

Doug - Iowa

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JMOR

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wisbaker

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buickanddeere

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David G

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TGP

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buickanddeere

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JMOR

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Janicholson

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JMOR

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buickanddeere

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TGP

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Doug - Iowa

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