Lookin for AC Amp Meter (or meters) with remote "donut" sensor. Not having much luck.

Thread Starter


Joined Dec 29, 2017
Background. My son and Daughter in Law run a business using various low load machines and are a buying a house with facilities for their business.
The former Airplane Hanger has a 75 amp main and a few sub breakers at this time. They are going all in at this time and have little for electric upgrade until October or so.

I would like to have ac amp meters to monitor the main breaker load on each 120V leg and on some or all of the sub breaker legs so we know if load balance is causing outages.

I have seen 30 amp analog donut amp meters used for generators at about $30 or so w 2 donuts and 2 meters in a 2x6x8 inch enclosure by reliance controls.
I have seen 200 amp digital donut amp meters (I think) on ebay for about $15 or so without enough information to tell if they are what I can use or if they are for 1 or 2 donuts and indications,

Any help would be greatly appreciated.

Thanks in advance.


Joined Mar 21, 2018
If the meters are supplied with the current transformers (donuts) then frequency (50Hz vs 60Hz) and maximum current indication are the only ratings you need to know.


Joined Jan 15, 2015
The devices you refer to, as mentioned, are Current Transformers and are a ratio metric transformer. A wire running through the center hole is the primary and there is a secondary output. They come in several different designs with a basic Current Transformer looking like the below image.

Coil 1.jpg

Coil 3.jpg

The upper transformer happens to be a 200:5 ratio meaning with 200 amps passing through the primary there will be a 5 amp output on the secondary. There is a caveat here in that the lead length of the secondary wires is limiter. This transformer is designed to work into a very low resistance load known as a "burden resistance". This all depends on the selected current transformer. When buying a turn key off the shelf solution including the current transformer and meter (display device) the CT and meter are designed to work together. If you link to any units that you are considering you will likely get more information. Years ago even a lowly CT could be an expensive affair but today with units off the boat from China they have become relatively inexpensive. There are also some tips and tricks. For example using a 60:5 ratio I can loop my primary conductor three times through the hole (amp turns) and now have a 20:5 CT. More on that if you need.



Joined Jan 15, 2015
Accuracy across 3 octaves! That's wild! What's the core made of?
Most inexpensive commercial grade current transformers use silicon-iron alloy as a magnetic material, commonly used for construction of current transformer cores. As the frequency increases so goes core loss, however, if we want really low core loss and something that will work at frequencies much above 400 Hz so also goes the cost. You can buy high permeability alloys as a core but when we want that we pay for it. Ferrite which is a ceramic compound consisting of a mixed oxide of iron and one or more other metals. Ferrite has ferrimagnetic properties making it suitable as a core material.

In my reply I also neglected to mention other types of current transducers or for that matter voltage transducers. You can have whatever you wish but with things like accuracy and measuring average RMS current or true RMS current comes cost. Just as an example I can go out and buy two of these at about $247 each USD or if I want True RMS current maybe two of these. Then if I want to see my split phase voltage I can get two of these. Using two current transducers and two voltage transducers gives me four channels of data so if I take each channel and scale them as 0 to 50 Amps and 0 to 150 Volts = 0 to 10 volts I could output that data to a data logger, even a small cheap one like this one and actually record all of the data. When all is said and done I can spend $100 or less to $1,000 all depending on what I want. :)