Measuring Split Phase Current With One or Two Current Transformers

Thread Starter

HighVoltageJoe

Joined Jun 1, 2017
3
Hello all,
Haven't posted here in a while but came up with a problem. I have a temporary 60 amp subpanel that I put on the front porch every year for my christmas display. I have a Bayite digital panel meter to measure current, voltage, power, etc. The problem I am having is getting it to measure split phase current. I want to measure both legs and it display the current of both legs on one meter. I do have two identical CTs but no matter how I wire them, I can't get it to display the total load. When I plug two strands of lights that draw 1 amp into each phase, the meter measures about 1.75 amps. Should be showing 2 amps. Any ideas on how I can get this to work?

Thank you,
Joe M
 

Reloadron

Joined Jan 15, 2015
4,501
Without seeing an actual schematic it's hard to say. You have two current transformers feeding a single channel of a meter? Have you measured a single string and then the other string on the split phase with a single string one at a time on each side. Since you have a panel and are likely running 240 VAC split phase the hot lines, L1 and L2 are 180 degrees out of phase from each other. Each phase having a current path to neutral. Therefore Current load Neutral = Current L1 - Current L2. If each line (L1 & L2) has 1.0 amp (equal current) the difference will be Zero. The only way to do this would be to measure each side current L1 and L2 and display them individually or use a system designed for this which will provide the sum of L1 and L2.

Without a drawing of your exact configuration I really have no idea why you would be seeing 1.75 amps.

Ron
 

ebeowulf17

Joined Aug 12, 2014
2,941
What are your voltages?
L1 - Neutral
L2 - Neutral
L1 - L2

Is there any chance you've got two legs from a three phase circuit? If your two legs were 120° out of phase instead of the opposite polarity you expect with split-phase, that would explain it almost perfectly. Wiring the two secondaries in parallel (or anti-parallel depending on your primary orientations) in such a way as to get 2A readout on split phase would read 1.73A on three phase - pretty close to what you're seeing.
 

Thread Starter

HighVoltageJoe

Joined Jun 1, 2017
3
What are your voltages?
L1 - Neutral
L2 - Neutral
L1 - L2

Is there any chance you've got two legs from a three phase circuit? If your two legs were 120° out of phase instead of the opposite polarity you expect with split-phase, that would explain it almost perfectly. Wiring the two secondaries in parallel (or anti-parallel depending on your primary orientations) in such a way as to get 2A readout on split phase would read 1.73A on three phase - pretty close to what you're seeing.
L1-Neutral = 120v
L2-Neutral = 120v
L1-L2 = 240

This is all single phase from residential power. The legs are 180 degrees out of phase. I've tried every possible combination of CTs and none have added up to the correct 2 amps. I figured that if I put them in the same direction on the two live wires and wire them in antiparallel that since they were 180 degrees out of phase they would just add. There has to be a loss somewhere. I think that the current is cancelling itself out at my two CT connections...but I can't figure out any other way to wire them!
 

ebeowulf17

Joined Aug 12, 2014
2,941
L1-Neutral = 120v
L2-Neutral = 120v
L1-L2 = 240

This is all single phase from residential power. The legs are 180 degrees out of phase. I've tried every possible combination of CTs and none have added up to the correct 2 amps. I figured that if I put them in the same direction on the two live wires and wire them in antiparallel that since they were 180 degrees out of phase they would just add. There has to be a loss somewhere. I think that the current is cancelling itself out at my two CT connections...but I can't figure out any other way to wire them!
Probably a dumb question, but have you confirmed the meter readings for each light strand individually, through this particular dual CT setup? In other words, when you've got both strands running, and the meter shows 1.75, does unplugging either strand yield a reading of 1.0? For that matter, what if you plug both strands in to one phase or the other? Do you get 2.0 in both those cases?

What I'm attempting with these questions is to eliminate any intrinsic errors in the individual CTs or the meter itself, and see if we really are dealing with something that only appears due to the split phase load combining.

The only other obvious thought I had would involve power factor differences, if one load was significantly more capacitive or inductive than the other, there would be phase shift which might account for the errors... but I'm guessing that you've plugged in two strands that are essentially identical, in which case there shouldn't be any significant power factor differences.
 

ebeowulf17

Joined Aug 12, 2014
2,941
P.S. Assuming none of my rambling above revealed an obvious answer, you'll probably get more help from others if you post a diagram of how you've wired this. Everyone loves nice, clean schematics, but even pencil sketches on a napkin would help.
 

Thread Starter

HighVoltageJoe

Joined Jun 1, 2017
3
Probably a dumb question, but have you confirmed the meter readings for each light strand individually, through this particular dual CT setup? In other words, when you've got both strands running, and the meter shows 1.75, does unplugging either strand yield a reading of 1.0? For that matter, what if you plug both strands in to one phase or the other? Do you get 2.0 in both those cases?

What I'm attempting with these questions is to eliminate any intrinsic errors in the individual CTs or the meter itself, and see if we really are dealing with something that only appears due to the split phase load combining.

The only other obvious thought I had would involve power factor differences, if one load was significantly more capacitive or inductive than the other, there would be phase shift which might account for the errors... but I'm guessing that you've plugged in two strands that are essentially identical, in which case there shouldn't be any significant power factor differences.
With my current setup, if I draw 1 amp on one phase with both CTs connected (I'll draw up a schematic later to show you) it registers correctly. As soon as I draw current from both CTs at the same time, I get losses.
 

Lo_volt

Joined Apr 3, 2014
78
My first guess is that when you use both phases, some of the return current is returning through the opposing phase and some is returning through the neutral. I think ebeowulf17 is on to something.

When you say "...I draw 1 amp on one phase with both CTs connected..." do you mean that you have zero current flowing through the other phase, i.e. it's disconnected?

If all of your loads are resistive and you measure 1 amp one each phase with the other disconnected, then you should still measure 1 amp when both are connected and turned on. This would be with that same load on both phases as the current drawn through one phase will return through the other. No current will run through the neutral.

Power factor of inductive or capacitative loads may affect the circuit, sending some amount of current through the opposing phase and some through the neutral, but I have a hard time believing it would be a difference of 0.75 amps. Some of the LED strings may have switching power supplies which will probably be capacitative, but unless they're really poorly designed, the difference shouldn't be that much.

Most likely, I think, your loads are not balanced and you're seeing some amount of current (perhaps 0.25 amps) return through the neutral and the rest return through the other phase.

Do you have a Kill-a-watt available? The one I have can measure power factor. Take a look at your lights, especially those with ballasts or power supply bricks. These almost always have a non unity power factor.
 
Last edited:

ebeowulf17

Joined Aug 12, 2014
2,941
... but I have a hard time believing it would be a difference of 0.75 amps.
For what it's worth, the stated difference is only 0.25, not 0.75. The expected meter reading is 2.0, and the actual meter reading is 1.75.

That said, I still agree with you that if the two lighting strands are of the same type, I'd expect power factor differences to negligible.
 
Top