US Split Phase - power calculation seems off (maybe)?

Loving finally getting visibility into our electricity usage with iotawatt. We have a standard US 240V split phase (so 2 x 120V) supply and I’m monitoring each 120V input wire to the distribution panel with an AcuCT-H100-200.

Everything looked fine initially, I have a total_watts output configured as main_1 + main_2 and I thought all was well until today when I used our Level 2 EV charger for the first time.

I was confused because the app that talks to the Juicebox was saying that the unit was drawing 28.6A at 240V and 6.89KW - but the iotawatt was showing no more than 4.7KW including everything else the house was doing at the time.

Looking at the data for when the charger came on, iotawatt shows that main_1 rose from 7.5A to 31.1A (+23.6A) and main_2 rose from 1.92 to 11.3A (+9.38A) which is a total extra draw of 32.98A. When the charger went off, main_1 fell from 30.1 to 5.93A (-24.17A) and main_2 fell from 11.5A to 1.45A (-10.05A) so a total drop of 34.22A. The change in power was from 4.84kW to 840W on turning off (-4kW) and from 979W to 4.98kW (+4.01kW) when turning on.

So the iotawatt seemed to measure the power consumption at 4kW but the Juicebox itself was reporting 6.89kW.

My questions are;

  1. Is it likely to be expected that the current pulled from each of the 120V legs is so imbalanced when the charger is on? I know they are imbalanced in general, it’s more that I would have expected the car to draw equally from each side - maybe that expectation is flawed.

  2. Might I have mis-configured something? I checked that my reference voltage transformer is set correctly and none of the inputs are showing a reverse symbol. The delta seems large, 4kW compared to 6.89kW (+72%)

I’d like the readings to be accurate to within maybe 10% as I’m measuring our critical load in advance of specifying a battery backup system so whilst I don’t need super accuracy, it needs to be in the ballpark otherwise I could get things quite wrong.

Wondering what I might have done wrong … or is the Juicebox simply lying?

Thanks for any pointers,


Something is wrong. You should be within 1%.

Can you put 50A CTs on each of the wires going to the charger to measure the power and current directly? If it’s a two-wire 240V they should show the same when the charger runs. If the charger is a three-wire, they may be different. In any event, if you could record what the charger says and post a plot of the two CTs and the two mains during the charge.

That should provide some indication of what the problem is. It could be that one of your mains CTs is not fully closed. At your earliest convenience, please read your meter and note the date and time. We can use that to validate the mains after 100 kWh or so.

No, that’s not likely, but if it’s three-wire I suppose it’s possible. If the charger is representing that the current is the 240V current, it should be the same on both legs.

Your numbers are pretty out of whack, so I expect it will be fairly easy to determine the cause.

I’m wondering about that too now - the electrician had problems fitting one of them as the type of panel we have means the 200 amp feeders come in very tight and to the same side of the main breaker, I’m not sure one of them was fully closed. I’ll take the panel off and check - having seen the procedure that part looks safe enough.

I’ll read the meter tomorrow too - thanks!

OK so the update is, between Dec 8 and Dec 17th, the exterior meter shows consumption of 183KWh but iotawatt shows only 140KWh for exactly the same period - so just 77% of the actual consumption. I’ll get an electrician back to take the panel cover off and check the CT’s. They were tricky to get in place and perhaps he missed fully closing one?

This is a simple diagnostic procedure that would answer any questions about the integrity of the mains CTs.

Sure, but A) I don’t have any 50A CTs and B) the same issue is evident for other 240V appliances such as the tumble dryer so I think it’s reasonably clear that it’s a more generic issue and hence more likely to be the main CTs. I’m also not inclined to take the panel off myself for (A) even if I had the 50A CTs available. The electrician has agreed to pop back and check and I’ll only be billed if everything is OK (which seems unlikely) so I should know more in a few days, well pending holiday arrangements I guess, it’s not urgent right now.

OK, got it.

This is a long shot, but is it possible that you have 120V/208V service? It’s uncommon with 200A mains but can be the way some apartments and condos are setup when the primary service is three-phase… I wouldn’t mention it except that the initial numbers that you posted are somewhat possible in that scenario. So at a minimum, ask the electrician if its 120V/240V or 120V/208V.

Learnt something there - never knew 120/208 was even a ‘thing’, thanks! The meter states 240v 3w which is hopefully an indication that it is indeed 240V, but if when the electrician comes the CT’s all look correctly closed, I’ll double check.

Problem was identified as one CT not being latched closed. Actually it was a larger problem - it’s not possible to fit the AccuCT 200a 25mm ones in our panel due to the way the cables come in to the same side of the main breaker. It’s the large rectangular part that is the problem.

Is it possible to use any other CT for 200a, for example we think the EKM SCT-025-200UL would fit OK but not sure it has the same FSD so may need some maths to correct on the input?

A picture would help. What is the diameter of the cable?

Not any other CT but several 200A CTs are supported. I would need to see the datasheet for EKM SCT-025-200UL. If your cable is less than 19mm, the SCT019-000 might fit. You can get them from the OEM shop.

So to conclude, we finally got the original CTs fitted, but at the expense of not being able to put the panel cover on. At least now I can measure the load for a while to get the data I need. You can maybe see the problem from the photo - one of the CT’s protrudes a little which is why the panel won’t fit back on.

With both now closed, the 240V loads are now looking perfectly balanced, thanks for putting me on the right track!