# Yet another, how can I combine multiple CTs into one question

Sorry, I know these are common here, and I have read through all the ones that I could find, but still want some confirmation of what I’m thinking.

I’m in the US with split phase 120 V service. I have an HVAC system with outside AC condenser and natural gas heat. There is a true 240V (30 amp breaker and 10/2 wire) circuit for the condenser. Then there is a 120V (15 amp breaker and 14/2 wire) circuit for the blower.

I currently have a 50 Amp CT on one leg of the 240V circuit and am doubling that reading in software, but I’d like to measure the entire HVAC system including the 120V blower portion using only 1 input. I don’t have the option to run all 3 wires through one CT.

I can see that the 240V circuit pulls about 8 Amps when the AC is on (from the CT on it), this is backed up by the increase in whole house’s draw on phase A. Phase B is the one that also has the 120V blower circuit on it, and the increase in the whole house’s draw when the AC is running is at most 15 Amps. This is backed up by the fact that when the heat runs there is a 6 Amp increase on phase B, and no change on phase A.

So to review it looks like the AC draws: 8 Amps on A + 8 Amps on B + 7 Amps on B = 28 Amps. So I think I’d be ok with three 50 Amp CTs combined in a splitter, despite the nameplate capacity of the breakers (30 and 15) being too high. What is the worst case scenario if there is a sudden draw of the full 30 + 30 + 15 Amps the breakers could provide on 3 50 Amp CTs combined in a splitter? Would it just ruin the CTs? Or could it damage the IotaWatt too?

I have two ideas:
Idea #1
Use 3 CTs, one on each of the 3 hots, and a headphones splitter.

Idea #2
Run the 120V circuit (which is phase B) and phase B of the 240V circuit through one CT, and then the phase A portion of the 240V circuit through a second CT, then combine those CTs in a 2 way splitter.

So in summary my questions are:
Do you think I can get away with 50 Amp CTs or should I just do 100 Amp to be safe?
What will happen if I (perhaps greatly) exceed the current rating of a CT?
Do both my ideas above make sense, would either be better than the other? If they are both equally good I will likely use idea 2 as that require one less CT.

Hi, I’m in the UK where the majority of small installations; domestic & small retail are 240vac single phase. You might find this link interesting reading:

https://learn.openenergymonitor.org/electricity-monitoring/ac-power-theory/use-in-north-america?redirected=true

I haven’t tried to answer your questions as there are others here, including Bob of course, who are experienced in the USA electrical system.

Rgds, Steve

Either should work provided you orient the CTs appropriately for phase. I recommend before combining that you plug each CT sans the splitter and insure you are getting positive power.

As far as the capacity issue, the CTs should be fine, even in your worst case scenario. The reason for oversizing is to insure the combined input doesn’t exceed the ADC capacity. The unit is conservatively rated at 50mA. There is about a 15% overcurrent tolerance so the IoTaWatt can actually handle about 57A before it maxes out. Overcurrent beyond that should only result in the an incorrect low reading.

If the condenser starts drawing 30A or even 25, you will probably have bigger problems.

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