It is my desire to totally disregard my center tapped conductor (3rd wire) of my split-phase mains for the purpose of a PV diversion here in the USA… My diversion loads are purely 240vac single phase.
Why would my PV surplus measurements require having two c.t.s to operate while this is commonly done for PV Diversion in the UK with just 1 c.t.?
With just one CT you can’t accurately measure your 120V loads but since they make up part of your consumption, you presumably want to include them in your balance calculation.
No,no, no, I’m diverting surplus here and the diverter wants to know, I don’t care myself what consumption is. Just want to get the diversion job done.
And for the record, my loads are remarkably balanced always and I’m not sweating any small random unbalance either.
But if the diverter is only looking at one leg (one CT) it won’t see any power being consumed by the 120V loads on the other leg. That power will flow back through the centre tap neutral.
In the UK case all the power flowing into/out-of the property is passing through the cable the CT is wrapped around so the diverter has all the information it needs. In your case the diverter will have no knowledge of the power being consumed by the 120V loads on the other leg.
He mentioned his diverter loads were 240 Volt devices. Viz:
In which case, a single CT should suffice, right?
I don’t think so. If you go back to the fundamentals: a diverter aims to keep the net power flow into/out-of the property at 0 right? If there are 3 wires coming into the property, then you need two power meters to measure that, compared with the UK case where there are only 2 wires coming into the property, so you only need one power meter.
Quite right. But…
A 240 Volt load on a US split-phase supply uses only two of those three wires.
i.e. just the two hot legs, no neutral.
After thinking about it for a bit, I remembered I have one of Robin’s prototype diverters.
I used it for only about 9 months while waiting for a net meter to be installed.
I ran my excess PV energy into an immersion heater in a 55 gallon drum of water as a dump load.
(4500 Watt, 240 Volt element) It used only one CT to measure the diverted energy, and reported
said energy on a 7-segment LED display.
(that was almost 5 years ago. Tempis sure Fugits when yer havin’ fun! 
)
Maybe I’m missing something, but I’m struggling to see how that can work assuming your house has some 120V loads as well.
Imagine it’s all nicely balanced and then somebody turns on an 800W 120V toaster. Presumably you now need to reduce the diversion load by 800W to keep things balanced, but if that toaster is on the un-monitored leg, how will the diverter know to do that?
That part I can believe. If your diverter load is 240V then you only need one strategically placed CT to measure it. But that’s not sufficient to keep the house balanced right?
They almost never are anyway. The arrangement of the breakers in the load center helps,
but only to an extent. Unbalanced operation in a typical US residential load center is par for the course.
The OP says his loads are “remarkably balanced” but if that’s the case, it’s very rare, and it’s not
that way all the time.
It’s kind of like the kWh meters we use on our split-phase system. They only have four terminals.
Two inputs and two outputs. The neutral line doesn’t run through the meter.
Sorry… I didn’t mean that sort of “balanced”. I meant balance between PV output and all loads (the diverter load plus all other loads including the toaster that comes and goes). So let me re-phrase…
Imagine the diverter is working brilliantly and there is no power flowing across the property boundary, then somebody turns on an 800W 120V toaster. To keep the power across the boundary at 0, the diverter now has to reduce the diverter load by 800W. What will inspire it to do that if the toaster is on the un-monitored leg?
Yes, but recall your exploded meter does have two CTs in it. They’ve taken a shortcut with the V measurement but they could never get away with that for the I measurement. It’s pretty much 2 power meters inside that thing… they’ve just made assumptions about V.
OK, gotcha.
In that case, yes, two CTs are needed. Three, if one wants to measure the diverted energy as well.
I concur.
The question I’d ask there is: Over what time period are your loads remarkably balanced? If you’re talking in terms of substantially less than the period of the diverter’s cycling rate (which will be dependent on the amount of energy being diverted but is in the order of seconds), then it might just be valid to assume that you don’t need a second c.t. But if it’s over a longer period - between your supplier’s meter readings say - then that assumption would be wholly unjustifiable.
There’s an easy way to see your imbalance. Put a c.t. on the neutral and record the current. If it is greater than negligible (define that as equivalent to the power you’re prepared to export or import when the diverter is operating) for more than a few seconds, your loads aren’t balanced.
Great follow up responses!
I have a greater understanding now.
Thanks to all whom replied.
I agree with all of the advice that’s been given here. dBC’s scenario with the 120V 800W toaster gets right to the heart of the matter. When the toaster is turned on, the diverter needs to reduce the consumption of its load by a similar amount because the amount of surplus power as measured by the utility meter will be 800W less than it was before. It can only do this by monitoring the current in both ‘hot’ legs for which two CTs are required.
To accurately monitor the power that is consumed by 120V appliances, both halves of the supply voltage would be needed to be measured. But because the utility meter only measures the 240V AC voltage - and assumes that both halves are equal - a diverter of surplus power can do likewise.