Hi All - when I put the CT around a live feed from the meter board that has more than 1 wire, it reads zero. The same CT put on any single wire is fine and reads correctly. Both are live feeds from 1 fuse box switch and go out to common lighting circuits. It should just read the addition of both wires right? (single phase, same phase, both live 230V etc). The CT is correctly closed around them- they’re only lighting feeds so not thick.
Welcome, Matt, to the OEM forum.
Something you’ve written there is clearly wrong, even though you believe it’s correct. It’s time to question everything.
As I think you understand, a wire carrying current generates a magnetic field around it. The c.t. measures this magnetic field. If you run two wires through the c.t., the fields add algebraically. If the currents in the two wires are equal and in phase, the fields will add, if opposite in phase, they’ll subtract and cancel completely. If they’re carrying almost the same current but out of phase, you’ll get almost no resultant field. You’re absolutely right, two wires carrying current in the same direction at the same instant (as it’s a.c.) should add their fields and the c.t. should see the sum current. And I know it works - take one wire through the c.t. with a 2-turns coil and you’ll read double.
To clarify what you mean here, you have one circuit breaker with two outgoing feeds. One feed to one group of lights (upstairs?) and one to another group (downstairs?), with their respective neutrals returning to a common neutral busbar? Other than the common neutral, the two circuits are completely separate? If you pull one wire out of the breaker, that group of lights doesn’t work, and likewise with the other?
So, who has done something wrong in your house in the past? would be my line of thinking. I suspect one of your live feeds is the line and the other is a neutral. Could this be the case?
The fallback theory is a broken wire on the output side of your c.t., which makes contact when it’s on a single wire and doesn’t when the c.t’s in a slightly different place on two wires. This is the Law of Natural Perversity in action, out to confuse both of us.
If you prove neither of these is the case, I’m completely baffled.
Hi thanks for the reply, yes it is two live feeds from 1 breaker, so one can’t be neutral or the breaker would blow…
It should indeed work so I’m taking a close look at the CT. Works fine on other lines but hasn’t had a jiggle yet…
You’ve got sight of both wires all the way from the breaker outgoing terminal as far as the c.t? I thought if somebody had maybe cut a wire short and used the black/blue as line and the red/brown as neutral for one circuit, and you’d put the c.t. on the wires of the same colour? I’m in super-cynic mode for this one - it’s weird.
To give you a guide, I’ve checked the resistance I see on the c.t. plug for the 3 “Shop” c.t’s I have - 10 Ω (100 A), 19.2 Ω (50 A) and 17.3 Ω (25 A). My guess is the 20 A one won’t be much different.
well, err, it helps if you put it on the right wire pair!!
Doh - end of problem.
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It seems I was right first time. A few 
for you.