Remember that in a "properly" loaded 120/240V system, which means that the 120V L2 loads are equal to the 120V L1 loads, then the net current in the mains neutral line will be zero. Why? Because the voltages (and the currents) of L1 & L2 are 180° out of phase. So when the current in L1 is at its positive peak, the current in L2 is at its negative peak, and they cancel each other out. When the current in L1 is at 90°, the current in L2 is at 270° - always exactly opposite.
Does that make sense to you? It's hard to envision without graphs, but good if it does.
Now for the fun part! You can almost never have perfect loading between the two! So even if you have "matched" loads on each, there's always going to be something a little out of whack which will unbalance things..
But seriously - any time you have a 120V load, you're "unbalancing" the system, and you'll end up with net non-zero current flowing through the neutral. But if you happen to have those "magic" matched 120V loads on L1 and on L2, you'll still have current flowing through each branch's neutral lead
, but when they meet up at the AC panel's neutral buss bar, the net current on the mains neutral back to the shorepower/genset will be zero
because they're exactly out of phase and cancel each other out. All your net current flow is between L1 and L2.
The only reason you'd need a third current transformer is if you have any 240V loads - like a heavy duty watermaker
, a scuba compressor
, large clothes dryer, etc...
The "fun" part mentioned above is where you begin to consider what's known as the power factor
(pf). There's a decent writeup over at Wikipedia
on it. Remember back in advanced math in high school
when you learned that the square root of -1, usually called i
, was an imaginary number? Well, that "imaginary" is actually very real, and comes into play when we're messing with alternating current - and inductors and capacitors. Inductors resist changes in current, and capacitors resist changes in voltage, so they're in essence "opposites", and you'll see when talking about power factors the terms inductive loads and reactive loads, and loads with a bit of each. But this stuff isn't needed to have a passing knowledge good enough to wire an AC system - just follow the ABYC recommendations and you'll be fine. If you want to dig in and actually understand the theory behind the recommendations - even better!
So, yes - install the two current transformers on L1 and L2 as indicated. Since you don't have any 240V appliances
, you don't need the third one. Remember that the xformers are polarity sensitive. When the electrician installed mine, one was backwards, so my panel's amp meter was measuring the difference between L1 and L2 rather than the sum - it took me a second to figure it out when the amps displayed on the meter dropped
when I switched on more AC loads!
I hope I haven't misstated anything here - it's been almost 30 years since I learned AC theory as an EE undergrad...