The neutral in a multi-wire branch circuit only carries the current imbalance, NOT the combined current of the two hots.

I assume you are in Canada?

Happy Canada Day BTW!!

When you add together things like the sine waves of voltage you have to know the phase difference between the waves.

The two legs are 180 degrees out of phase.

If you put equal loads on each 120 V leg, the resulting current in the neutral wire is zero.

The advantage of the 'Edison system' like this is that we can have 120 V to ground for safety, but between the legs get 240 V for larger loads.

Yes it was Edison that did the three wire service, but it had naught to do with sine waves nor 240V loads. It was DC and all the loads were 100V (the nominal voltage being 110 to deal with resistive loss). The reason for a +110/0/-110 distribution system was to save on wire. The entire system was a giant MWBS.

"...but it had naught to do with sine waves nor 240V loads"

We use the system since it allows for larger loads at 240 V while maintaining the safety of having a maximum of 120 V to ground.

It is true that it also works for DC systems, but only if the DC is configured in the same way.

Edison's chief problem with using DC was the inability to take advantage of higher distribution voltages to reduce line loss.

The lowest distribution voltage in the US is 7.2 kV.
It runs up to almost 500 kV for a very few long distance lines, and up to 1 MV for some special DC lines (we now can convert the DC back to AC using SCRs, large stacks of SCRs).

I'm a bit confused. if I plug in a toaster and a kettle in the split receptical, the common doesn't carry both loads? Am I missing something? and yes I'm in Canada. thanks for the fast reply's. I'm glad I found this sight. thanks again

basically the toaster will return through the kettle. Any difference in amperage between the two will be carried on the neutral wire. Just like the whole house, 3 wires come in (all the same size, sometimes even smaller for the neutral). One leg carrying 80 amps and the other carrying 95 amps (just an example), the neutral only sees 15 amps. In that scenario the neutral is allowed to be smaller because it is unlikely that 200 amps would ever be used on one leg with 0 amps being used on the other leg (200 amp service in example). In your 15 amp scenario, the most your neutral will see is 15 amps. If some moron were to move breakers around so that the black and red are on the same phase, the neutral code potentially see 30 amps. That would not happen since you have to use a double pole breaker anyway. Don't forget to check GFCI requirements, if GFCI protection is required, you will have to use a double pole GFCI breaker.

Thank You Spencer. The panel I'm using is a stablock and it is possible to put a double breaker in this make of panel on the same leg. I found it out when I hooked up an oven/range and the oven wouldn't heat (but the clock worked). I'll have to make sure that I put the breaker in the right position in the panel. Sounds like that type of panel needs a safety upgrade. Thanks so much

> The panel I'm using is a stablock ...

Change it. I wouldn't knowingly sleep in a house with one of those panels.

Here is a link that might be useful: FPE Stab-lok Panels

For some reason, the FPE panels in Canada are not the same problem as the are in the US. I believe they are still newly installed to this day.
Maybe one of our regulars from Ca can shed some light.

Oops, sorry, I didn't see that he was in Canada.

I've read too that the Canadian FPE panels were OK. No idea what the difference was / is.

"I've read too that the Canadian FPE panels were OK. No idea what the difference was / is. "

The panelsare not so much a problem as the breakers,

FPE in Canada is Fereral Pioneer as oppposed Federal Pacific in the USA.
FPE is still a very popular panel in Canada. They were made by a different company than the ones in the USA. Schneider Electric makes them now.