We were doing some wiring in my son's 95 year old house (complete rewire in the 60s) and found a common neutral where the 2 legs were fed from 2 breakers on the same phase. Of course we move things around.
You know, I couldn't understand why so many people say make sure multiconductors are on opposite 'phases' to protect the neutral conductor. Figuring, ofcouse they will be opposite when using double pole breakers. So, I wrote off the warning.
But as time goes on I hear from electricians they run into many times homeowners tossing in two breakers, and only using the spaces provided. Somtimes not having the breakers next to each(vertically) but spread out in the panel. I've even heard about one homeowner putting in mini-tandem breakers, and using that for a three conductor cable.
So very good advice!
later,
tom @
Yes, that's true, but you're talking about resistances measured in tenths of ohms, in series with resistances measured in hundreds of ohms. As a practical matter, except on *very* long runs, the resistance of the conductors is not really a concern.
I'm *not* ignoring the neutral. But you're missing the fact that the voltage drop between the source and the load is almost completely independent of the resistance *elsewhere* in the circuit. The only effect that the resistance of the neutral has on the circuit is a *minuscule* difference in the current that flows through it.
The effect on voltage drop is still so small as to be utterly negligible.
Do you know just *how* low the resistance of copper conductors actually is?
I don't think I ever said it wasn't "real". But it certainly isn't important.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
Ouch. Good catch!
It's perfectly fine, as long as both of these conditions are met: a) the two hot conductors are on *opposite* legs of the service, and b) both hot conductors are disconnected by the same device. Both requirements are easily met by using a standard double-pole (240V) breaker.
That'll definitely be easier in the long run, and less confusing to the next homeowner.
-- Regards, Doug Miller (alphageek at milmac dot com)
Nobody ever left footprints in the sands of time by sitting on his butt. And who wants to leave buttprints in the sands of time?
Just what he said it does. A multi wire branch circuit will operate all but the imbalanced portion of it's load at 208, 240, 480, or what have you rather than at 120, or 277. Using a single phase multi wire branch circuit supplied from a single phase 120/240 volt center tapped service with twelve amps on one leg and fifteen amperes on the other the neutral will only carry the difference or in this case three amperes. On a circuit that is fifty feet long the three amperes has a voltage drop of the round trip distance of one hundred feet the twelve remaining amperes from both legs will have the voltage drop of only fifty feet because it is using the other ungrounded conductors load to complete the trip. You get the same amount of work done for roughly half of the voltage drop.
-- Tom Horne
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