Balancing Breaker Box Circuits

Need help from the brain trust.

I've always heard you should balance 120v loads in a U.S. breaker box between the two phases coming in, and that balancing loads reduces the current through the neutral.

Got a friend who has a situation where he has two 15 amp motors on fans which he may run one or both. He wanted to use 12/3 w/ground and run each motor from separate legs of the box, basically run them on a 240v circuit but run them back on the neutral. He was concerned 30 amps would overload the neutral. I told him with two identical fans running on separate legs there would be practically zero current on the neutral.

Is what I'm saying actually true? I was basing the idea off of balancing a breaker box to limit neutral current.

The neutral isn't a concern. You try to balance the draw of electricity on both the legs of power coming in so you don't overload one while the other isn't doing much.

(quoted from post at 19:17:59 06/24/19) The neutral isn't a concern. You try to balance the draw of electricity on both the legs of power coming in so you don't overload one while the other isn't doing much.

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That's how I've always understood it.

You are correct. If both loads are perfectly balanced, there will be zero neutral current. And if the loads are unbalanced, worst case the neutral current will be the same as the current on the leg with the greatest current.

Yes, you are correct. The two breaker handles
should be tied together so they are both shut off
at the same time. This reduces the risk of getting
shocked by a "live" neutral when servicing one
motor.

(quoted from post at 19:24:17 06/24/19) You are correct. If both loads are perfectly balanced, there will be zero neutral current. And if the loads are unbalanced, worst case the neutral current will be the same as the current on the leg with the greatest current.

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Thanks. I'm not sure the other posters understood the question, that being "how does balancing the two legs affect the neutral?".

Our garage was wired that way when we bought the place, 12-3 wg feeding it, 2 circuits, but the breakers were not tied
together. When I added a 3rd stall (my shop) I ran a new 100a circuit to a breaker panel and made it right. It was a cheap way
to put 2 separate circuits in a garage.

You are correct. The load on the neutral will never be higher than the difference in the two loads.

this is called multiwire branch circuit and code requires the two breakers be equipped with a handle tie. that assures that both are disabled if turned off for service.

reference:
https://www.twielectric.com/safety-and-energy-saving-tips/national-electrical-code-multiwire-branch-circuit/

Andy, another great electrical question (they draw the most responses). In a nutshell and for most cases (my short answer) YOU ARE CORRECT. Any time I attended NEC Seminars and other training and in my years designing power distribution systems BALANCING LOADS AMONG DIFFERENT PHASES WAS A MAJOR DESIGN CRITERIA. Since the two legs in a 120/240 Volt Single Phase Three Wire service are 180 out of phase from the other, if BOTH legs are drawing the same current NEUTRAL CURRENT IS ZERO. Anytime current is carried in a conductor there are I Squared R Heat Energy losses, so if there's no current those are avoided.

THAT BEING SAID there's more to the story!! While the above holds true for resistive loads, if the loads were inductive and if the inductance isn't the same on both legs the circuits may not actually be quite "perfectly" balanced. When I considered utilizing what are referred to in the trade if I recall correctly "Multi Wire Branch Circuits" (single common Neutral for two legs) if there were complex multiple (never the same) highly inductive loads I was trained to avoid their use.

All in all and even though what you spoke of sure will "work" and your basic premise (if loads are balanced Neutral current is zero) is true, WHEN DESIGNING MOTOR CONTROL BRANCH CIRCUITS in our shop and in the training I received multi wire branch circuits where a common Neutral was used for two legs wasn't favored.

But don't let this discourage you, a couple small motor loads isn't all that much and any such resulting minor mis matches isn't gonna break the grid lol...........What I was talking about was bigger and more complex power distribution NOT a couple small motors in a shop.

John T

Thanks for all the great responses, guys!!

Electricians that tie this into the panel must understand staggered phasing or they will have two 3 wire circuits on phase A and end up doubling the neutral current. One must be phase A and one must be phase B . In all cases including 3 phase A, B C . no 3 wire will be fed from two of the same phase. Yet every electrician has seen it wrong.

John T, I always appreciate and respect your knowledge on all things electrical. I agree with everything you state about circuit balancing. I could be wrong, and I am not an E E, but it is my understanding that typical split phase 240 residential service does not have 2 phases 180 out, but a single phase of the secondary transformer winding tapped at both ends (each end is a hot leg) and center tapped (neutral). This is why it is typically referred to as 240 v split or single phase. I am going to paste an explanation of this from the website "all about circuits". I am certainly willing to be corrected if I am wrong.

Split-phase Power System

Instead of a single 240-volt power supply, we use two 120 volt supplies (in phase with each other!) in series to produce 240 volts, then run a third wire to the connection point between the loads to handle the eventuality of one load opening. This is called a?split-phase?power system. Three smaller wires are still cheaper than the two wires needed with the simple parallel design, so we?re still ahead on efficiency. The astute observer will note that the neutral wire only has to carry the?difference?of current between the two loads back to the source. In the above case, with perfectly ?balanced? loads consuming equal amounts of power, the neutral wire carries zero current.
Explanation of split phase 240 v Service