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Nestor_Kelebay 08-28-2009 06:04 PM

Sharing white neutral wires
OK, I've posted enough responses, now I'm posting a question.

In 1960, when my building was built, the heating system (boiler transformer, and water circulating pump) were on a 15 amp screw-in fuse in a fuse box in my electrical service room.

Since then, we've added an circuit breaker panel for the parking lot plugs in that same electrical service room that neighbors the boiler room.

In August of 2006, we replaced the old boiler with two new high efficiency boilers and an indirect fired water heater. Each boiler has it's own circulating pump, there's a pump to pump hot boiler water through the heat exchanger coil inside the water heater, and there's still the main circulating pump for heating the building in winter. So, there's a lot more load on that one screw in fuse.

Now, it seems that there's a problem with the undirect fired water heater that can only be corrected by replacing the water pump for the water heater with a larger one. The existing pump draws 2.5 amps, and the proposed pump will draw 4.5 amps.

There is concern that replacing the pump is going to result in the new heating system blowing 15 amp screw-in fuses, so we've run another circuit into the boiler room from the circuit breaker panel. I know how to free up the power going to the existing water pump on the original circuit. Also, I know how to wire the new power source to the boiler control so that the new pump runs off the power from the new circuit instead of the old one.

The white neutral wire from the existing pump joins all the other white wires from all the other 120 volt pumps and controls and connects to the white bus bar in the original fuse box.

My question is this:

Will it be necessary to ALSO trace back the white wire from the pump location, disconnect it from all the other white wires and connect it to the new white wire from the circuit breaker? Or, is it appropriate to leave it connected as is.

I'm thinking the white return lines from both voltage sources all get connected together inside the circuit breaker panel, and in the fuse box, so sharing white wires that way isn't a problem.

However, in this case, I'm sharing white neutral wires BETWEEN electrical panels. That is, the power going from the new circuit breaker panel will go through the new pump and end up going back to the original fuse box for the building, which is a different electrical "panel".

Obviously, the most rigorous way to do this would be to trace the white wire to the pump back, disconnect it from all the rest of the white wires it's pigtailed to, and connect it to the white wire from the circuit breaker panel. But, is it actually necessary to do that?

kok328 08-28-2009 07:02 PM

The sharing of the neutral wire can only be done when sharing it between the opposing phases in the load center and shared between two breakers not multiple breakers. I'd play it safe and separate it from the other neutrals due to the fact that we don't know if these are also on opposing phases in the load center. You don't want to add to the problem and surely don't want to cause problems with the pump circuit.

Nestor_Kelebay 08-28-2009 10:48 PM

I understand what you're saying, Kok, especially the part about not wanting to cause problems.

But the two hemispheres of my own brain are arguing over this one, and right now the other guy is saying: "Look, if all the red lines all ultimately connect to the red line coming into the building, and all the black lines ultimately connect to the black wire coming into the building, and all the white wires ultimately connect with the white wire coming into the building, then what difference does it make (provided you don't exceed the amperage rating of the panel) where you draw the power from and what panel's white bus bar you connect the white wire to, as long as it connects back somehow to that white wire in the main panel. As long as you connect between black and white or red and white, and don't exceed the amp rating of the panel, and you have a fuse on that circuit, why does the geographic location of the connections matter?

So, one half of my brain is telling me there shouldn't be a problem connecting the white wire to a different panel. And the other half agrees with you. And, the real difficulty I'm having is that half isn't coming up with any reason not to do it except that old time favourite: "Cuz no one else does it that way, so there's got to be SOME reason not to do it."

And, unfortunately, that arguement just doesn't make as much sense as the half I'm listening to right now.

After all, electricity moves at the speed of light, which is 300,000 kilometers per second. In the 1/120'th of a second it takes the black wire to go from +120 volts to -120 volts, the electricity has gone 2500 kilometers or 1500 miles. My building is barely 100 feet by 100 feet, so distances aren't going to come into play to cause timing problems. Any voltages in that white wire are going to arrive at the main panel at the same time regardless of which panel I connect the white wire to.

A similar arguement would be: If we had the main power cables coming into a main panel, and then added a sub-panel, where would be the problem in drawing power off the red or black voltage source of the sub-panel, and connecting the white to the white bus bar in the main panel? Or, vice versa? I just don't see any problem that would arise from doing that, so why would there be a problem if both were sub-panels?

Prolly tomorrow I'll be leaning the other way.

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