NOT SAFE? Losing a Neutral in a Multiwire-Branch Circuit

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Have you ever wondered what would happen if you removed the neutral wire on a multiwire branch circuit? Would it be as simple as your toaster not working or would it be something much worse? In today’s episode of Electrician U, Dustin explains what would happen if you did.

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In a normal 120v branch circuit, I think we all know what would happen if we removed the neutral from the circuit. Since current flow needs a complete path from source, through the load, and back to source, the circuit would NOT function if the neutral was removed. It’s also helpful to understand that the neutral conductor carries the full load of the circuit in a normal branch circuit.
On a multiwire branch circuit things are very different. A multiwire branch circuit is where you have multiple phase (hot) conductors that share a common neutral. The neutral conductor carries only the unbalanced load between the 2 loads. There are rare cases where the 2 circuits are drawing the EXACT same amperage and the neutral conductor has ZERO load on it, but again usually NOT the case. If you were to remove the neutral conductor on a completely balanced set of devices in a multiwire branch circuit, you are in essence adding the loads together in series. Yes you have double the voltage, but keep in mind that you have double the resistance working against it. Actually, things would function as normal.
But if the loads were unbalanced (as they almost always are) things could get ugly! Let’s say we had an 1800w toaster on one 120v circuit (1800w, 15a, 120v) and a 600w TV on the other (600w, 5a, 120v). The neutral conductor carries the unbalanced load between the two, so it would carry 10a and everything functions normally. If you were to take that neutral away, you would in essence have a complete 240v circuit with 2 different sets of loads in series. The resistances would need to be added together (as is the case in series circuits) which for our example would wind up being 32 ohms. Dividing the voltage (240v) by the 32 ohms we would end up with 7.5 amps (and in series circuits, amperage is equal among the loads). Again, it is helpful to understand that in series circuits, amperages are added together to make up the total amperage of the circuit (for our case it is 240v with the neutral removed).
So, taking the 7.5 amps and applying the INDIVIDUAL resistance to the equation, we can start to see how damaging removing the neutral would be. Our 1800w toaster now is operating on 60 volts (7.5 x 8 ohms of resistance) and our 600w TV is operating on 180 volts (7.5 x 24 ohms of resistance). The toaster is not receiving the proper voltage to generate the 1800 watts of heat so it would not get very hot and is under performing. BUT, the TV is operating at much higher voltage than it is designed for and will most likely be damaged as the circuit boards would probably fry, things may melt, etc. Using the power wheel equations we can finish painting the picture of what our equipment is actually doing. Our 1800w toaster is now consuming less than a third of what its supposed to at 450w (60v squared divided by 8 ohms of resistance) and our 600w TV is consuming more than double of what it is designed to at 1350w (180v squared divided by 24 ohms of resistance). While the toaster is not suffering quite as much damage as our TV (again, the toaster will not get hot and may not last as long as it’s struggling to produce the heat without enough resources to do so), the TV will burn up quickly as it’s being pushed entirely too hard!
We hope this has been helpful in understanding the loss of the neutral conductor on a multiwire branch circuit. Is there a topic you would like to see discussed on Electrician U? Leave us a comment in the comments section and let us know. Please continue to follow Dustin Stelzer and Electrician U as we are constantly updating our content to assist our followers in becoming the best electricians that they can be.

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