2 Circuits, 1 Neutral, 2 Legs?

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aelectricalman

aelectricalman

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KY
A panel I went into today had this written in the (220/110) panel "Garage Doors circuit and lights circuit share a neutral and must be placed on seperate legs". Can someone please tell me what the reasoning is. Does it create a parellel inductance. If so, how? Why can't the shared neutral circuits both be placed on the same circuit?
 
Re: 2 Circuits, 1 Neutral, 2 Legs?

When the circuits are on opposite "phases," the neutral only carries the imbalance of current that is between the "hot" (ungrounded conductors). This priciple applies to any shared neutral circuit.

Say you have 10 amps on one circuit, and 5 amps on the other. The "neutral" will be carrying 5 amps.

Now, same circuit, but now both hots are carrying 10 amps, then these currents will cancel each other out and the neutral current will be 0 amps.

Take the same situation, but now consider the "hots" are on different breakers but the same phase. Each "phase" is carrying 10 amps. Now the neutral will carry 20 amps. If we are using #14 wire, the neutral will be overloaded.

It should be noted that almost every service in the United States is a multiwire cicuit of some type, either single phase or 3-phase. The service "neutral" carries the imbalance of current from the phase conductors.

[ January 18, 2005, 07:50 PM: Message edited by: peter d ]
 
Re: 2 Circuits, 1 Neutral, 2 Legs?

If the two ungrounded conductors of the three-wire circuit were supplied from the same panel bus, their load currents would add in the common neutral and overload it.

When they are supplied from opposite busses, the currents cancel, and the neutral only carries the difference.

Ed

[ January 18, 2005, 07:41 PM: Message edited by: Ed MacLaren ]
 
Re: 2 Circuits, 1 Neutral, 2 Legs?

Special rules apply to multiwire branch circuits:

1) Continuity of the grounded conductor cannot depend on the device (ie: neutral wires cannot be attached directly to screws on a receptacle).

2) If more than one ungrounded conductor is connected to a device on the same yoke or strap, the circuit must be fed from a 2-pole breaker or breakers attached with an approved handle tie.

I am doing this from memory, can anyone think of others?

Mark :roll:
 
Re: 2 Circuits, 1 Neutral, 2 Legs?

It is the best circuit used.

Roger
 
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