Wiring method-common neutral

[rich000]

I have been seeing this wiring method used in alot of the new homes being built in Northern VA.

Two 15A circuits being fed from the panel via a 14/3 wire.

An example would be that the 14/3 would run to a room. The black wire would power the lights and the red would power receptacles.

I know a common neutral is allowed, but using 14AWG, isn't the ampacity only 20A (table 310-16)?

Isn't the potential for 30A to be on that neutral without the breakers tripping?

Thanks.

 

[Ed MacLaren]

Re: Wiring method-common neutral

The two circuit currents do not add in the neutral of a properly installed three-wire circuit - they cancel.

Generally, multi-wire circuits save materials, labor, and energy, but may not be advised for loads with high harmonic content unles the neutral wire is oversized.

For the same load current, distance, and wire size, the losses are reduced, and power delivered is increased, by using multi-wire circuits.

(The values in the sketch are chosen for ease of calculation.)

Ed

[ August 29, 2003, 08:39 AM: Message edited by: Ed MacLaren ]

 

[gregoryelectricinc]

Re: Wiring method-common neutral

Perfect, I love your diagrams Ed.

 

[websparky]

Re: Wiring method-common neutral

Nice drawing ED

The neutral in Ed's first drawing is refered to as a "shared" neutral. A "common" neutral is a horse of a different color!

[ August 29, 2003, 07:21 PM: Message edited by: websparky ]

 

[george t. everett]

Re: Wiring method-common neutral

What's the differance between shared & common?

 

[bennie]

Re: Wiring method-common neutral

Shared neutrals are applied to active conductors in different time domains.

Common neutrals are applied to conductors in the same time domain.

Simply stated; Shared is part time. Common is forever