14/3 wiring

#1
09-15-06, 08:39 AM
Member
Join Date: Sep 2006
Posts: 11
14/3 wiring

I read a post and replies regarding the use of 14/3 wiring. All responses referenced utilizing separate phases for each load to eliminate overheating the neutral line. Although I understand how it could normally be a problem for most applications, i.e. feeding circuits, I don't see how it is a problem to run 14/3 cable to a fan/light combination. Since the wiring in the fan connects the black from the fan and the black from the light to the incoming line and shares a neutral I don't understand how using a 14/3 cable so I can run separate swith for fan and light would be a problem. Please let me know if I am off base on this. thanks.

#2
09-15-06, 08:57 AM
Member
Join Date: Aug 2004
Location: NA
Posts: 1,065
Received 1 Vote on 1 Post
The difference is that the 14 3 in the situation your speaking is serving a combined load that isnt over 15 amps total. In a multiwire 14/3 circuit with shared neutral each hot wire serves "different 15 amp loads" or at least the possibility... so you have the potential to place 30 amps or more on the neutral if both hots are coming from the same buss in the breaker panel. The neutral is 14 awg so it isnt capable of 30 amps only 15 amps in simple terms. This is why in a multiwire it is critical to use a double pole or two single pole breakers on different legs (busses) this has the effect of canceling neutral current in the shared neutral. In other words if I had 10 amps on one hot and 10 amps on the other hot the current will be zero in the neutral or if 10 amps on one hot and 5 amps on the other...5 amps will flow in the neutral the difference between the two loads. In contrast if both breakers are on the same buss serving different loads of 10 amps then 20 amps will be in the neutral....the current adds in this situation.

roger

#3
09-15-06, 08:57 AM
Member
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
You are not off base.

The use of a 14-3 cable for supplying power to a fan/light combination so that both can be separately controlled is done all the time. Similarily, 14-3 is used to provide switched and unswitched power in the same cable, such as to feed a receptacle with both switched and unswiotched power. These uses are all for a single 120 volt circuit that originates at the panel as one hot wire and one neutral wire and one ground wire.

The posts you read and are confused about are discussing using 14-3 (or more typically 12-3) for a multi-wire circuit. This use of three conductor cable (plus ground) does indeed require that the two hot wires be on separate legs of the incoming 240. When done properly, the maximum current on the neutral is the rating of the breaker, 15 amps (20 amps if 12 gage wire). This occurs when the load on one hot wire is zero amps and the load on the other hot wire is the maximum, 15 amps. The minimum current on the neutral is zero amps, and occurs when the current on one hot wire equals the curent on the other hot wire.

However, if a multi-wire circuit is improperly wired, then the maximum current on the neutral could be twice the rating of the breaker (30 amps if 15 amp breakers). This would be a very real fire hazard.

#4
09-15-06, 09:15 AM
Member
Join Date: Aug 2004
Location: NA
Posts: 1,065
Received 1 Vote on 1 Post
I believe that it would be safe to say that if we use a 14/3G NM-B (romex) multiwire to supply two 15 amp loads via 2 breakers on the same bus there will be 30 amps on the neutral...and the issue here is that neither breaker will trip, so the wire just overheats till you have a fire.

EDIT:

I should say...do not confuse this with what your doing.. as Bob has explained.. your situation is not the same, is safe, and is done all the time.

Roger

Last edited by Roger; 09-15-06 at 09:27 AM.
#5
09-15-06, 09:55 AM
Member
Join Date: Sep 2006
Posts: 11
Thanks for the responses. Much appreciated!

#6
09-15-06, 10:09 AM
Member
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
The 3-conductor cable in your "example" is being used in wiring a 2-wire, 120 volt, Branch-Circuit , protected by a fuse or circuit-breaker which limits the current to the ampacity of the cable conductors.

If the Neutral (White wire) is #14 wire = 15 amps, and the rating of the fuse/CB is 15 amps, you can't overload the Neutral of a 3-conductor cable connected at an outlet box to a 2-wire, 120 volt "Feed In" cable , Black & White wires, because the "live" Black conductor of the 2-wire "Feed-In" cable to the switch outlet box is connected to a fuse/CB rated at 15 amps.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Pingbacks are Off
Refbacks are Off