Replies

1. 
   cap | Jul 27, 2006 08:59am | #1

   It's an efficatious and safe arrangement.  It's the standard configuration for wiring a ceiling fan/light, or a bath fan/light, or a large light fixture with separately switched sets of lamps--separately switched supply conductors (hots) from the same circuit and a common return (neutral) on that circuit.

   Efficatious because it works; the shared neutral conductor provides a return path for current whether it's passed through the light or through the fan.  Compare the arrangement to a string of receptacle outlets on one circuit.  One neutral wire serves as the return path for all the current from all loads connected to any or all receptacles.  And it's efficient as well--running another neutral conductor so that the fan and the light have separate neutrals to the switchbox would be a waste of resources, because it's not needed.  

   Safe because the breaker protects both the hot and the neutral wires of the circuit.  The neutral can't carry any more current than the hot (or supply) conductor in that circuit, because the hot is the source of any current in the circuit. 

   In a properly wired circuit, the hot and the neutral conductors should be the same gage and be sized to the overcurrent protection device.  In other words, if the hot (or ungrounded) conductor is protected appropriately, e.g., a 15 amp breaker for 14 gage copper wire,  that neutral between the fan/light and the switch is safely protected against overload, short circuit, and ground fault conditions.

   There are situations where, due to miswiring, a neutral can carry more current than it's rated for.  This is dangerous.  It most commonly happens when a neutral is shared between two different circuits, both of which are supplied from the same pole or leg of a standard 240/120V service.  This miswire often results from someone rearranging circuit breakers in a panel, and putting the two breakers of a multiwire or Edison circuit on the same pole.  (A search of "multiwire circuit" or "Edison circuit" will get you several good explanations of this set up).  With the two breakers on the same pole of the supply, the common neutral carries the sum of the currents of the two circuits, which can lead to the neutral carrying twice it's rated current.  The wire overheats, insulation melts off, maybe you get a short or ground fault, and maybe a fire. 

   But the big difference in the situation you described is that the shared neutral is carrying current from two loads supplied by the same circuit.   

   Capisco?

   Cliff

1. 
   BarryO | Jul 27, 2006 10:22am | #3

   Given that the light and the fan are in the same piece of equipment, I guess 210-4(b) would apply(?).  In other words, you need a handle tie between the two breakers, to ensure that when you power down one, that you power down the other.

1. 
   cap | Jul 27, 2006 05:56pm | #4

   Barry,

   The two feeds to the switches are pigtailed off of one supply conductor in the switchbox, the way I read the original poster's description.  So we're only dealing with one circuit here.

   You're right on, if the two loads were being fed from two circuits.  That new (2005 NEC) requirement for handle-tied (or two-pole) breakers for multi-wire circuits.

   Cliff

1. 
   BarryO | Jul 27, 2006 09:33pm | #6

   Oops, you're right; 'shoulda read the post more carefully.  I guess when I hear 'shared neutral' I automatically think of multiwire circuit.

2. 
   peedee | Jul 27, 2006 08:34pm | #5

   Cliff and all, Yes, thank you for the insight and replies. You are correct these runs/ cables are on the same circuit, but I appreciate knowing what to do if they were not should I have the opportunity and experience to work in/ on such a situation. Once again thanks.David

2. 
   user-86517 | Jul 27, 2006 09:08am | #2

   It sounds safe to me too. All the neutral wires end up connected together at the panel anyway. There is no issue in sharing the return (neutral).

   --Andy

3. 
   MattMacBOS | Oct 15, 2020 01:20am | #7

   CAP - great response and explanation - thank you.

4. 
   unclemike42 | Oct 15, 2020 06:20am | #8

   since a single breaker feeds the whole thing, you are ok.

   the only issue to consider is that the remote three way switch would need a neutral if you wanted it to be a dimmer, or have a light on the switch to help you find it in the dark