A material and man-hour (read MONEY) conservation technique I have noticed more and more recently is called multi-circuit wiring. The purpose of this technique is to save money on wires and man-hours by pulling only one wire for two circuits.
Seems like a good idea and there are definitely a fair amount of contractors that agree. During normal operation of most circuits no significant problems will be noted. However if too many watts are added, the circuits could be prone to overheating.
The issues come from the neutral wire. With a single circuit wire there is a black wire for the hot, and a white wire for the neutral. For simplicity sake let’s look at these as the supply=hot(black) and the return=neutral(white) for the current respectively.
A 15amp circuit should use a 14 gauge wire for supply and return. That means that if the maximum amount of power (allowed by the breaker) is called for, the wire on the supply and on the return are both thick enough to hold all of 15 amps or 1800watts at 120 volts. [(15amps)x(120volts)=1800watts]
Now let’s save some time and materials during installation by running a multi-circuit wire. This wire has two supplies and only one return. Remember each one of the 14 gauge wires are designed to hold 15 amps but now you have two wires that could be called on to hold all of 15 amps and only one 14 gauge return/neutral wire for 30amps! (15+15=30amps) Normal alternating current modulation will prevent the neutral from having to carry both loads at the same time but if something goes wrong and the neutral does experience over current in this manner the breaker will not trip, because breakers only protect the hot/supply wires.
The deal is the authorities that say this is ok are betting that there will never be a situation that causes both wires to call for the entire load at the same time. As a home inspector here in Salem, Oregon I am paid to be a “worst case scenario,” guy. It is my job to alert my clients to not only issues but also potential issues and educate them as best I can.