The Dangers of Multi-Wire Branch Circuits
Introduction
Multiwire branch circuits have the following advantages:
More cost effective.
Fewer conductors required for a given number of circuits.
Smaller raceways required.
However, extreme care must be taken to ensure safety is not compromised.
The element of multiwire branch circuits that makes them potentially dangerous is the “shared neutral.” When sharing a neutral between two circuits, you must remember to use opposing power bus connections at the source. In other words, always make sure to use L1 + L2 + shared neutral in order to cause the two currents to subtract (or cancel each other out) and produce a net neutral current of zero. This is called a “balanced” system. NEVER use either L1 + L1 + shared N or L2 + L2 + shared N. Using “non-opposing hots” will cause the two currents to add and, thus, create a neutral current which could exceed the ampacity of the neutral conductor (possibly causing a fire).
Normal Neutral Current
The desired condition if two circuits must share a neutral is for the load currents to “subtract” and then return “zero amps” to the source through the neutral (N) conductor. This occurs when the loads connected between L1 and N vs L2 and N are balanced (or L1 and L2 currents are equal and opposite in polarity). If the loads are different (or unbalanced), a “neutral current” is created since the difference is no longer zero amps. Excessive imbalances will create excessive neutral currents, which can overload the current-carrying capacity of the neutral conductor (possibly causing a fire).
NOTE: The peaks of each AC (Alternating Current) sine wave at L1 and L2 are always 180 degrees out of phase with each other. That 180-degree phase-shift has everything to do with why there is 240VAC difference in potential between L1 and L2 at the positive and negative peaks of each separate sine wave. Since the neutral (N) is at the center tap on the incoming power transformer secondary, the L1 and L2 voltages to neutral (N) are both 120VAC. Remember: AC (Alternating Current) can only flow in one direction at a time…positive for the first “half-cycle”…negative for the second “half-cycle.” If current flows into L1, then it must flow out of L2, and vice versa.
Open Neutral
Let’s assume two “correctly wired” circuits are sharing a neutral and the loads (or appliance) on each circuit is different (i.e. a Television and a hair dryer) or unbalanced loads. Each load has their own “hot” wire for power. The L1 to N load receives 120VAC. Likewise, the L2 to N load also receives 120VAC. However, if the neutral becomes disconnected for any reason while power is applied to both circuits, the two “unequal” loads which once shared a neutral are now in series with each other between L1 and L2. This increases the total series voltage to 240VAC and that series voltage will divide based on the “unequal” internal resistance of each load. The sum of the two “unequal” voltages is still 240VAC. However, since the voltage drop on each appliance is based on its internal resistance (which does not change), the new voltage across each load will drastically increase on one load and decrease on the other. So both appliances are no longer operating at their “design voltage.” Furthermore, the appliance operating at the higher voltage (the television) will experience an astronomical increase in dissipated power and will likely burn up almost instantaneously. The other appliance operating at a lower voltage (the hair dryer) could also receive damage to the motor windings due to the effects of under-voltage causing excessive heat.
NOTE: Always use an additional pigtail when connecting any device to a shared neutral connection to avoid opening the neutral conductor path while power is ON, thus, preventing an “open-neutral” condition which can cause a fire.
Because of the dangers associated with shared-neutral circuits, I avoid this wiring method whenever possible. The reason is because you never know if the next person to work on the circuit will be aware and take the necessary precautions. However, if the situation requires its use, then precautions must be taken to ensure safety and prevent fire hazards. It is always a good idea to include safety notes on electrical drawings with circuits using shared neutrals. Finally, ALWAYS clearly AND permanently mark circuits that share neutrals AND bundle those circuits in the panel using wire ties.
