Multi-wire branch circuits can help to increase available energy and reduce costs, and proper installation can increase safe usage, but the hazards they entail may make them cost-prohibitive in the long run.
There’s a reason multi-wire branch circuits exist. These electrical conduits, consisting of two or more ungrounded, or hot, conductors connected to a single-grounded, or neutral, conductor help to facilitate greater load at less expense. Say, for example, you wanted to plug in multiple devices that suck up a lot of energy.
With a multi-wire system, you could get double the power (or more) to electrical outlets, allowing you to power myriad electronics and appliances without the fear of blowing the breaker when you operate more than one at a time. Normally, you’d have to set up multiple independent circuits to make this happen, including multiple ground wires, and that would entail a greater expense than simply splitting a system with a single ground wire.
Unfortunately, there is high danger when using multi-wire branch circuits. If the system isn’t set up correctly and the load isn’t split evenly between the two circuits, one of the hot wires or the shared neutral wire could end up carrying much more of the load than intended, and this can be extremely dangerous. So, what are the dangers surrounding multi-wire branch circuits? Can this system be safely implemented?
Why are Multi-Wire Branch Circuits Dangerous?
The presence of unintentional voltage could result in increased risk of shock (especially for electricians working on the system), but there are also dangers for users. In multi-wire branch circuits, the ungrounded conductors are out of phase and the ground wire facilitates sharing the load so that the voltage is equalized. Any mistake in setting up the system could lead to one portion having more voltage and the other having less.
When this occurs, certain electronics could experience lower voltage while others get a much higher dose. At the very least, this could lead to electronics that don’t function properly, but the more dangerous aspect is the potential for electronics to overload and overheat as they receive added voltage and wattage that they are unequipped to handle, leading to ultimate (and irreversible) failure of the device. Even instances of minor overload can add up and reduce the life of the electronic device.
The greatest danger, of course, is the damage users won’t notice occurring—the damage to the neutral conductor. Overloading can wear material components, including insulative materials, possibly resulting in arc-fault fires that originate in unseen areas throughout the home. This type of house fire could be devastating.
Is There a Way to Safely Use Multi-Wire Branch Circuits?
The National Electric Code (NEC) has provisions pertaining to the safe and proper usage of multi-wire branch circuits, notably section 210.4(B), which states: “Each multi-wire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates.” The best means of complying with this safety regulation is the use of a handle tie applied to two single-pole breakers.
There are dangers inherent to using multi-wire branch circuits, even when properly installed with required fail-safes. The NEC, however, does have guidelines in place to make them as safe as possible. The best way to avoid the hazards of using multi-wire branch circuits, of course, is to simply spend the money to install multiple grounded conductors to handle energy demands.