The “shared neutral” is the principle by which both 240V and 120V are provisioned on the same cable for the service entrance in North American homes. Ordinary receptacles can also be wired in this manner, providing twice the power from each outlet pair. This allows you to operate, for example, a table saw and a shop-vac simultaneously, without tripping the breaker. It also prevents one tool from drawing down the voltage that is available to the other.
Furthermore, it allows a combination of 120V and 240V receptacles to be installed on the same cable. If you decide in the future that you want to upgrade to a 240V tool, you can simply replace the 120V receptacle with a 240V one.
These are big advantages in work shops, where high amp loads may be used, and where a combination of 120V and 240V receptacles may be desired. It is more economical than installing many dedicated circuits, and provides a great deal of flexibility to accommodate future needs.
An ordinary 120V circuit has a ground, neutral and hot conductor. With 12 AWG wire, it can deliver 20 amps.
A shared neutral circuit has an additional hot conductor, which is connected to the opposing phase. This allows a further 20 amps to be delivered, with the addition of only one conductor. There will be 240V between the two hot conductors, and 120V between either hot and neutral. The neutral conductor only has to return the imbalance between the two hots, so that for example:
- If one outlet is delivering 20 amps, and the other 0, then 20 amps will return on the neutral.
- If both outlets are delivering 20 amps, then 0 amps will flow on the neutral.
Therefore we see that it is impossible to exceed 20 amps on the neutral, even though the circuit can deliver a total of 40 amps at 120V.
Step 1: Install 12-3 Romex
This cable includes a ground (bare), neutral (white), and two hots (red and black).
It is also OK to use larger 10-3 cable for 20 amp receptacles. This will reduce resistive loss on the cable, improving the performance of high-torque tools. However, 10 AWG costs more, and is also more difficult to fit neatly into a single-gang box, so I chose to use 10 AWG only for a couple of dedicated circuits, and not for all receptacles in the shop.
Step 2: Install a Two-pole 20 Amp Breaker
A two-pole breaker is required by code, for several good reasons. It ensures that both hots:
- are on opposing phases, so it's impossible to overload the neutral.
- will be tripped in the event of a fault on either leg.
- are deactivated when the breaker is manually opened for service.
Step 3: Break Off the Tab, and Install Receptacle.
Receptacles have a tab between the top and bottom terminals, which can be broken off to allow them to be wired separately. Usually the purpose of this is to allow one outlet to be switched separately from the other. In this application we will be wiring the top and bottom outlets to opposing phases.
Break off the bridging tab ONLY on the hot side.
Step 4: Caveats:
There is a risk with shared neutral circuits that should a break occur in the neutral line, unpredictable voltages (between 0 and 240V) will occur at the 120V outlets. For this reason, great care should be taken to ensure that the neutrals are firmly connected at every point in the circuit. In this application, I avoid the use of any wire-nuts on the neutral conductor. I recommend Leviton 5352 heavy duty receptacles, which provide a mechanically solid, clamp-down connection for up to four neutral conductors.
Also, because the box will be a little more cramped with the added hot connection, it is best to avoid having more than two cables coming into each box. Arranging the receptacles in a simple daisy-chain path will keep the wiring orderly and easy to connect, with no need for wire nuts.
The presence of a shared-neutral circuit may be confusing to a future homeowner or electrician who looks at your installation. For this reason, two-pole breakers serving 120V receptacles should be marked as shared-neutral.
Shared neutral wiring is governed by section 210.4 of the NEC (Multi-wire branch circuits).