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Multi-wire Branch Circuit Wiring
F–2 976-0043-01-02
Multi-wire Branch Circuits
Problem A potential safety problem exists when installing stand-alone 120 Vac
inverters into existing 120/240 Vac wired panels where multi-wire branch
circuit wiring methods were used.
Legacy situation Multi-wire branch circuits are wired differently from “home run” type
wiring (Figure F-1) in that only one neutral wire is used to provide the
neutral-return path for each circuit connected to both phases of the AC
grid. This method has been employed by electricians in recent years to
keep construction costs down by saving copper and labor costs involved
in running a separate Romex™ for each circuit.
Normal condition Under normal conditions, this technique is quite safe and meets code
requirements. When used as originally installed, the current for each
circuit is 180° out-of-phase with each other, so the neutral wire never
receives more current than it was designed to handle as the current from
each circuit subtracts (or cancels out, leaving only the difference current
between the two circuits). Refer to Figure F-2.
Safety issue A safety problem occurs when a stand-alone 120 Vac inverter is installed
to power these circuits, causing the one neutral wire to now carry the in-
phase currents for both circuits. Since the current is in-phase, the two
circuits add instead of subtract, potentially doubling the current flow in
the neutral return wire! Refer to Figure F-3. The branch circuit breakers
do not protect the neutral wire from overload under this condition. This
excess current will overheat the neutral wire, potentially creating a fire
hazard.
Figure F-1
Conventional Home-type Wiring
Load Center
L1
L2
Neutral
Ground
120 Vac
15 A
Bre aker
15 A
Breaker
240 Vac
from Grid
Bare - Ground
White - Neutral
(Current Flow 15 A)
Black - Hot
(Current Flow 15A)
120 Vac
Black - Hot
(Current Flow 15A)
White - Neutral
(Current Flow 15 A)
Bare - Ground