Multiwire Branch Circuits; Figure 5-1 Conventional "Home-Run" Type Wiring - Xantrex7 UX - REV A Owner's Manual

5.0 Troubleshooting

Multiwire Branch Circuits

©2002 Xantrex Technology Inc. All Rights Reserved.
P/N 975-0026-01-01 Rev A 06/2002
A potential safety problem exists when installing stand-alone 120 Vac inverters
into existing 120/240 Vac wired panels where multiwire branch circuit wiring
methods were used.
Multiwire branch circuits are wired differently from "home run" type wiring 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 separate "romex" for each circuit.
See Figure 5-1 below for a diagram of conventional "Home-run" type wiring.
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).
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! 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.
See Figure 5-2 on page 60 for a diagram of multiwire branch circuit wiring
and current flow.
L2
240 VAC
from Grid L1
Neutral
Ground
25 A
25 A
Figure 5-1
Conventional "Home-run" Type Wiring
Load Center
25 A
25 A
Breaker
Breaker
Black–Hot
25 A
Black–Hot
120 VAC
White–Neutral
White–Neutral
Bare–Ground
Bare–Ground
25 A
120 VAC
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