5.0 APPENDIX
66
©2000 Xantrex Technology Inc.
Multiwire Branch Circuits
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 (Figure 49) 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.
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 outleaving only the difference current between the two circuits). Refer to
Figure 50.
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 51. 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.
L1
L2
Neutral
Ground
Black–Hot
White–Neutral
Bare–Ground
120 VAC
120 VAC
240 VAC
from Grid
25 A
25 A
25 A
25 A
25 A
Breaker
Black–Hot
Bare–Ground
White–Neutral
Load Center
062949-001
25 A
Breaker
Figure 49
Conventional Home-run Type Wiring
L1
L2
Neutral
Ground
Black–Hot
Single White–Neutral
Bare–Ground
120 VAC
120 VAC
240 VAC
from Grid
25 A
0 A
25 A
25 A
Breaker
(Ganged)
Red–Hot
Load Center
062949-002
25 A White–Neutral
Splice
The out-of-phase
currents subtract
at this point
25 A
Breaker
(Ganged)
Bare–Ground
Splice
Figure 50
Multiwire Branch Circuit Wiring and Current Flow
L1
L2
Neutral
Ground
Black–Hot
Single White–Neutral
Bare–Ground
120 VAC
120 VAC
240 VAC
from Grid
25 A
10 A
15 A
25 A
Breaker
(Ganged)
Red–Hot
Load Center
062949-002a
25 A White–Neutral
Splice
When unbalanced
current flows through
each leg, only the
difference current
flows through the
neutral return wire.
25 A
Breaker
(Ganged)
Bare–Ground
Splice
Summary of Contents for Trace DR Series
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