Conductors in Parallel
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The designations "National Electrical Code” and “NEC” refer to the
National Electrical Code®, which is a registered
trademark of the
National Fire Protection Association.
Top 50 NEC Rules
Conductors in Parallel
By Mike Holt
Ungrounded and grounded neutral conductors sized 1/0
and larger can be connected in parallel (i.e., electrically joined at
When conductors are run in parallel, the current must be evenly
distributed between the individual parallel conductors. This is
accomplished by ensuring that all ungrounded and grounded neutral
conductors within a parallel set are identical. Each conductor of a
parallel set must:
In addition, raceways or cables containing parallel conductors must
the same physical characteristics and the same number of conductors in
each raceway or cable. Conductors for one phase (ungrounded conductor)
or the grounded neutral conductor, aren't required to have the same
physical characteristics as those of another phase or grounded neutral
conductor to achieve balance.
- Be the same length.
- Be made of the same conductor material (copper/aluminum).
- Be the same size in circular mil area (minimum 1/0 AWG).
- Use the same insulation material (like THHN).
- Terminate in the same method (set screw versus
The equipment grounding (bonding) conductors for circuits in
must be identical to each other in length, material, size, insulation,
and termination. In addition, each raceway (where required) must have
equipment grounding (bonding) conductor sized in accordance with
250.122. The minimum 1/0 AWG rule of 310.4 doesn't apply to equipment
grounding (bonding) conductors [250.122(F)(1)].
When more than three current-carrying conductors are run together in
a raceway longer than 24 inches, the ampacity adjustment factors of
Table 310.15(B)(2)(a) must be applied. See 310.10 and 310.15 for
Editor's note: This information was extracted from Mike
the National Electrical Code
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What's Wrong Here?
By Joe Tedesco
Think you know how this installation violates the
NEC? Visit EC&M's
Web site to see the answer.
Hint: Think Too-Tall Jones
By Mike Holt
Q. When are insulated bushings required on
Visit EC&M's Web
site to see the answer.
By Steven Owen
Q. A cable tray system has been installed to
support service entrance conductors. The service entrance conductors
consist of six sets of three-conductor 500kcmil interlocked armored
cables installed in parallel in a single layer in the tray. An
additional feeder, consisting of three sets of parallel three-conductor
500kcmil interlocked armored cables, has been installed in a single
layer in the same cable tray. The cable tray is wide open (i.e., no
barriers or dividers). Is this installation NEC compliant?
- Yes, because all cables are interlocked armored cables.
- No, because conductors other than service entrance conductors are
not permitted in the same cable tray with service entrance conductors,
unless a solid fixed barrier of a material compatible with the cable
tray is installed to separate the service entrance conductors from the
- No, because interlocked armored cables are not permitted to be used
as service entrance conductors.
- No, because cable trays cannot be used to support service entrance
Web site for the answer and explanation.
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