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Note: The answers to these questions are based on the 2017 NEC.
Underlined text indicates a change in the rule for the 2017 NEC.
Q1. What is the Code requirement that allows paralleling of circuit conductors?
A1. Ungrounded and neutral conductors can be connected in parallel only in sizes 1/0 AWG and larger where installed in accordance with 310.10(H)(2) through (H)(6) [310.10(H)(1)].
Q2. When conductors are installed in parallel, what does the Code require regarding the conductor and installation characteristics?
A2. When circuit conductors are installed in parallel, the conductors must be connected so the current will be evenly distributed between the individual parallel conductors by requiring all circuit conductors within each parallel set to [310.10(H)(2)]:
(1) Be the same length.
(2) Consist of the same conductor material (copper/aluminum).
(3) Be the same size in circular mil area (minimum 1/0 AWG).
(4) Have the same type of insulation (like THHN).
(5) Terminate using the same method (set screw fitting versus compression fitting).
Author's Comment:
•
When installed in raceways or enclosures, paralleled conductors must be grouped to prevent inductive heating.
Q3. What does the Code require for separate raceways or cables that contain parallel conductors?
A3. Separate raceways or cables containing parallel conductors must have the same electrical characteristics [310.10(H)(3)].
Conductors of one phase, neutral, or equipment grounding conductor aren't required to have the same physical characteristics as those of another phase, neutral, or equipment grounding conductor.
Author's Comment:
• If one set of parallel conductors is installed in a metal raceway and the other conductors are installed in PVC conduit, the conductors in the metal raceway will have an increased opposition to current flow (impedance) as compared to the conductors in the nonmetallic raceway. This results in an unbalanced distribution of current between the parallel conductors.
• Parallel conductor sets must have all circuit conductors within the same raceway [300.3(B)(1)].
Parallel sets of conductors aren't required to have the same physical characteristics as those of another set to achieve balance.
Author's Comment:
• For example, a 400A feeder with a neutral load of 240A can be paralleled as follows: Figure 01
▫ Phase A, Two”250 kcmil THHN aluminum, 100 ft
▫ Phase B, Two”3/0 THHN copper, 104 ft
▫ Phase C, Two”3/0 THHN copper, 102 ft
▫ Neutral, Two”1/0 THHN aluminum, 103 ft
▫ Equipment Grounding Conductor, Two”3 AWG copper, 101 ft*
*The minimum 1/0 AWG requirement doesn't apply to equipment grounding conductors [310.10(H)(5)].
Q4. What is the Code required sizing of equipment grounding conductors for circuits in parallel?
A4. The equipment grounding conductors for circuits in parallel must be sized in accordance with 250.122(F). Sectioned equipment grounding conductors smaller than 1/0 AWG are permitted in multiconductor cables, if the combined circular mil area of the sectioned equipment grounding conductor in each cable complies with 250.122 [310.10(H)(5)].
Author's Comment:
• The minimum 1/0 AWG parallel conductor size rule of 310.10(H) doesn't apply to equipment grounding conductors.
(6) Bonding Jumpers. Equipment bonding jumpers and supply side bonding jumpers are sized in accordance with 250.102.
Author's Comment:
• The equipment bonding jumper isn't required to be larger than the largest ungrounded circuit conductors supplying the equipment.
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