366.20 Conductors Connected in Parallel.
Code Change Summary: A new code section provides requirements for parallel conductors in an auxiliary gutter.
Parallel conductor installations are covered in NEC® 310.10(H) and are permitted for each phase, polarity, neutral, or grounded conductor in sizes 1/0 AWG and larger. Joining conductors in parallel is like having two or more smaller conductors connected at each end to make one larger conductor. This is often done to make wire pulling easier.
In order to install conductors in parallel, the paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, equipment grounding conductor, or equipment bonding jumper must comply with all of the following:
(1) Be the same length.
(2) Consist of the same conductor material.
(3) Be the same size in circular mil area.
(4) Have the same insulation type.
(5) Be terminated in the same manner.
The above rules ensure that each set of parallel conductors carries the same ampacity. Where run in separate cables or raceways, the cables or raceways must have the same number of conductors and must have the same “electrical characteristics”.
The phrase “electrical characteristics” refers to the properties of the raceway such as raceway material. Using the same raceway type (nonmetallic or ferrous metal) for each parallel set of conductors ensures that electrical current is not transferred to the raceway through induction.
In the 2017 NEC®, additional code language was added to address proper grouping of parallel conductors inside an auxiliary gutter. Before the code change, the requirements for grouping of the parallel conductors was limited to within each raceway or cable but no mention was made about what goes on inside the gutter box.
Section 300.20(A) states the following: Where conductors carrying alternating current are installed in ferrous metal enclosures or ferrous metal raceways, they shall be arranged so as to avoid heating the surrounding ferrous metal by induction. To accomplish this, all phase conductors and, where used, the grounded conductor and all equipment grounding conductors shall be grouped together.
Most auxiliary gutters are made of ferrous metal therefore it only makes sense to keep parallel conductors grouped properly behind the gutter box cover.
A similar code change occurred in Article 376 for metal wireways.
Below is a preview of Article 366. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC® edition of NFPA 70.
2017 Code Language:
N 366.20 Conductors Connected in Parallel. Where single conductor cables comprising each phase, neutral, or grounded conductor of an alternating-current circuit are connected in parallel as permitted in 310.10(H), the conductors shall be installed in groups consisting of not more than one conductor per phase, neutral, or grounded conductor to prevent current imbalance in the paralleled conductors due to inductive reactance.