NEC 310.4 Compliance: Rules for Parallel Conductors

310.4 Conductors in Parallel

(A) General

Aluminum, copper-clad aluminum, or copper conductors of size 1/0 AWG and larger can be connected in parallel for each phase, polarity, neutral, or grounded circuit conductor.

Exception No. 1

Conductors smaller than 1/0 AWG are allowed for specific applications, such as control power for instruments, contactors, relays, etc., or for frequencies of 360 Hz and higher, provided:

• They are in the same raceway or cable.
• Each conductor can carry the entire load current.
• Overcurrent protection ensures no conductor exceeds its ampacity if one or more parallel conductors are disconnected.

Exception No. 2

Under engineering supervision, grounded neutral conductors of 2 AWG and larger can be run in parallel for existing installations to address overheating due to triplen harmonic currents.

FPN to Exception No. 2: This exception can be used to alleviate overheating of neutral conductors in existing installations due to high content of triplen harmonic currents.

(B) Conductor Characteristics

Parallel conductors must meet the following requirements:

1. Same length.
2. Same conductor material (e.g., all copper or all aluminum).
3. Same size in circular mil area.
4. Same insulation type.
5. Same termination method.

(C) Separate Cables or Raceways

If parallel conductors are in separate cables or raceways:

• Each cable or raceway must have the same number of conductors and electrical characteristics.
• Conductors of one phase, polarity, neutral, etc., do not need to match the physical characteristics of another phase, polarity, etc., to achieve balance.

(D) Ampacity Adjustment

Parallel conductors must comply with the ampacity adjustment rules in 310.15(B)(2)(a).

(E) Equipment Grounding Conductors

Parallel equipment grounding conductors must be sized according to 250.122.

Sectioned equipment grounding conductors smaller than 1/0 AWG are permitted in multiconductor cables, provided the combined circular mil area complies with 250.122.

Key Considerations:

• Parallel conductors are typically used to increase current-carrying capacity.
• Strict adherence to the NEC requirements ensures safety and proper functionality.
• Exceptions allow flexibility for specific applications, such as control circuits or addressing harmonic issues in existing installations.