Sizing Parallel Conductors
Last month, we discussed selecting and sizing a conductor and limiting the loads to accommodate the temperature termination rating rules outlined in the National Electrical Code (NEC). But when an installation requires conductors to be loaded in excess of 250A to 300A, it's often more cost effective to install the conductors in parallel (electrically joined at both ends). The total cross-sectional area required for the circuit when you install parallel conductors will be less than if you install a single conductor. Because smaller conductors have a greater current-carrying capacity (per 100,000 circular mil) than larger conductors, it's more cost effective to install parallel conductors.
Example No. 1 - single conductor: Q. What size 75øC service entrance conductor does the NEC require for a 3-phase commercial building, with a calculated demand load of 570A?
(a) 500kcmil (c) 1000kcmil (b) 750kcmil (d) 1250kcmil
A. The answer is (d), 1250kcmil, which is rated for 590A, per Table 310-16.
Example No. 2 - parallel conductors: Q. What size 75øC service entrance conductor does the NEC require for a 3-phase commercial building that has a calculated demand load of 570A? But this time, you are to install conductors in parallel in two different raceways.
(a) two 300kcmil conductors (b) two 250kcmil conductors (c) two 500kcmil conductors (d) two 750kcmil conductors
A. The answer is (a), two 300kcmil conductors 4285A2two conductors4570A (Table 310-16).
Step 1: Divide the load by number of raceways.
570A load32 raceways4285A for each parallel phase conductor.
Step 2: Select the appropriate wire size from Table 310-16.
Table 310-16, 75øC Column, requires the use of a 300kcmil conductor, rated 285A.
When we parallel the circuit conductors in two different raceways for the 570A load, we can use two 300kcmil parallel conductors, which have a total cross-sectional area of 600,000 circular mils, instead of one 1,250,000 circular mil conductor. A 300,000 circular mil is more common, easier to handle (reduced labor cost), and less expensive per circular mil than a 1,250,000 circular mil conductor.
NEC parallel requirements. When paralleling conductors, you must install them in accordance with the requirements in Sec. 310-4. This rule permits you to connect each phase, neutral, or grounded circuit conductor (No. 1/0 and larger) in parallel (electrically joined at both ends to form a single conductor).
Exceptions to this rule permit conductors smaller than No. 1/0 to be paralleled for elevator travel cables, supply control power to indicating instruments, contactors, relays, solenoids, and an existing grounded neutral conductor to alleviate overheating from a high content of triplen harmonic currents.
Each paralleled phase, neutral, or grounded circuit conductor shall:
- Be the same length.
- Have the same conductor material.
- Be the same size in circular mil area.
- Have the same insulation type.
- Be terminated in the same manner.
- Have the same physical characteristics, where run in separate raceways or cables.
Conductors of one phase, neutral, or grounded circuit conductor are not required to have the same physical characteristics as those of another phase, neutral, or grounded circuit conductor, to achieve equal division of current between the paralleled conductors.
Where you install equipment-grounding conductors in parallel, they shall comply with the above requirements - except you must size them in accordance with Sec. 250-122(f). This means that equipment-grounding conductors in parallel can be smaller than No. 1/0.
Sizing parallel equipment-grounding conductors - Sec. 250-122(f). Where you run conductors in parallel in multiple raceways or cables as Sec. 310-4 permits, you shall run the equipment-grounding conductors, where used, in parallel in each raceway or cable. Size each parallel equipment-grounding conductor based on the ampere rating of the overcurrent device protecting the circuit conductors in the raceway or cable, per Table 250-122.
Example No. 3 - Sizing grounding conductors in parallel: Q. A building has a 600A protection device for a feeder with a demand load of 570A. You're to install the feeder conductors in parallel in two different raceways. What size equipment-grounding conductor does the NEC require in each raceway?
(a) No. 3 (c) No. 1 (b) No. 2 (d) No. 1/0
A. The answer is (c), No. 1, based on the 600A overcurrent protection device [Sec. 250-122(f)].
The No. 1/0 minimum for parallel conductors in Sec. 310-4 doesn't apply to equipment-grounding conductors. When installing conductors in parallel in a single raceway, the ampacity adjustment factors in Sec. 310-15(b)(2)(a) apply.