When installing feeder conductors for a stage or set (per Art. 530), an issue arises regarding the size of the overcurrent protection devices for the feeder conductors. Here's the situation.
The building is used primarily for motion picture production. The feeder conductors from the substation to the stages are protected by overcurrent protective devices (OCPDs) that are located in the substation. The overcurrent devices are multi-pole units. The calculated load is 500A, continuous. The feeder conductors consist of 2 sets of 400kcmil per phase, THHN or THWN conductors (the dual insulation rating on the conductor) installed in parallel in two separate raceways. For the purposes of ampacity adjustment, the neutral conductor in this example is not considered to be current-carrying.
What is the minimum conductor size required for the feeder conductors, and what is the maximum size OCPD setting permitted per the above-referenced section of the Code?
- 4/0 AWG; 600A rating or setting of the OCPD.
- 300kcmil; 2,280A setting of the OCPD.
- 400kcmil; 800A rating or setting of the OCPD.
- 400kcmil; 2,680A setting of the OCPD.
As per 530.18(B), "In buildings used primarily for motion picture production, the feeders from the substations to the stages shall be protected by means of overcurrent devices (generally located in the substation) having a suitable ampere rating. The overcurrent devices shall be permitted to be multi-pole or single-pole gang operated. No pole shall be required in the neutral conductor. The overcurrent device setting for each feeder shall not exceed 400% of the ampacity of the feeder, as given in the applicable tables of Art. 310."
In our example, the calculation would go as follows:
If we follow the requirements of 215.2(A)(1), a 125% multiplier is required for the continuous load.
500A x 1.25 = 625A
625A ÷ 2 conductors per phase = 312.5A per conductor, per each individual phase.
From the 75°C column of Table 310.16 [also reference 110.14(C)], a 400kcmil conductor is rated 335A.
335A x 2 = 670A
670A x 400% = 2,680A
This is the maximum setting permitted for the OCPD. [Refer to 240.6(A) for fixed trip circuit breakers or fuses and 240.6(B and C) for adjustable trip settings.] Note: 2,680A is the maximum setting permitted per this section of the Code. Lower settings may be used.
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at email@example.com.