When run in parallel using two conduits, what is the minimum kcmil size permitted for conductors between a 480-volt, 3Ø generator rated at 500 kVA and a distribution panelboard?

When run in parallel using two conduits, what is the minimum kcmil size permitted for conductors between a 480-volt, 3Ø generator rated at 500 kVA and a distribution panelboard?

• A. 350
• B. 500
• C. 400
• D. 600

Answer: (B)

When conductors are run in parallel, you have to apply derating because multiple current-carrying conductors share the load in a raceway. With two conduits used for a three‑phase feeder, there are six current-carrying conductors in total, so the ampacity of each conductor is reduced by the derating factor for 4–6 conductors (0.8). First find the total load current. A 500 kVA, 480 V three‑phase source draws about 601 A (S divided by √3 times V). If you split this between two parallel banks, each bank must carry roughly 300 A per phase. After applying the derating factor (0.8), the per‑conductor ampacity must be at least 300 / 0.8 ≈ 375 A. Looking at common copper conductor ampacities at 75°C, sizes smaller than 500 kcmil don’t reliably provide 375 A after derating, while 500 kcmil copper easily meets and exceeds this requirement. Therefore, the minimum standard size that satisfies the derated load for parallel conductors in two conduits is 500 kcmil.

When conductors are run in parallel, you have to apply derating because multiple current-carrying conductors share the load in a raceway. With two conduits used for a three‑phase feeder, there are six current-carrying conductors in total, so the ampacity of each conductor is reduced by the derating factor for 4–6 conductors (0.8).

First find the total load current. A 500 kVA, 480 V three‑phase source draws about 601 A (S divided by √3 times V). If you split this between two parallel banks, each bank must carry roughly 300 A per phase. After applying the derating factor (0.8), the per‑conductor ampacity must be at least 300 / 0.8 ≈ 375 A.

Looking at common copper conductor ampacities at 75°C, sizes smaller than 500 kcmil don’t reliably provide 375 A after derating, while 500 kcmil copper easily meets and exceeds this requirement. Therefore, the minimum standard size that satisfies the derated load for parallel conductors in two conduits is 500 kcmil.