All questions and answers are based on the 2005 NEC.
Q. What is the minimum depth for placing Schedule 40 PVC conduit below a concrete slab?
A. If it's under a building, there is no depth requirement. If it's placed outside under a concrete slab, then the minimum cover is 4 inches [Table 300.5].
Q. I want to come into a power distribution block with one feeder fused at 400A and feed three 150A panels. How do I size the tap conductors for each panel if they don't exceed 10 feet in length?
A. The tap conductors must be sized not smaller than the calculated load in accordance with Art. 220 — and not smaller than the rating of the overcurrent protective device at the termination of the tap conductors. Also, the tap conductors must have an ampacity not less than 10% of the ampacity of the overcurrent protection device that protects the feeder [240.21(B)(1)].
Example: What size tap conductor would be required from a 400A overcurrent protection device supplying a 150A circuit breaker if the calculated continuous load was 100A?
1/0 AWG tap conductors would be required to supply the circuit breaker. A 1/0 AWG “feeder” can be protected by a 150A overcurrent protection device because of the “next-size-up” rule [240.4(B)]. However, this rule does not apply to feeder “taps” (see Figure).
Q. How high does a meter need to be mounted above grade? What about the service disconnect?
A. The NEC does not specify a minimum or maximum height for meters, but it does require switches and circuit breakers to be installed so the center of the grip of the operating handle of the switch or circuit breaker (when in its highest position) isn't more than 6 feet 7 inches above the floor or working platform.
Note: Section 550.32(F) requires the service disconnecting means enclosure for mobile and manufactured homes to be mounted a minimum of 2 feet above the finished grade.
Q. What size THHN conductor and protection device is required for a 3-phase, 4-wire feeder that supplies a 184A continuous load, assuming all conductors are current carrying?
A. The answer is 250kcmil copper with 250A overcurrent protection. The minimum feeder-circuit conductor ampacity, before the application of any adjustment and/or correction factors, must be no less than 125% of the continuous load based on the terminal temperature rating ampacities as listed in Table 310.16 [110.14(C), 215.2(A)(1)].
Because the load is 184A continuous, the feeder conductors must have an ampacity not less than 230A (184A × 1.25), according to Table 310.16, 75°C column — before the application of any conductor ampacity adjustment. According to this table, 4/0 AWG conductors are suitable, because they have an ampere rating of 230A at 75°C.
Feeder overcurrent protection devices must have an ampacity not less than 125% of the continuous loads [215.3]. The minimum overcurrent protection for an 184A continuous load must be rated not less than 230A (184A × 1.25). Because there is no 230A-rated protection device, you must use the next larger size overcurrent protection device, which would be 250A in accordance with 240.6(A) [240.4(B)].
Now it gets a little complicated. The ampacity of a 4/0 AWG conductor at 75°C without adjustment is 230A. This conductor is permitted to be protected by a 250A protection device, but you need to see if this conductor's ampacity after conductor ampacity adjustment is permitted to be protected by the 250A protection device. In this case, you need to adjust the conductor ampacity for four current-carrying conductors in a raceway or cable.
Conductor ampacity after adjustment factors = Table 310.16 conductor ampacity (based on conductor insulation rating) × conductor ampacity adjustment factor [Table 310.15(B)(2)(a)].
4/0 THHN ampacity after adjustment factors = 260A × 0.80 = 208A.
Because the ampacity of 4/0 THHN is only 208A, it's not permitted to be protected by the required 250A protection device. Remember the next-size-up rule — the next size up for a conductor ampacity of 208A would be a 225A device, so the 250A device cannot be used after de-rating. Therefore, you have to increase the conductor one size larger to 250kcmil. However, you need to see if this conductor's ampacity after conductor ampacity adjustment is permitted to be protected by the 250A protection device.
250kcmil THHN ampacity after adjustment factors = 290A × 0.80 = 232A. As you can see, this option is okay.
Q. When can a transfer switch be installed ahead of the service disconnect?
A. Only when the transfer switch is rated as suitable for use as service equipment [230.66]. Suitable for use as service equipment means that the transfer switch contains a service disconnecting means. It's also supplied with a main bonding jumper so that a neutral-to-case connection can be made, as required in 240.24(B).