Stumped by the Code

Apr 1, 2009 12:00 PM, By Mike Holt, NEC Consultant

All questions and answers are based on the 2008 NEC.

Q. We have a 400A, 3-phase feeder that we want to wire with two parallel sets of 3/0 AWG conductors in the same raceway. Is this acceptable?

A. As shown in the Figure , each current-carrying conductor of a paralleled set of conductors must be counted as a current-carrying conductor for the purpose of conductor ampacity adjustment, in accordance with Table 310.15(B)(2)(a) [310.4(D)].

Each current-carrying conductor of a paralleled set of conductors must be counted as a current-carrying conductor.

Here's an example if the neutral is considered a current-carrying conductor:

Ampacity 3/0 THHN is 225A [Table 310.16 at 90°C)

Adjustment factor [Table 310.15(B)(2)(a)] = 0.70

Adjusted ampacity = 225A × 0.70

Adjusted ampacity = 158A

For this scenario, you could not use 3/0 THHN in parallel in the same raceway because the combined ampacity is only 316A (158A × 2), which is not permitted to be protected by a 400A overcurrent device [240.4].

Here's an example if the neutral is not considered a current-carrying conductor:

Ampacity 3/0 THHN is 225A [Table 310.16 at 90°C)

Adjustment factor [Table 310.15(B)(2)(a)] = 0.80

Adjusted ampacity = 225A × 0.80

Adjusted ampacity = 180A

For this scenario, you could use 3/0 THHN in parallel in the same raceway because the combined ampacity is 360A (180A × 2), which is permitted to be protected by a 400A overcurrent device [240.4(B)].

Note : When adjusting conductor ampacity, the ampacity used is based on the temperature insulation rating of the conductor as listed in Table 310.16, not the temperature rating of the terminal [110.14(C)].

Q. Is it true you can only have one bedroom on an AFCI-protected circuit?

A. No. The NEC doesn't limit the number of receptacle outlets on a general-purpose branch circuit in a dwelling unit. Although there's no limit on the number of receptacle outlets on a dwelling's general-purpose branch circuits, the NEC does require a minimum number of circuits to be installed for general-purpose receptacles and lighting outlets for the dwelling [210.11(A)]. In addition, the receptacle and lighting loads must be evenly distributed among the required circuits [210.11(B)].

Q. What are the requirements for installing receptacles face up on countertops in a commercial occupancy?

A. None. However, in dwelling units, receptacles must not be installed in a face-up position in countertops or similar work surface areas [406.4(E)].

Q. I'm selling my 30-year-old home, and the home inspector has asked me to replace the existing breakers with AFCIs in order to maintain Code compliance. Is it required by law to update a structure to make it compliant with new codes?

A. No. The electrical system is grandfathered to the standards it was constructed, as long you leave the structure in its original condition — such as no additions or conversions of unfinished space into living space. However, all modifications must be installed to the standards in effect at the time.

Q. Can we use flexible metal conduit for the emergency lights in a hospital?

A. Generally, only nonflexible metal raceways or Schedule 80 PVC conduit can be used for the mechanical protection of emergency systems in a hospital [517.30(C)(3)]. But flexible metal conduit can be used if it is fished into an existing wall and isn't subject to physical damage [517.30(C)(3)(c)].

Q. If I have 25 current-carrying conductors in a wireway, do I have to derate them?

A. No, only when you bundle more than 30 current-carrying conductors in a wireway does the conductor ampacity as listed in Table 310.16 need to be adjusted as per Table 310.15(B)(2)(a) [376.22(B)]. Please note that signaling and motor control conductors between a motor and its starter used only for starting duty aren't considered current carrying for conductor ampacity adjustment.

Q. We recently sized a circuit to a commercial oven. The manufacturer says the total load per phase is 58A, single-phase, 240V and it's not capable of being used continuously. What size conductor and protection can I use?

A. The rating of a branch circuit is not permitted to be less than the marked rating of the appliance [422.10(A)]. In this case, that rating is 58A. Therefore, 6 AWG conductors rated 65A at 75°C [110.14(C) and Table 310.16] are acceptable for this equipment, given the fact that is not capable of continuous use. The circuit protection device would be 60A [240.4 and 240.6(A)].