NEC Questions

By Mike Holt for EC&M Magazine

Q1. I have a 1600A switchboard that is protected with ground-fault protection in accordance with 230.95. The only neutral load on the switchboard is from two 225A lighting panelboards. What size feeder, neutral, and bonds wires are required for this installation if we use rigid nonmetallic conduit (RNC)?

A1.
Ungrounded Conductor - According to 240.4(C), the ungrounded conductors must have an ampacity not less than 1600A. This can be accomplished by any of the following parallel sets (conductors rated at least 75C):
Six sets of 300 kcmil = 285A x 6 = 1710A
Five sets of 400 kcmil = 335A x 5 = 1675A
Four sets of 600 kcmil = 420 x 4 = 1680A

Note: Four sets of 500 kcmil is only rated 1520A (380A x 4), therefore it cannot be used.

Neutral Conductor - Section 220.22 states that the neutral demand load shall be “the maximum unbalance computed load between the neutral and any one ungrounded conductor.” I will assume that all of the loads on two 225A panelboards to be nonlinear line-to-neutral loads.

Based on this worst case assumption, we cannot apply the “over 200A, 70% demand factor” contained in 220.22. Therefore the neutral must be sized at 100% of the line-to-neutral loads, which in this case is 450A.

Assuming that we parallel the feeder in four raceways, the neutral conductor in each raceway must have an ampacity of no less than 450A/4 = 113A. According to Table 310.16, 2 AWG has a rating of 115A x 4 = 460A. But, the parallel rules contained in 310.4 require each parallel neutral conductor to be sized no smaller than 1/0 AWG.

Note: Fine Print Note No. 2 of 220.22 states “A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral currents.” This means that “good design” dictates that we should consider “up-sizing” the neutral conductor to accommodate the odd triplen harmonic currents, which do not cancel.

The industry practice is to “double the neutral.” If we followed this practice, the neutral conductor would then be sized based on 450A x 2 = 900A. We can accommodate this by installing a 4/0 AWG neutral conductor in each of the four parallel raceways.

Equipment Grounding (bonding) Conductor- An equipment grounding (bonding) conductor must be installed in each of the rigid nonmetallic conduits. Each shall be sized to the circuit’s overcurrent device protecting rating, in accordance with Table 250.122. For a 1600A protected feeder, this would require a 4/0 AWG bond wire in each of the raceways.

Q2. A metal wire trough, used as a pull point, is installed above an electrical panel. The inspector wants us to terminate all of the equipment grounding (bonding) conductors to the metal trough instead of pulling them continuous to the panel ground bar. This just does not make sense to me. What do you think?

A2. The rule on terminating equipment grounding (bonding) conductors was clarified in the 2002 NEC. 250.148 states that “where conductors are spliced or terminated on equipment within a box, the equipment grounding conductors associated with those circuit conductors shall be spliced or joined within the box or to the box with devices suitable for the use.”

Since the circuit conductors run through the trough without spice or termination, the equipment grounding (bonding) conductors are not required to terminate to the trough. Terminating the grounding conductor twice (the panelboard and trough) is just asking for trouble.

Q3. What is the working space requirement for 120V industrial machinery?

A3. Where conditions of maintenance and supervision ensure that only qualified persons will service the installation, the working space from live parts for 120V circuits shall be a minimum of 2 ft [670.5].

Exception: Where the enclosure requires a tool to open, and where only diagnostic and troubleshooting testing is involved, the clearances can be less than 2 ft. How much less? This is a judgment call by the authority having jurisdiction.

Note: The 2 ft working space rule only applies to 120V circuits. If the equipment is rated 208, 240, 277, or 480V, then the working space requirements shall comply with the more stringent requirements contained in 110.26.

Q4. I’m under the impression that all raceway and cable support fittings must be listed for the purpose. I keep telling my boss that we can only use listed nonmetallic sheath cable stables. Am I correct?

A4. Nope. The NEC does not require support systems (including cable trays) to be listed. However, the NEC requires all equipment to be approved by the authority having jurisdiction [110.2]. In addition, 90.4 specifies that the authority having jurisdiction have the responsibility for deciding on the approval of equipment and materials.

What’s does “approved” mean? According to Article 100, “approved” means acceptable to the authority having jurisdiction.

Note: I don’t think UL lists supports systems. Does anybody know for sure?

Q5. Section 310.4 defines conductors in parallel as “electrically joined at both ends to form a single conductor.” If we have two sets of parallel conductors in a single raceway (total of six physical conductors), must we apply the ampacity adjustment factors contained in 310.15(B)(2)(a)? Some engineers feel that if we electrically join the conductors to form a single conductor, we only have three conductors, instead of six!

A5. I hate these simple questions. They are not that simple to explain because we have to consider conductor size to accommodate continuous loading, overcurrent protections as well as conductor ampacity. But here I go.

Conductor Ampacity - If you have six physical conductors in a raceway, then you have six conductors in the raceway for conductor ampacity adjustment purposes. It’s really just that simple. The definition of a parallel conductor contained in 310.4 only applies to 310.4 for the purpose of applying the requirements contained in 310.4.

Example: Can 3/0 AWG THHN, paralleled in a single raceway, and protected by a 400A protection device be permitted to supply a 320A continuously load on a panelboard?

Continuous Load - The maximum continuous load shall not exceed 80 percent of the conductor ampacity before the application of any adjustment factors [215.2(A)(1)]! The 3/0 AWG THHN parallel set has an ampacity of 400A at 75C before ampacity adjustment; therefore it meets the continuous load size requirements.

Conductor Ampacity - The ampacity of six 3/0 AWG THHN conductor installed in the same raceway must be adjusted by the 80% multiplier contained in Table 310.15(B)(2)(a).
3/0 AWG THHN Adjusted Ampacity = 225A* (at 90C) x 0.80 = 180A x 2 = 360A

* Conductor adjustment is based on the conductor ampacity at the temperature rating of the conductor insulation (in this case 90C), not the 75C terminal rating of the equipment. Read the last sentence in the first paragraph of 110.14(C).

Overcurrent Protection - Section 240.4(B) permits the 360A rated conductors to be protected by a 400A protection device, “next size up rule.”

Q6. Are hospital grade receptacles required in doctor, chiropractic, or dentist examination rooms? What about isolated ground receptacles?

A6. Oh good! An easy one. No and No.
Hospital Grade Receptacles - Hospital grade receptacles are only required for patient bed locations, defined in 517.2 as an inpatient sleeping bed; or the bed or procedure table used in a critical patient care.

Isolated Ground Receptacles - Isolated ground receptacles, receptacles incorporating an isolated grounding connection intended for the reduction of electrical noise [250.146(D)], are never required by the NEC.

In fact the FPN to 517.16 cautions against the indiscriminate use of receptacles with insulated grounding terminals since such a practice forfeits the benefit of parallel grounding paths that otherwise would occur.

Author’s Comment: 517.13(B) requires all receptacles installed in "patient care areas" (including not only patient bed locations but also doctor, chiropractic and dentist exam rooms) to be grounded by an insulated copper conductor connected to the grounding terminal of the receptacle.

Q7. Section 250.56 states that “A single electrode consisting of a rod that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode.” Where did the 25 ohm value come from?

A7. I have no clue, but my research indicates that it was added to the 1918 NEC. Since my personal Code book collection only goes back to 1925, I’m not sure if this statement is true. Maybe someone knows where I can get a 1918 NEC.

Q8. What is the minimum size branch circuit conductor to a motor and what is the largest breaker I can use to protect these conductors?

A8. Motor circuit conductors shall be sized no smaller than 125% of the motor full-load current rating listed in Table 430.147 through 150 [430.6(A)(1) and 430.22(A)]. The maximum size inverse time circuit breaker for short-circuit ground-fault protection shall not exceed 250% of the motor full-load current rating [430.52(C)(1)].

Example: 10 hp, 230V, three-phase motor having a Service Factor 1.15 used for continuous duty application has a FLC rating of 28A [Table 430.150].

Note: Terminals rated 75C.

Branch Circuit Conductors - 28A x 1.25 = 35A. Typically this would be 10 AWG THHN/THWN.

Branch Short-Circuit and Ground-Fault Protection - 28A x 2.5 = 70A

Yes, it’s okay to have the 10 AWG conductors protected by the 70A protection device [240.4(G)] because the motor is protected by an overload protection device (heaters) sized not larger than 125% of the motor nameplate rating [430.6(A)(1) and 430.32].

Q9. How do you size the protection device and conductor for a 40 hp, 230V, three-phase fire pump motor?

A9. Overload protection is not permitted for the fire pump motor circuit [240.4(A) and 695.6(D)], but short circuit protection is required and it must be set to carry indefinitely the locked-rotor current of the fire pump motor [695.4(B)(1)].
Table 430.151(B) lists the locked-rotor current for the 40 hp, 230V, three-phase motor as 580A. Therefore the short-circuit protection device must be sized at least 600A.

The branch circuit conductors for a fire pump motor are sized just like any continuous duty motor, 125 percent of the motor full-load current rating as listed in Table 430.150 [695.6(C)(2)].

For a 40 hp, 230V, three-phase motor (FLC or 104A), the branch circuit conductors shall have an ampere rating of not less than 104A x 1.25 = 130A. According to Table 310.16, this would work out to be a 1 AWG conductor [110.14(C)(1)(b)].

So the short-circuit protection device is set at least 600A, and the conductors to the fire pump motor are sized no smaller than 1 AWG.

Note: Voltage drop considerations might require the branch circuit conductors to be sized larger than 1 AWG. See 695.7 for details.

Q10. Can I use one single-pole 15A, 125V AFCI breaker and one 15A, 125V nonAFCI circuit breaker with 14/3 nonmetallic sheath cable to supply a AFCI protected dwelling unit bedroom circuit and another circuit that is not AFCI protected?

A10. No. A single-pole AFCI circuit breaker, just like a single-pole GFCI circuit breaker, is not designed to operate on a multiwire branch circuit. I suggest you use the new 14/4 NM cable that is manufactured for this purpose.

 

Copyright © 2002 Mike Holt Enterprises,Inc.
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