Code Quandaries is back, and Mike Holt, EC&M's NEC Consultant, is here to answer your most difficult questions. Need some help with an installation? Clashing with your local inspector? E-mail Mike at mike@mikeholt.com with a description of the situation, and your question may show up in a future column.

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

A. Oh good! An easy one. No and No. 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 area (Figure at right).

Isolated ground receptacles, or receptacles that incorporate an isolated grounding connection intended for the reduction of electrical noise [250.146(D)], are never required by the NEC.

An isolated ground receptacle isn't actually isolated from the system ground. This type of device has its grounding contacts insulated from the metal mounting yoke. Art. 517 properly calls an “isolated ground receptacle” an “insulated ground receptacle.” 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.

Q. I'm an electrical designer in Greensboro, N.C. I recently worked on a new car dealership project. The engineer and I decided that to power the vehicle lifts, the contractor would have to provide an SO cord from a junction box above to the lift motor. The contractor says that the inspector told him that the cord must have a twist-lock connection and receptacle. I've searched the NEC but can't find where this is required. Can you help?

A. A twist-lock connector (locking type) isn't required for a cord used for this purpose. However, 400.7(B) requires an attachment plug and receptacle (twist-lock not required) for any flexible cord used for the connection of utilization equipment to facilitate frequent interchange [400.7(A)(6)]. However, if the manufacturer's installation instructions call for a twist-lock plug and receptacle, then it must be provided, per 110.3(B).

Q. I have a 1,600A 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 bond wires are required for this installation if we use rigid nonmetallic conduit (RNC)?

A. Ungrounded conductor. According to 240.4(C), the ungrounded conductors must have an ampacity not less than 1,600A. This can be accomplished by any of the following parallel sets (conductors rated at least 75°C):

Six sets of 300kcmil=285A×6=1,710A

Five sets of 400kcmil=335A×5=1,675A

Four sets of 600kcmil=420×4=1,680A

Note: You can't use four sets of 500 kcmil, because they would only be rated 1,520A (380A×4).

Neutral conductor. Sec. 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 the two 225A panelboards are nonlinear line-to-neutral loads.

Based on this worst-case assumption, you can't 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 about 113A (450A÷4). According to Table 310.16, 2 AWG has a rating of 460A (115A×4). But the parallel rules contained in 310.4 require each parallel neutral conductor to be sized no smaller than 1/0 AWG.

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 allows for the possibility of high harmonic neutral currents.” This means that “good design” dictates that you should consider “up-sizing” the neutral conductor to accommodate the odd triplen harmonic currents, which don't cancel.

By following the industry practice of “doubling the neutral,” the neutral conductor would then be sized based on 450A×25900A. You 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 1,600A-protected feeder, this would require a 4/0 AWG bond wire in each of the raceways.

Q. I thought 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?

A. Nope. The NEC doesn't require support systems — including cable trays — to be listed. However, the NEC requires all equipment to be approved by the AHJ (110.2). In addition, 90.4 specifies that the AHJ has the responsibility for deciding on the approval of equipment and materials, which, according to Art. 100, means acceptable to the AHJ.

According to UL, metal cable trays can be classified as to their suitability as an equipment grounding conductor in accordance with 318.3(C) and 318.7(B). Nonmetallic cable trays are listed and tested in accordance with the performance and construction requirements of NEMA FG 1-1993.

Q. Some 480V-120/208V, 3-phase transformers come shipped with a bonding strap that connects the XO terminal to the case of the transformer. My boss told me to install a bonding jumper sized to Table 250.66 and based on the secondary conductors and leave this factory-bonding strap in place. Is this really required by the NEC?

A. No. You're not required to add an additional bonding jumper if the transformer is listed by a qualified electrical testing laboratory (90.7). It's true that 250.30(A)(1) requires a bonding jumper sized in accordance with Table 250.66 to bond the XO terminal to the case. But if the manufacturer has installed one, then there's no need for you to add an additional bonding connection.

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

A. Motor circuit conductors must be sized no smaller than 125% of the motor full-load current rating listed in Tables 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 must not exceed 250% of the motor full-load current rating [430.52(C)(1)].

Example: A 10-hp, 230V, 3-phase motor with a Service Factor 1.15 used for continuous duty application has a FLC rating of 28A [Table 430.150]. Its terminals are rated 75°C.

Branch circuit conductors. 28A×1.25=35A. Typically this would be 10 AWG THHN/THWN.

Branch short-circuit and ground-fault protection. 28A×2.5=70A

Yes, it's OK to protect the 10 AWG conductors with a 70A protection device [240.4(G)] because the motor is protected by an overload protection device (heaters) no larger than 125% of the motor nameplate rating [430.6(A)(1) and 430.32].

Q. Can I use one single-pole 15A, 125V AFCI breaker and one 15A, 125V non-AFCI circuit breaker with 14/3 nonmetallic sheath cable to supply an AFCI-protected dwelling unit bedroom circuit and another circuit that isn't AFCI protected?

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

Q. What is the proper way to replace two-prong receptacles with a grounding type receptacle in an older house where no ground wire is located in the outlet box?

A. Where no grounding means exists in the outlet box, a nongrounding-type receptacle can be replaced with a GFCI receptacle, if marked “No Equipment Ground,” or a grounding type receptacle, if GFCI protected and marked “GFCI Protected” and “No Equipment Ground.” See 406.3(D)(3) for details.

Q. We recently designed an upgrade to an existing imaging center that has 208V, 3-phase service. We're adding additional equipment that requires 480V, 3-phase, and we designed a separate service to supply the new 480V equipment. The permit plan reviewer, stating that the NEC didn't allow two services to the same building, rejected the plans. It's my belief that 230.2(D) permits this design for differing voltages. What's your opinion?

A. You're correct. Per 230.2(D), “additional services shall be permitted for different voltages.” You also need to comply with 230.2(E), which requires a building supplied by more than one service to have a permanent plaque or directory at each service disconnect location denoting all other services supplying that building and the area served by each. In addition, be sure you use the same grounding electrode for both services (250.58).

Q. How do you size the conductor and protection device for a 15 kVA, 240V, 3-phase space heater?

A. Conductors and protection devices for electric space heating equipment must be sized no smaller than 125% of the ampere rating of the equipment [424.3(B)].

Equipment Ampere Rating=kVA/(Volts×1.732)=15,000/(240V×1.732)

Equipment Ampere Rating=36A

Conductor and Protection not Less Than=36A×1.25=45A

Conductor=6 AWG for 60°C terminals or 8 AWG for 75°C terminals [110.14(C)]

Protection=45 or 50A device [240.6(A)]

Q. I have a situation where a contractor used 15A, 125V receptacles on a 20A circuit with 12 AWG conductors. In addition, the contractor used 14 AWG pigtails to the 15A receptacles. I feel that a 20A rated receptacle is required and the conductors to the receptacle shall be a minimum 12 AWG. Am I right?

A. Yes and no. A 15A rated receptacle can be connected to a 20A branch circuit that supplies two or more receptacles. This would include a single duplex receptacle, because it contains two receptacles [210.21(B)(3)]. However, the tap rule contained in Exception No. 1 of 210.19(A)(4) is intended to prohibit the use of a 14 AWG pigtail from a 12 AWG branch circuit conductor protected by a 20A overcurrent protection device. So it's okay to install a 15A, 125V rated receptacle on a 20A circuit, but you're not permitted to install a 14 AWG pigtail on a 20A circuit.