Code Quandaries

Is it legal to supply a 1,200A main breaker with three sets of 500 kcmil THHN/THWN conductors (with no major motor loads)? Do 15A receptacles for a 20A branch circuit that feeds modular office furniture receptacles and is rated 15A have to be replaced with 20A receptacles?

Apparently the Code was more confusing than we thought. After lying dormant for months, 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 with a description of the situation at [email protected], and your question may show up in a future column.

Q. Is it legal to supply a 1,200A main breaker with three sets of 500 kcmil THHN/THWN conductors (with no major motor loads)?

A. No. Where the overcurrent device is rated higher than 800A, the conductors must have an ampacity not less than its rating [240.4(C)]. It's true that 500 kcmil THHN/THWN conductors at 90°C have an ampacity that meets the requirement of 240.4(C) (430A×3=1,290A), but you must meet all NEC requirements. The rules of 110.14(C)(1)(b) call for you to size the conductors at terminals based on the 75°C column of Table 310.16, which requires the conductors at the terminations to be sized no smaller than 600 kcmil.

Q. I have a 20A branch circuit that feeds modular office furniture receptacles rated 15A. The inspector tells me that I must replace the 15A receptacles with 20A receptacles. Is he right?

A. No. When connected to a branch circuit that supplies two or more receptacles, a 15A receptacle can be installed and protected by a 20A overcurrent protection device [Table 210.21(B)(3)].

Q. Is AFCI protection required for switches located in a bedroom that control a lighting outlet in another space?

A. If the switch controls utilization equipment in the bedroom, then it must be AFCI protected. However, if the switch operates lighting outlets for an outdoor luminaire, closets, or other loads not terminated in the bedroom space, then the NEC doesn't require AFCI protection. A switch isn't considered an outlet. According to Art. 100, an outlet is defined as a point on the wiring system at which current is taken to supply equipment that uses electric energy for electronic, electromechanical, chemical, heating, lighting, or similar purposes (100). This would include a receptacle outlet and a lighting outlet, but not a switch.

Phase-control dimmers may cause nuisance tripping when used with AFCIs. This nuisance tripping is dependent on the load wattage and the number of high-wattage lamps on each AFCI. The use of high-wattage lamps or large-wattage loads will increase the chance of nuisance tripping when a dimmer turns on. This is only an issue when a dimmed circuit turns on and isn't encountered during dimming or fades.

My contacts at Eaton/Cutler-Hammer also note that the UL 1699 standard for 15A and 20A AFCIs includes testing with 1,000W incandescent lamp dimmers. This is appropriate for residential circuits. Staying at or below 1,000W of total dimmed lamps per AFCI circuit should result in nuisance-free performance. Additional loads can be on the circuit, as long as they're not dimmed loads.

Q. I've noticed a new trend toward residential, under-the-counter, washer/dryer installations. I don't think the cord is accessible to be used as a disconnecting means when the appliance is installed. Am I correct in my interpretation of the term “accessible?”

A. No. The word “accessible” means capable of being removed or exposed without damaging the building structure or finish, or not permanently closed in by the structure or finish of the building. Therefore, the installation is fine (422.33).

Q. The Code requires all branch circuits that supply 125V, 15A or 20A outlets in dwelling unit bedrooms to be AFCI protected. Does this apply to smoke detectors and wall air conditioning units connected to a 125V, 15A or 20A circuit?

A. AFCI protection is required for all 125V, 15A and 20A outlets, which includes the outlet for smoke detectors as well as wall-mounted air conditioners. However, reports from the field indicate that AFCI breakers will nuisance trip when they supply motor loads. The UL 1699 AFCI standard requires nuisance tripping testing with a capacitor-start motor drawing a minimum peak current of 120A. Underwriters Laboratories uses an air compressor for this test that draws 145A peak. Note that an air conditioner shouldn't experience a nuisance tripping problem.

Q. My inspector wants me to splice the 8 AWG bonding jumper in a pool pump motor with an exothermic welding or irreversible compression type connector. Is he correct?

A. No. The 8 AWG pool bonding conductor must be connected with exothermic welding or by pressure connectors or clamps that are labeled as suitable for the purpose [680.26(C)].

Q. I want to replace the existing two-prong receptacles in an old home with grounding-type (3-wire) receptacles, even though the wiring method only has two wires. What do I have to do?

A. According to the requirements of 406.3(D)(3), where no grounding (bonding) means exists in the outlet box, such as old 2-wire NM cable without a ground, non-grounding type receptacles can be replaced with:

  • Another non-grounding receptacle.

  • A GFCI-receptacle, if marked “No Equipment Ground.”

  • A grounding-type receptacle, if GFCI-protected and marked “GFCI Protected” and “No Equipment Ground.”

Note that GFCI protection devices function properly on a 2-wire circuit without an equipment grounding conductor because the equipment grounding conductor serves no purpose in the operation of the GFCI protection device.

Permission to replace non-grounding type receptacles with GFCI-protected grounding type receptacles doesn't apply to new receptacle outlets extending from an existing ungrounded outlet box. Adding a receptacle outlet (branch-circuit extension), necessitates a grounding type receptacle. The grounding terminal must be grounded according to 250.130(C).

Q. During an infrared inspection, I found a 400A breaker that had nine conductors per phase on the load side of the breaker, sized at 10 AWG, 8 AWG, and 1/0 AWG. The customer claimed it was legal per the 10-ft tap rule.

A. This installation isn't a violation of the 10-ft feeder tap rule, assuming all of the requirements contained in 240.21(B)(1) are followed. However, the requirements of 110.14(A) only permit one conductor per terminal unless the terminal is so identified. Naturally, no 400A breakers are identified for use with nine conductors.

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