NEC Questions Based on the 2008 NEC

March

Here is the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent yesterday so you can see how you did.

 

By Mike Holt for EC&M Magazine

 

Q1. How does the code address diesel fuel as it relates to hazardous locations?

A1. Diesel fuel is only a concern when it is handled or stored at or above its flash point. The flash point of diesel fuel is 100°F, so very rarely does an area involving diesel fuel need to be classified as a hazardous location. If you are dispensing diesel fuel, note that 514.3(A) states that flammable liquids with a flash point below 100°F can be unclassified if the AHJ allows, so liquids with a flash point above 100°F can certainly be unclassified as well, in most applications. The only exception to this that I could see would be in areas such as Phoenix or Las Vegas, where summer temperatures often exceed 110 degrees.

 

Q2. Can a receptacle be installed above a suspended ceiling?

A2. Receptacles are permitted above a suspended ceiling, but a flexible cord is not [400.8(5)]. Why install a receptacle above a ceiling if the flexible cord is not permitted in this space? Because the receptacle can be used for portable tools; it just can’t be used for cord-and-plug-connected equipment fastened in place, such as a projector.

 

Q3. Can a light fixture and its associated wiring be mounted to a tree?

A3. Trees can be used to support luminaires [410.36(G)], but they must not be used to support overhead conductor spans [225.26].

 

Q4. Can a three way switch be used as a generator transfer switch for a single circuit in a house?

A4. Transfer equipment must be suitable for the intended use and must be designed and installed to prevent interconnection of the utility and generator supply [702.6]. According to the UL White Book (category QJQR), “snap switches have not been investigated for switching a load between two alternate sources of supply”, so 110.3(B) would prohibit using a three way switch for transfer equipment.

 

Q5. Where does the code require removal of line voltage conductors and raceways that are no longer in use?

A5. Abandoned line voltage conductors are only required to be removed in cellular concrete floor raceways [372.13], cellular metal floor raceways [374.7], underfloor raceways [390.7], and information technology equipment [640.5(F)].

 

Q6. Are AFCIs required in a college dorm room?

A6. All 15A or 20A, 120V branch circuits in dwelling units supplying outlets in family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, or similar rooms or areas must be protected by a listed AFCI device of the combination type [210.12(B)]. If your college dorm room meets the definition of a dwelling unit, you must comply with this rule. Remember that a dwelling unit is a space that provides independent living facilities, with space for eating, living, and sleeping; as well as permanent facilities for cooking and sanitation [Article 100]. It is often the presence of permanent provisions for cooking that determines whether or not a dorm room is a dwelling unit.

 

Q7. Can service conductors go directly to a transfer switch, without a disconnect upstream of the transfer switch?

A7. Electrical equipment must not be connected to the supply side of the service disconnect enclosure, except for the equipment listed In 230.82, which does not include transfer switches. If the transfer switch is the service disconnect, and is marked as "suitable for use as service equipment" [230.66], it can be used as the service disconnect, which complies with this rule.

 

Q8. We use a portable generator to provide power to a construction type trailer that is moved every few days. The generator feeds a panel in the trailer. Do we have to connect the generator to earth even though it is portable and is going to be disconnected every couple of days?

A8. Yes. Portable generators are not required to be grounded (connected to the earth) if [250.34(A)]:

(1) The generator only supplies equipment or receptacles mounted on the generator, and

(2) The metal parts of the generator and the receptacle grounding terminal are connected to the generator frame.

 

Q9. I am having a hard time understanding the feeder tap rules. Can you please do an example, and show how the 1/10 rule for 10 foot taps applies?

A9. Feeder tap conductors up to 10 ft long are permitted without overcurrent protection at the tap location if [240.21(B)(1)]:

(1) The ampacity of the tap conductor must not be less than:

a. The calculated load in accordance with Article 220, and

b. The rating of the device or overcurrent device supplied by the tap conductors.

(2) The tap conductors must not extend beyond the equipment they supply.

(3) The tap conductors must be installed in a raceway if they leave the enclosure.

(4) The tap conductors must have an ampacity not less than 10 percent of the ampacity of the overcurrent device that protects the feeder.

 

Example: A 400A breaker protects a set of 500 kcmil feeder conductors. There are three taps fed from the 500 kcmil feeders that supply disconnects with 200A, 150A, and 30A overcurrent devices. What are the minimum size conductors for these taps?

• 200A: 3/0 AWG is rated 200A at 75°, and is greater than 10 percent of the ampacity of 500 kcmil, which is rated 380A at 75°.
• 150A: 1/0 AWG is rated 150A at 75°, and is greater than 10 percent of the ampacity of 500 kcmil, which is rated 380A at 75°.
• 30A: 8 AWG is rated 50A at 75°, and is greater than 10 percent of the ampacity of 500 kcmil, which is rated 380A at 75°. Anything smaller than 8 AWG can’t be used, as it will have an ampacity of less than 10 percent of 380A (38A) in the 75° column of 310.16.

 

Q10. We are being told that we can’t use PVC conduit underground to provide power to the gasoline pumps we are wiring. Is this true?

A10. PVC conduit with an equipment grounding conductor can be used below the Class I location if buried under not less than 2 ft of earth, concrete, asphalt, etc. Threaded rigid metal conduit or threaded intermediate metal conduit must be used for the last 2 ft of the underground run [514.8 Ex No. 2].

 

Q11. I am remodeling an existing commercial building that has Type NM cable installed above the suspended ceiling. With this in mind, can I also install Type NM cable above the suspended ceiling?

A11. Type NM Cable isn’t allowed exposed in dropped or suspended ceilings in other than one- and two-family and multifamily dwellings [334.12(A)(2)]. Although this was allowed in many buildings under previous code editions, you must comply with this rule now that it is no longer permitted.

 

Q12. We have an apartment complex and are wondering about which table in the code to use when sizing the service conductors. The service is 400A, and we want to use the conductor sizes in Table 310.15(B)(6). Can we do this?

A12. No, Table 310.15(B)(6) is only allowed for conductor sizing for individual dwelling units of one-family, two-family, and multifamily dwellings, and it can't be used for  service conductors supplying multifamily dwellings.

 

Q13. We are remodeling a commercial building that has way too many receptacles on each circuit. The owner says they don’t need the receptacles, and I really want to have this building Code compliant if my name is on it. Can I simply remove the receptacles and install blank covers on them, or do I need to start removing wires and everything else?

A13. The calculations in 220.14(I) apply to receptacles, not outlet boxes: Each 15A or 20A, 125V general-use receptacle outlet is considered as a 180 VA per mounting strap. With that said, removing the receptacles and installing a blank cover on them would result in code compliance. Assuming that the circuits are 20A, the maximum number of receptacles on a circuit is 13.

Circuit VA = Volts x Amperes

Circuit VA = 120V x 20A

Circuit VA = 2,400 VA

Number of Receptacles = 2,400 VA/180 VA

Number of Receptacles = 13

 

Q14. Are hospital grade receptacles required in a dentist office?

A14. No. Receptacles for inpatient sleeping beds or procedure table beds used in a critical care area (patient bed location) must be listed as “hospital grade" [517.18(B)]. “Hospital grade” receptacles aren’t required in treatment rooms of clinics, medical and dental offices, or outpatient facilities because these locations don’t have a “patient bed location” as defined in 517.2.

 

Q15. Where does the code talk about grounding equipment racks for computer servers?

A15. It doesn’t. As long as the computers themselves are connected to an equipment grounding conductor [250.4(A)(3)], you should be fine.