NEC Questions and Answers - March Part 1 of 2
By Mike Holt for EC&M Magazine
Q1. Is it legal to make a mechanical transition from electrical metallic tubing to flexible metal conduit with a fitting that joins these two raceways instead of installing a box?
A1. Yes, if the fitting is identified for the use and the conductors are not spliced or terminated within the fitting [300.15(F)]. Otherwise a box is required.
Q2. Is the electrician required to leave us a way to bond the satellite dish to the electrical system?
A2. Yes, an accessible bonding point must be provided at service equipment and at the disconnecting means of separate buildings or structures for connecting intersystem bonding and grounding electrode conductors [810.21(F)(1)(c)]. According to Section 250.94, that point can be any one of the following:
- An exposed, nonflexible metallic raceway.
- An exposed grounding electrode conductor.
- An external connection approved by the authority having jurisdiction.
Note: The 2008 NEC contains some new rules for this application.
Q3. Can I use a single receptacle without GFCI protection to supply a refrigerator in a kitchen?
A3. If this is a dwelling unit, the answer is yes; either a single or duplex receptacle can be used to supply the refrigerator as long as it is not installed to serve the countertop according to 210.8(A)(6). But all 15 and 20A, 125V receptacles installed in commercial kitchens, even those that do not supply the countertop surface, must be GFCI protected. In other than dwelling units, a kitchen is defined as an area with a sink and permanent facilities for food preparation and cooking [210.8(B)(2)].
Q4. The NEC has a requirement that the grounding conductor for satellite systems be as short as possible. Does this mean the satellite dish must be located as close as possible to the electrical service ground?
A4. No, it does not. Actually, 810.21(F)(1) which covers satellite dishes requires that the grounding conductor be connected to the nearest accessible location of the following:
- Building or structure grounding electrode system [250.50].
- Interior metal water piping system, within 5 ft from its point of entrance [250.52(A)(1)].
- Accessible service bonding means [250.94].
- Metallic service raceway.
- Service equipment enclosure.
- Grounding electrode conductor or the grounding electrode conductor metal enclosure.
If a ground rod is installed to serve as the ground for the dish, it must be connected to the building’s power grounding electrode system with a minimum 6 AWG conductor [810.21(J)].
Q5. I have a 480V to 120V, single-phase 1.5kVA transformer. The engineer tells me that no overcurrent protection is required on the secondary side, because it’s provided by the primary protection device. Is this true?
A5. Yes, as long as the primary overcurrent protection does not exceed 5A. The rule is as follows: “The primary overcurrent protection device sized in accordance with 450.3(B) can protect the secondary conductors of a 2-wire system, provided the primary protection device does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio [240.21(C)(1)].
Step 1. Calculate the primary current for a 1.5 kVA, 480V single-phase transformer:
I = VA/E
I = 1,500VA/480V
I = 3.13A
Step 2. Size the primary overcurrent protection device in accordance with 450.3(B). The primary overcurrent protection must not exceed 167% of the 3.13A primary current rating.
Primary Protection (max) = 3.13A x 167%
Primary Protection (max) = 5.2A
Confirm that the 5A primary overcurrent protection is suitable to protect the secondary conductors.
Step 1. Calculate the secondary current rating:
I = VA/E
I = 1,500VA/120V
I = 12.5A
Step 2. Size the secondary conductors:
According to Table 310.16 and 110.14(C), we must use a minimum of 14 AWG, which has an ampacity of 20A in the 60 degree C column of Table 310.16.
Step 3. Determine that the primary overcurrent protection device of 5A does not exceed the value determined by multiplying the secondary conductor ampacity (20A) by the secondary-to-primary transformer voltage ratio (120V/480V) [240.21(C)(1)].
Primary Protection (max) = 20A x 120/480
Primary Protection (max) = 20A x 0.25
Primary Protection (max) = 5A
Q6. Can I secure signal or communications cables to ceiling-support wires or ceiling grid?
A6. No. Ceiling-support wires and ceiling grids are not permitted to support raceways and cables. Therefore you must provide independent support wires secured at both ends for this purpose in accordance with 300.11(A) [800.24].
Q7. Does the NEC require fluorescent lay-in luminaires to be supported independent of a suspended ceiling?
A7. No, the NEC does not require support of lay-in luminaires independent of the ceiling grid. However the luminaires must be attached to the suspended-ceiling framing with screws, bolts, rivets, or clips listed and identified for such use [410.16(C)].
CAUTION: local building codes might have a specific requirement to independently support the luminaires from the ceiling grid.
Q8. I’m confused… 210.12(B) requires AFCI protection for all bedroom outlets, and understand that this applies to the smoke alarm outlets, but 760.21 and 760.41 state that you can’t protect fire alarm circuits with an AFCI protection device. What is the deal?
A8. The wiring for smoke alarm outlets located in dwelling unit bedrooms must be AFCI protected as per 210.12(B). The Article 760 GFCI/AFCI limitation only applies to the circuit that supplies a nonpower-limited [760.21] or power-limited [760.41] fire alarm system. Smoke detectors connected to a 15 or 20A, 120V circuit must be AFCI protected if located in the bedroom of a dwelling unit [210.12(B)], because this circuit isn’t supplied by a fire alarm panel in accordance with NFPA 72, National Fire Alarm Code.
Q9. Can I use a 3-pole breaker to protect a single phase load?
A9. Yes, because three-phase breakers have a ‘straight voltage rating’ [240.85].
Straight Voltage Rating. A circuit breaker with a straight voltage rating, such as 240V or 480V, is permitted on a circuit where the nominal voltage between any two conductors (line-to-neutral or line-to-line) doesn’t exceed the circuit breaker’s voltage rating.
Slash Voltage Rating. A circuit breaker with a slash rating, such as 120/240V or 277/480V, is permitted on a solidly grounded system where the nominal voltage of any one conductor to the metal case doesn’t exceed the lower of the two values and the nominal voltage between any two conductors doesn’t exceed the higher value.
Caution: A circuit breaker with a slash rating, such as 120/240V, can only be used on a circuit where the nominal voltage of any one ungrounded conductor to metal case doesn’t exceed the lower of the two values. A 120/240V slash circuit breaker cannot be used on the high-leg of a solidly grounded 120/240V delta system, because the line-to-ground voltage of the high-leg is 208V, which exceeds the 120V line-to-ground voltage rating of the breaker.
Q10. Can a circuit breaker be used to switch fluorescent lighting?
A10. Yes, but the circuit breakers must be listed and marked SWD or HID. High-intensity discharge lighting can be switched only by circuit breakers marked HID [240.83(D].
Q11. Can I use aluminum rigid metal conduit in a Class 1, Division I hazardous (classified) location?
A11. Yes, 501.10(A)(1) requires threaded rigid metal conduit or threaded steel intermediate metal conduit. There is no mention in the NEC that the rigid metal conduit be steel, so aluminum rigid metal conduit is acceptable as long as it’s suitable for the environment.
Q12. Does the NEC require metal manhole covers to be grounded to an effective ground-fault current path as per 250.4(A)(3)?
A12. No, not specifically. Metal handhole covers are required to be bonded in accordance with 250.96(A) by 314.30(D). A similar reference dealing with manhole covers in section 110.75(D) does not include the bonding requirement. A 2008 NEC proposal to require manhole covers to be connected to an equipment grounding conductor was rejected. Code Making Panel 1 states “The construction requirements of manholes… virtually exclude the covers from contacting energized parts”.
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