NEC Questions and Answers May Part 1 of 2

NEC Questions and Answers

May Part 1 of 2

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. I want to install fluorescent strip lights on a sheetrock ceiling. Can I support the fixtures directly over the recessed outlet box and connect to the branch circuit wiring through a chase nipple?

A1. No. When an electric-discharge luminaire is surface mounted over a concealed outlet box, and not solely supported by the outlet box, the luminaire must be provided with suitable openings that permit access to the branch-circuit wiring within the outlet box [410.14(B)].

Q2. Where in the code book does it say that sharing a neutral is acceptable?

A2. Article 100 defines a multiwire branch as two or more ungrounded circuit conductors with a common grounded (neutral) conductor. There must be a voltage potential between the ungrounded conductors and an equal voltage potential from each ungrounded conductor to the grounded (neutral) conductor.

Multiwire branch circuits offer the advantage of fewer conductors in a raceway, smaller raceway sizing, and a reduction of material and labor costs. In addition, multiwire branch circuits can reduce circuit voltage drop by as much as 50 percent. However, because of the dangers associated with multiwire branch circuits, the NEC contains additional requirements to ensure a safe installation. See 210.4, 300.13(B), and 408.41 for more information.

Q3. What size THHN conductor and protection device is required for a three-phase 4-wire feeder that supplies an 184A continuous load, assuming all conductors are current carrying?

A3. 250 kcmil copper with 250A overcurrent protection. The minimum feeder-circuit conductor ampacity, before the application of any adjustment and/or correction factors, must be no less than 125 percent of the continuous load based on the terminal temperature rating ampacities as listed in Table 310.16 [110.14(C), 215.2(A)(1)].

Since the load is 184A continuous, the feeder conductors must have an ampacity not less than 230A (184A x 1.25) according to Table 310.16, 75ºC column, before the application of any conductor ampacity adjustment. According to this table, 4/0 AWG conductors are suitable, because they have an ampere rating of 230A at 75ºC.

Feeder overcurrent protection devices must have an ampacity not less than 125 percent of the continuous loads [215.3]. The minimum overcurrent protection for an 184A continuous load must be rated not less than 230A (184A x 1.25). Since there is not 230A rated protection device, we must use the next larger size overcurrent protection device, which would be 250A in accordance with 240.6(A) [240.4(B)].

Now it gets a little complicated… 4/0 AWG ampacity at 75ºC without adjustment has an ampacity of 230A and this conductor is permitted to be protected by a 250A protection device, but we need to see if this conductor’s ampacity ‘after conductor ampacity adjustment’ is permitted to be protected by the 250A protection device. In this case, we need to adjust the conductor ampacity for four current carrying conductors in a raceway or cable.

Conductor Ampacity after Adjustment Factors = Table 310.16 Conductor Ampacity (based on conductor insulation rating) x Conductor Ampacity Adjustment Factor [Table 310.15(B)(2)(a)]

Conductor 4/0 THHN Ampacity after Adjustment Factors = 260A x 0.80

Conductor 4/0 THHN Ampacity after Adjustment Factors = 208A

Since the ampacity of 4/0 THHN is only 208A, it’s not permitted to be protected by the required 250A protection device. Remember the “next size up rule” - the next size up for a conductor ampacity of 208A would be a 225A device so the 250A device cannot be used “AFTER” derating. We therefore have to increase the conductor one size larger to 250 kcmil, but we need to see if this conductor’s ampacity ‘after conductor ampacity adjustment’ to permitted to be protected by the 250A protection device.

Conductor 250 kcmil THHN Ampacity after Adjustment Factors = 290A x 0.80

Conductor 250 kcmil THHN Ampacity after Adjustment Factors = 232A, okay.

Q4. What is the minimum service drop clearance above a roof?

A4. In general, overhead service conductors must maintain a minimum vertical clearance of 8 ft above the surface of a roof for a minimum distance of 3 ft in all directions from the edge of the roof [230.24(A)]. If the slope of the roof exceeds 4 in. for every 12 in, 120/208V or 120/240V conductor clearances can be reduced to 3 ft over the roof [230.24(A) Ex 2].

If the service mast is used in a through-the-roof application, 120/208V or 120/240V conductor clearances can be reduced to 18 in ft over the overhang portion of the roof [230.24(A) Ex 3]. There must not be more than 6 ft of service-drop conductors over the overhang portion of the roof and the service mast can’t be located more than 4 ft from the edge of the roof overhang measured horizontally.

Q5. How high does a meter need to be above grade? What about the service disconnect?

A5. The NEC does not specify a minimum or maximum height for meters, but it dos required switches and circuit breakers to be installed so the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, isn’t more than 6 ft 7 in. above the floor or working platform.

Note: 550.32(F) requires the service disconnecting means enclosure for mobile and manufactured homes to be mounted a minimum of 2 ft above the finished grade.

Q6. What are the NEC requirements for the minimum distance receptacles and switches must be from the bathtub/shower space?

A6. Receptacles must not be installed within or directly over a bathtub or shower stall according to 406.8(C) and switches can be located next to, but not within, a bathtub, hydromassage bathtub, or shower space [404.4, 680.70,680.72].

Note” Switches must be located not less than 5 ft from pools [680.22(C)], outdoor spas and hot tubs [680.40], and indoor spas or hot tubs [680.43(C)]. Receptacles must be located no less than 5 ft from any spas or hot tubs [680.22(A)(1) and 680.43(A)(1)]. Hydromassage bathtubs are treated like bathtubs [680.72].

Q7. Does 200.6 intend that the color gray can be used to identify the neutral conductor of one system while the color white is used to identify the neutral of another system when conductors of different systems are installed in the same raceway?

A7. Basically the answer is yes. The NEC states that where neutral conductors of different wiring systems are installed in the same raceway, cable, or enclosure, each neutral conductor must be identified to distinguish the systems by one of the following 200.6(D)]:

(1) A continuous white or gray outer finish along its entire length.

(2) The neutral conductor of the other system must have a different outer covering of continuous white or gray outer finish along its entire length. A white conductor with a colored stripe (other than green) running along the entire length of the conductor is also permitted.

(3) Other means of identification as permitted by 200.6(A) or 200.6(B) that will distinguish each system neutral conductor.

The means of identification must be permanently posted at each branch-circuit panelboard.

Note: Where a premise has branch circuits supplied from more than one voltage system, each ungrounded conductor must be identified by system [210.5(C)].

Q8. Does the Code allow the use of single pole light switches for single line-to-line lighting, or must I use two-pole switch?

A8. The Code does not have any requirements for the number of conductors needed to be opened to control a single line-to-line load. However, if you are using a circuit breaker for switching, you must use a two-pole circuit breaker or two individual single-pole circuit breakers with handle ties identified for the purpose [240.20(B)(2)].

Q9. When can a transfer switch be installed ahead of the service disconnect?

A9. Only when the transfer switch is rated as suitable for use as service equipment [230.66]

Suitable for use as service equipment means that the transfer switch contains a service disconnecting means and it’s supplied with a main bonding jumper so that a neutral-to-case connection can be made, as required in 240.24(B).

Q10. Can I run underground telephone line in the same trench with underground UF cable without separation?

A10. Yes.