NEC Questions and Answers

April Part 2 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. In a dwelling unit, what are the receptacle requirements for island counter tops?

A1. In dwelling unit kitchens, at least one receptacle outlet must be installed at each island countertop space with a long dimension of 2 ft or greater, and a short dimension of 1 ft or greater. When breaks occur in countertop spaces for appliances, sinks, etc., and the width of the counter space behind the appliance or sink is less than 1 ft, each countertop space is considered as a separate island for determining receptacle placement [210.52(C)(2), and (4)].

 

The receptacle outlet for the island countertop space can be installed below the countertop where no wall space or backsplash is available, as long as the required receptacle(s) is located no more than 1 ft below the countertop surface and no more than 6 in. from the counter’s edge, measured horizontally [210.52(C)(5) Ex].

 

Q2. What is the code requirement for the outdoor clearance height of temporary electrical feeders above the ground at a construction site?

A2. First of all, remember that all requirements of the NEC apply to temporary installations unless specifically modified in Article 590 [590.2(A)].

 

The following vertical clearances apply to outdoor feeders and branch circuits [225.18]:

(1) 10 ft above finished grade, sidewalks, platforms, or projections from which they might be accessible to pedestrians for 120V, 120/208V, 120/240V, or 240V circuits.

(2) 12 ft above residential property and driveways, and those commercial areas not subject to truck traffic for 120V, 120/208V, 120/240V, 240V, 277V, 277/480V, or 480V circuits.

(4) 18 ft over public streets, alleys, roads, parking areas subject to truck traffic, driveways on other than residential property, and other areas traversed by vehicles (such as those used for cultivation, grazing, forestry, and orchards).

 

Q3. When a lighting and appliance panelboard is fed from the secondary of a three-phase transformer, is the panelboard required to have a main breaker installed in it?

A3. Yes, lighting and appliance branch-circuit panelboards supplied from a transformer, as permitted in 240.21(C), must have overcurrent protection for the panelboard on the secondary side of the transformer. The required overcurrent protection can be in a separate enclosure ahead of the panelboard, or it can be in the panelboard. [408.36(A), 408.36(A) Ex. 1, 408.36(D)].

 

Exception: A panelboard supplied by a 2-wire system or a 3-wire, delta/delta-connected, three-phase system, is considered protected by the primary protection device when installed in accordance with 240.4(F) and 240.21(C)(1).

 

Note: A lighting and appliance branch-circuit panelboard is one with more than 10 percent of its overcurrent protection devices protecting “lighting and appliance branch circuits”, which are 30A or less circuits with neutral connections [408.34(A)].

 

Q4. Are there any restrictions on the outside conductor length between the secondary of a transformer and a panelboard? 

A4. Outside secondary conductors can be of unlimited length without overcurrent protection provided the conductors terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors in accordance with 240.21(C)(4).

 

Q5. What are the requirements on the location of the primary protection device for a transformer? Must it be within sight of the transformer and/or the secondary panelboard?

A5. Section 450.3 provides details on sizing the overcurrent protection for a transformer primary, but there are no requirements that the overcurrent protection device be within sight of either the transformer or panelboard, nor other location requirements.

 

Q6. Can Type MC cable be installed from fixture to fixture without support above a suspended ceiling?

A6. Type MC cable can be run unsupported where the cable is not more than 6 ft long from the last point of cable support to the point of connection to a luminaire within an accessible ceiling 330.30(D)(2). Type MC cable fittings are permitted to be used as the last means of cable support.

 

Q7. If a raceway contains three 120V circuits and each ungrounded conductor has its own white wire, instead of a common neutral, how many current carrying conductors would the NEC count as current carrying for conductor ampacity adjustment?

A7. On a 2-wire circuit, the current is the same on both conductors. Your example raceway contains six current-carrying conductors and Table 310.15(B)(2)(a) adjustment factors must be applied. 310.15(B)(4)(a) has two examples of circuits where the neutral is not considered a current carrying conductor. The neutral conductor of a 3-wire, single-phase, 120/240V system, or 4-wire, three-phase, 120/208V or 277/480V wye-connected system, isn’t considered a current-carrying conductor and are not required to be counted for ampacity adjustment.

 

Q8. On which phase of a disconnect must I land the high leg conductor of a three-phase, 4-wire delta-connected system?

A8. The Code does not specify which phase to terminate the high-leg conductor in disconnects, but the high-leg conductor must terminate on the “B” (center) phase within a panelboard or switchboard [408.3(E)].

 

It’s my understanding that the ANSI standard for meter equipment requires the high-leg conductor to terminate on the “C” (right) phase of the meter enclosure. This is because the demand meter needs 120V and it gets this from the “B” phase.

 

Warning: When replacing equipment in existing facilities that contain a high-leg conductor, care must be taken to ensure that the high-leg conductor is replaced in the original location. Prior to 1975, the high-leg conductor was required to terminate on the “C” phase of panelboards and switchboards. Failure to re-terminate the high-leg in accordance with the existing installation can result in 120V circuits inadvertently connected to the 208V high-leg, with disastrous results. A re-termination error can also cause three-phase motors to run in the opposite direction which can cause significant damage.

 

Q9. What is the maximum distance I can locate the electric meter from the service disconnect?

A9. There is no maximum distance of service conductors when run on the outside of a building or structure. However, once service conductors enter the building, the NEC requires them to terminate in a service disconnecting means placed at a readily accessible location nearest the point of service conductor entry (the NEC does not specify the maximum length within a building) [230.70(A)(1)].

 

Q10. Can I install the meter and service disconnect directly below a window, if it is supplied by an underground service lateral?

A10.  The Code does not restrict the area below a window for the installation of service equipment. Interestingly enough, the Code does not specify a minimum mounting height for the service disconnect except for mobile homes which is 2 ft minimum [550.32(F)], but it does specify that the maximum height from the center of the grip of the operating handle of a switch or circuit breaker used for service equipment, in its highest position, must not be more than 6 ft 7 in. above the floor or working platform [240.24(A)].

 

Note: The Code does not define an acceptable height for the meter, but most utility companies require it to be mounted near eye level, so check on local requirements that may apply to the meter height.

 

Q11. When are bonding bushings required on metal electrical raceways?

A11. Bonding bushings are one of the methods that can fulfill the NEC requirements for bonding of service raceways, for bonding raceways of 277/480V systems, and for bonding in hazardous (classified) locations. Here are some details:

 

When a metal service raceway terminates to an enclosure with a ringed knockout, a listed bonding device, such as a bonding wedge or bonding bushing, must bond one end of the service raceway with a bonding jumper sized in accordance with Table 250.66 [250.92(B)(4) and 250.102(C)]. If ringed knockouts are not encountered, a bonding locknut can be used instead of a bonding wedge or bonding bushing.

 

Metal raceways or cables, containing 277V or 480V circuits, terminating at ringed knockouts must be bonded to the metal enclosure with a bonding jumper sized in accordance with Table 250.122, based on the rating of the circuit overcurrent protection device [250.97, 250.102(D)].

 

Because of the explosive conditions associated with electrical installations in hazardous (classified) locations, electrical continuity of the effective ground-fault current path (metal parts of equipment and raceways) must be ensured by one of the methods specified in 250.92(B)(2) through (4). A bonding bushing is one of the acceptable methods [250.100].

 

Q12. When sizing the feeder conductors on the load side of a 1200A adjustable trip breaker, can the conductors be sized to the 1100A adjustable long time trip setting of the circuit breaker?

A12. As a general rule the ampere rating of an adjustable circuit breaker is equal to its maximum long-time pickup current setting, and that maximum would be used for sizing conductors [240.6(B)]. However, if the adjustable-trip circuit breaker has restricted access to the adjusting means, then the ampacity of the conductors can be sized based on the adjusted long-time pickup current settings [240.6(C].

 

In sizing conductors, remember that section 240.4(C) says that if the circuit’s overcurrent protection device exceeds 800A, the conductor ampacity, after ampacity adjustment and/or correction must have a rating not less than the rating of the overcurrent device.

Example: An 1100A protection device can be used to protect three sets of 500 kcmil conductors per phase, where each conductor has an ampacity of 380A at 75°C, in accordance with Table 310.16.

 

Q13. What are the rules for grounding the metal mounting straps of snap switches?

A13. Section 404.9(B) requires that the metal mounting yokes for switches, dimmers, and similar control switches, must be grounded to an effective ground-fault current path, whether or not a metal faceplate is installed. The metal mounting yoke must be grounded by metal screws mounting it to a metal box or by connection of an equipment grounding conductor to the grounding terminal on the yoke.