If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty in understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions can be sent to email@example.com.
Field-assembled lighting control panel
I have installed a lighting control panel consisting of a Hoffman weatherproof enclosure, a six-circuit main lug-only panel, a 2-pole contactor, a photocell and associated parts, all sized according to the National Electrical Code (NEC). A 30-ampere (A) circuit approximately 20 feet away feeds the contractor which feeds the MLO panel. The MLO panel has two 2-pole, 20A circuit breakers that feed six 150-watt (W) HPS fixtures. All components are Underwriters Laboratories-listed and assembled in the field and installed per the manufacturer’s instructions and the NEC. Does a field-assembled lighting control cabinet made up of UL-listed components need to be listed or field--certified as an assembly?
There is no NEC requirement that a field-assembled lighting control panel, consisting of listed components, be listed by a recognized testing laboratory. These same components could be installed on a wall without being in an enclosure. Installing them together in an enclosure does not subject them to additional listing requirements.
Calculating an overcurrent device
Why is it permitted to protect 10 AWG motor circuit conductors supplying a 7 horsepower (hp), 230-volt (V), FLA 19.2, three-phase motor with a 60A overcurrent device?
The motor branch-circuit conductors that supply the motor are required by 430.22(A) to have an ampacity of not less than 125 percent of the motor’s full-load current. In accordance with 430.6(A)(1), the motor full-load current rating is not taken from the motor nameplate but is taken from the Motor Full Load Current Tables 430.247, .248, .249 or .250. Since your motor is a 7 horsepower, 230V, three-phase, alternating current (AC) motor, Table 430.250 must be used, and it shows a full-load current rating of 22A, which, when multiplied together (125 percent 22), equals 27.5 ampacity for the motor branch-circuit conductors. According to Table 310.16, using the 75 degree column, 10 AWG conductors can be used.
Now we must determine the motor branch-circuit overcurrent protection. This is called the branch-circuit short-circuit and ground-fault protection and is determined using Table 430.52. For a polyphase motor, when using an inverse time breaker, you are permitted to use a rating 250 percent of the motor full-load current shown on the motor full-load current tables. Multiplying the full-load current rating of 22A by 250 percent equals 55A. Since 55A is not a standard rating, 430.52 Exception No. 1 permits the use of the next higher rating, which according to 240.6, is 60A.
Then, we must determine the motor overload-protection rating. NEC 430.32(A)(1) requires that an overload device rated at 125 percent of the motor nameplate full-load current rating must be provided for this motor. Note that for overload device rating, the motor nameplate full-load current rating is used. The motor nameplate full-load current rating is 19.2A, multiplied by 125 percent, which equals 24A-rating for the motor-overload device. This means the motor can run at a 25 percent overload for an indefinite period of time without damage to the motor circuit conductors, which are sized at 125 percent of the motor full-load current.
Based on the above, the 10 AWG motor branch-circuit conductors are protected by an inverse-time circuit breaker rated at 60A. This is permitted because a short circuit or ground fault is of such magnitude that the inverse-time circuit breaker will open before any damage would occur to the motor insulation, and the motor-overload protection will protect these conductors in case of a prolonged overload.
Transfer switch purpose
I have a question about installing a standby generator and transfer switch. The building is a single-family residence with a 200A, single-phase, 120-–240V service. The installer wired a 200A transfer switch with no main disconnect between the meter can and the transfer switch.
There are numerous ways to connect a standby generator and an automatic transfer switch. In this installation, the standby generator apparently is going to serve the entire load. Service equipment must be installed upstream of the automatic transfer switch or it can be incorporated within it. This would require the transfer switch to be listed as suitable for use as service equipment.
Buried cable requirement
Does the NEC require underground medium-voltage 14-kilovolt (kV) cable to be in conduit, encased in concrete?
Medium-voltage cables identified for direct burial are not required to be in concrete-encased conduit. Underground cables installations are covered in 300.50. The 14-kV direct burial cable must be buried 30 inches and in accord with a new footnote 3d that was added to Table 300.50 for the 2008 NEC: cables that are buried 30 inches or more and not encased in concrete must have their location identified by a warning ribbon.
Two-pole circuit breaker concerns
On the issue of three-wire circuits requiring a 2-pole circuit breaker: Multiwire branch circuits of this type help to reduce labor/material costs and help to balance loads, etc. Now, with this ruling, I would find it difficult to want to use it for the very reason stated. I would not want this in my business place if a problem were to occur (and shut off two instead of one circuit). So, now I would have to install all two-wire circuits, larger conduits, etc. Your comments would be appreciated.
The new rule in the 2008 Code requires that each multiwire circuit must be provided with a means to simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates (210.4B). This requirement was put in the 2008 Code to reduce the risk of shock to people working on equipment supplied by a multiwire circuit. An example is a kitchen countertop receptacle with the break-off tab removed being supplied by a multiwire circuit. A workman doing maintenance on this receptacle could turn off one circuit breaker not knowing that it is fed from two sources.
Yes, multiwire circuits reduce labor and material costs, but shared neutrals add to harmonic problems; however, the main concern should be balancing out safety as opposed to cost.
Branch circuits serving utilization equipment
I am going to install an individual 125V branch circuit in a kitchen of a single-family dwelling to serve a cord-and-plug-connected dishwasher. The dishwasher load is 8.5A. My duplex receptacle is rated 15A. Does the NEC permit 14 AWG Type NM cable protected by a 15A-rated circuit breaker to serve this dishwasher?
An individual branch circuit is defined as a branch circuit that supplies only one piece of utilization equipment. An individual branch circuit cannot supply a duplex receptacle where it would be possible for two loads to be supplied. A single receptacle must be used and must have a rating not less than that of the branch circuit 210.21(B)(1), which states a 15- or 20A individual branch circuit, as described in (Table 210.21(B)(2)), can protect a 15A-rated single receptacle. So the NEC permits a 14 AWG NM cable protected by a 15A rated circuit to serve this dishwasher.
Circuit conductor compatibility
Is there any rule that would allow Class 2, 24V circuit conductors in the same field-installed conduit with 120V or higher circuit conductors?
NEC 725.136 does not permit Class 2 conductors to be placed in raceways with conductors of electric light and power unless it is permitted by 725.136(B) through (I).
Cable serving residential HVAC
Can Type NM cable be installed in 6 feet of liquidtight flexible-metal conduit used to feed a residential HVAC unit that is located outdoors?
A new Section 300.9 in the 2008 Code requires that where raceways are installed in wet locations above grade, the interior of these raceways shall be considered to be a wet location. Note that this new section does not include interiors of enclosures, as does 300.5(B) for underground installations. Based on the above, the interior of the liquidtight flexible-metal conduit is considered to be a wet location and 334.12(B)(4) does not permit Type NM cable to be used in a wet or damp location.
TROUT answers the Code Question of the Day on the NECA NEIS website. He can be reached at firstname.lastname@example.org.