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 firstname.lastname@example.org. Answers are based on the 2011 NEC.
Connecting the grounding electrode conductor
Does the grounding electrode conductor have to be connected to the neutral bus, or can it be connected to the ground bus?
NEC 250.24(4) permits the grounding electrode conductor to be connected to the ground bus in the panelboard if there is a wire from the ground bus to the neutral bus. Using the panelboard housing as a conductor is not permitted.
Explain how, when using Column “C” in Table 430.72(b), they say a 60-ampere (A) overcurrent device can protect a 12 AWG copper control circuit conductor?
The motor control circuit conductors are protected from physical damage by the raceway in which they are enclosed. The coil circuit current is small, and overload is not a problem. Short-circuit and ground-fault currents rise rapidly and will open the motor branch-circuit overload protective devices before any damage could occur.
Branch-circuit size for a water heater
I have a 40-gallon water heater in a single-family dwelling. The nameplate rating is 4,500 watts (W) for each of two elements. What size branch circuit do I need for this heater?
The arrangement of the thermostat in the heater will only permit one 4,500W element to be connected at one time. Using a nominal 240-volt (V) circuit, 4,500 ÷ 240 = 18.75 amperes (A). Section 422.13 requires a water heater with a capacity of less than 120 gallons to be supplied by a branch circuit that has a rating of not less than 125 percent of the nameplate rating of the water heater. Based on the previous computation, 18.75 × 1.25 = 23.4A. Although Table 310.16 shows a 12 AWG conductor as having an ampacity of 25A, the asterisk directs us to Section 240.4(D), where we find that a 12 AWG conductor is limited to 20A for our purposes. Therefore, the minimum conductor size is 10 AWG and the overcurrent protection or branch-circuit rating is either 25 or 30A.
In the January 2013 Code FAQs, under “Sizing conductors and overcurrent protection,” you write, “30.8 × 2.5 = 61.6.” Either this is wrong or you meant to size to 2.0. Which is correct?
Sorry, our computation was in error. Our reference here is to 430.52(C), which shows the rating of motor branch-circuit short circuit and ground-fault protection devices intended to protect the motor branch-circuit conductors. Table 430.52 shows the percentage of full-load current permitted for the rating. Our motor is a 10 horsepower, 208V, alternating current (AC) polyphase type with a full-load current of 30.8A, as determined from Table 430.250. Going down the left-hand column of Table 430.52 to AC polyphase motors and then across to the right to inverse-time breakers, which we had indicated we were going to use, we find a rating of 250 percent permitted. Multiplying 30.8A × 2.5 = 77A. Following the requirements of Table 430.72(B), we use Column C.
Support for NMC
Please explain the support requirements for nonmetallic cable (NMC). I think I read that each cable entering a box must be secured to the box. Is this a requirement? I don’t see this on the job.
It depends on whether the box is metal or nonmetallic, and, if nonmetallic, it depends on the size of the box. Section 370.17(a) requires that, where cable is used with metal boxes, the cable must be secured to the box. Section 370.17(b) requires that, where cable is used with a nonmetallic box, it must be secured to the box; however, there is an exception for single gang nonmetallic boxes no larger than a nominal size 2¼-by-4 inches, which permits the cable to enter the box without securing the cable to the box. It also permits multiple cable entries in a single cable knockout opening. Where not fastened to the box, the cable must be fastened within 8 inches of the box. Each cable entering an enclosure covered by Article 373 must be securely fastened to the enclosure. There is an exception for surface-mounted enclosures.→
Heating cable around pool
I have a customer who wants me to install heating cable in the deck around his pool. Is this permitted?
No, no, no! NEC 680.27(C)(3) does not permit radiant heating cables to be embedded in or below a pool deck within 20 feet from the inside walls of the pool. Doing so might make the pool deck an involuntary dance floor!
Junction boxes above electrical panels
On commercial jobs, we often install junction boxes above electrical panels and then bring MC cables into those junction boxes. Often, the length of conduit from the electrical panel to the junction box is more than 24 inches. I think that, per Table 310.15 (B)(3)(a), the wire from the breaker to the junction box needs to be derated, or you need to increase the wire size when you have more than three current-carrying conductors in the raceway. My fellow electrician disagrees and says that the wire from the breaker to the junction box is part of the electrical panel.
NEC 310.15(B)(3)(a) requires that adjustment factors shown must be applied for conductors where more than three current-carrying are installed for a continuous length longer than 24 inches.
Installing bunched cable through knockout
Am I permitted to install nonmetallic sheathed cables, bunched together, through a knockout or chase nipple into a panel enclosure?
The requirements for cables entering enclosures (panels) are outlined in Section 312.5(C). The basic rule is that each cable must be secured to the panel. The UL White Book indicates that, except for duplex connectors or when otherwise marked on the carton, connectors are recognized for connecting one cable only. There is an exception to 312.5(C), which permits NMC to enter the top of a surface-mounted enclosure through nonflexible raceways provided all of the conditions of (a) through (g) are met. But read this section carefully and note that the exception applies only to surface-mounted panels. Remember the practice of putting a bunch of cables in one large connector is not permitted.
Need for full-size grounding conductor?
Please explain 250.122(F), showing the need of the full-size grounding conductor in parallel installations. If the grounded conductors can be paralleled to get to the equivalent size needed to carry the supply current, then it follows that the equivalent size grounding conductors would be sufficient. They are all connected at a common point at both the load and supply ends, so any fault current should flow evenly on all conductors not just on one, as you suggest.
You’re correct up to the point where you say the conductors “are connected at a common point at both the load and supply ends so any fault current should flow evenly on all conductors.” That’s not exactly correct. While connected at a common point at the load and supply ends, the length of each of the equipment grounding conductors is different.
Assume there are two parallel nonmetallic conduits, and a fault occurs at an enclosure where an ungrounded conductor in one of the parallel conduits accidentally touches the enclosure. The fault is fed by both conductors of the same phase.
The shortest path to the grounded conductor in the supply panelboard from the fault is through the equipment grounding conductor in the conduit enclosing the faulted phase conductor. The longer the path through the equipment grounding conductor in the other paralleled conduit results in a higher impedance and causes a greater amount of ground-fault current to flow in the equipment grounding conductor in the conduit with the faulted conductor.
For this reason, each equipment grounding conductor must be capable of carrying enough current to allow the overcurrent device to operate without the equipment grounding conductor burning open.
Bare copper conductor in metal raceway
Can the grounded (neutral) service-entrance conductor be installed as a bare copper conductor in a metal raceway?
NEC Section 230.41 Exception (1) permits a bare copper grounded conductor to be used in a raceway or as part of a cable assembly.
Do I need an equipment grounding conductor?
If I use electrical metallic tubing as the raceway for a motor branch circuit and flexible metal conduit for the connection to the motor, do I need to install an equipment grounding conductor?
NEC 348.60 requires that, when flexible metal conduit is used to connect equipment for reasons of flexibility, an equipment grounding conductor must be installed.