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, send in your questions and we will let the Code decide. Questions can be sent to firstname.lastname@example.org. Answers are based on the 2011 NEC.
Panel in the bathroom
Is it legal to have an electrical panel located in a bathroom?
An electrical panel containing the service disconnecting means cannot be located in a bathroom [230.70(A)(2)]. In dwelling units and guest rooms or suites of hotels and motels, overcurrent devices cannot be located in bathrooms [240.24(E)]. In other than dwelling units and guest rooms in motels or hotels, panelboards containing overcurrent devices can be located in bathrooms. The requirements of 110.26(A) regarding working space, (D) illumination and (E) dedicated space must be followed.
Grounding many service panels
In a 20-unit multifamily building, do I run a grounding electrode conductor from the service panel in each unit to the closest water pipe?
The panelboard in the unit isn’t a service panel. It is a subpanel supplied by feeder conductors from the main service. The main service disconnecting means is where you connect the grounding electrode conductor. The main service disconnect is fed by the service entrance conductors, and the grounding electrode conductor is sized according to the size of the service-entrance conductors (250.66).
How much free conductor?
Am I required to leave 6 inches of conductor extending from a box where I am just pulling through the box? In other words, I’m not terminating at a device installed in that box.
NEC 300.14 requires that at least 6 inches of free conductor, measured from the point in the box where it emerges from the raceway or cable sheath, be left at each outlet box and be long enough to extend at least 3 inches outside the box opening. If the conductors are not terminated or spliced at the outlet box, there’s an exception to this requirement. This means that these conductors can be pulled in tightly. However, I always followed the “better safe than sorry” idiom and left enough slack so that I could use the conductors in the future if necessary. NEC 314.16(B)(1) permits each conductor that passes through the box without termination or splice to be counted once but doesn’t restrict the length of the conductor loop.
Supplying a gas furnace
Where does it say that a gas-fired furnace must be supplied from a separate circuit?
Section 422.12 Central Heating Equipment, requires central heating equipment to be supplied by an “individual” branch circuit [210.21], not a separate one. As defined, an individual branch circuit is one that supplies only one utilization equipment. An exception to 422.12 permits auxiliary equipment directly associated with the heating equipment, such as a pump, valve, humidifier or electrostatic air cleaner to be connected to the same branch circuit.
Bonding around knockouts
Is bonding required around concentric and eccentric knockouts in metal enclosures when using metal raceways?
The last paragraph of Section 250.92(B) refers to the use of bonding jumpers around concentric or eccentric knockouts at the service. Section 250.92(B), “Method of Bonding at Services,” is referred to by 250.97 for determining bonding methods for other than service conductors. This reference is a requirement that bonding jumpers be used around concentric and eccentric knockouts in boxes and other enclosures for systems over 250 volts (V) to ground.
Hot tub and NM cable
Can a hot tub installed outdoors be wired with Type NM cable?
Type NM cable is not permitted by 334.12(B)(4) to be used in wet or damp locations, so no.
Replacing a ground connection
I have a situation where a ¾-inch EMT conduit contains No. 12 conductors on a 20-ampere (A) circuit. The installation also includes a No. 12 green grounding conductor. I want to add a 50A, 240V circuit to this conduit using two No. 8 THHN conductors. I’m allowed to use the metallic raceway as the sole ground connection for a circuit, so a separate grounding conductor isn’t required. Since one already exists, am I required to replace it with a No. 10 sized for the 40A circuit? Can I just leave the No. 12 ground in place and connect my 50A receptacle’s ground connection to the metallic box with a No. 10 green wire?
If I understand you correctly, your installation is properly grounded by the use of metallic conduit and a metal box. Grounding continuity is ensured by connecting the yoke of the receptacle to the metal box, which is grounded.
I have to drive a ground rod for an isolated ground panel. How close can I drive the ground rod to my service ground rod?
250.53(B) requires that the electrode spacing be not less than 6 feet from any other electrode.
Lighting versus motor
Which has a higher short-circuit rating: a breaker feeding lighting or a breaker feeding a motor?
This may help you. A circuit breaker is assigned an interrupting rating based on its construction. For example, it could be 22,000A root-mean-square (rms) symmetrical. Let’s say that the system is able to deliver 20,000A fault current to a point on the load side of where the breaker is installed. That would make it “legal” to use the 22,000 interrupting rating breaker. A fault on the load side of the breaker would result in the breaker seeing whatever the source available fault current is. However, when motors are involved, the motors act as a generator when a short circuit occurs. Now you have more than one source of fault current: the source (utility) and the motor(s). While the motor is still turning, the motor pumps back fault current. So, depending on how the circuits are connected, you could have a problem. Draw out a diagram to trace to just where the fault current travels. Let’s say that, in a motor control center, the motor contribution to the fault current is 3,000A. Add the source 22,000 and 3,000 for motor contribution, and you have now exceeded the interrupting rating of the breaker. A main breaker in the motor control center might only “see” the source fault current. The branch circuit breakers might see the source fault current and the motor contribution to the fault. This simply means that the branch breakers might “see” more fault current than the main breaker. It all depends on the circuitry and where that fault occurs. NEC 240.86 addresses series rating and motor contribution. Motor contribution to a fault involves a short time period: transient and subtransient. But for all practical purposes, usually the starting current of a motor is considered to be the motor’s contribution to a fault. For example, if a 100A full-load ampere motor is running at the time of a fault, the motor contributes approximately 500A to the fault.
Motor circuit sizing
I have a 400-horsepower (hp), 460V motor, which has a full-load current (FLC) of 477A, according to Table 430.250. To determine the size of the motor circuit conductors and the motor circuit overcurrent protection, do I follow 240.4(C)? Using Table 430.52, if I choose a breaker at 150 percent, it would be 715A. Should I follow 240.4(C) and size my conductor to 800A, or should I use 430.22 and size my conductor to 125 percent?
To determine the size of the motor branch-circuit conductors, you must use 430.22, which requires that the conductors shall have an ampacity of not less than 125 percent of the motor’s full-load current as determined by 430.6(A)(1). For your motor, this means using Table 430.250, which shows a full-load current of 477A for a 460V motor. This requires conductors with an ampacity of not less than 477 × 1.25 = 596A. The overcurrent protection for these conductors must be provided, according to 430.52. If you are using an inverse-time breaker, the maximum rating would be 250 percent of the full-load current or 477 × 2.5 = 1,192A, which can be modified using Exception No. 1 to 430.52 to a rating of 1,200A. For more information on installing motors and controllers, you may want to get my book, “Electrical Installation and Inspection,” (Delmar Publishers). Or you may want to get a copy of NECA 230 2010, “Standard for Selecting, Installing, and Maintaining Electric Motors and Motor Controllers (ANSI),” which I also authored. Both publications describe the six basic steps to follow when installing a three-phase electric motor.