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. Answers are based on the 2011 NEC.
Grounding electrode conductor too small?
I’ve been instructed that, if only one grounding electrode is available, which is a 5⁄8-inch, 8-foot-long ground rod (25 ohms or less), the grounding electrode conductor isn’t required to be larger than a 6 AWG copper. This rule would apply even if a 2,000-ampere (A) service is supplied by properly sized conductors. Why is the grounding electrode conductor sized so small in this application? I believe that this section applies when it is a supplemental grounding electrode and not the primary and only electrode.
Section 250.66(A) applies wherever that portion of the grounding electrode conductor is the sole connection to the rod electrode. We mean that, if other electrodes are involved, requiring various sizes of grounding electrode conductors, the bonding together of all the electrodes must be accomplished without using a smaller conductor to feed a larger conductor. The grounding electrode conductor is sized based on the short-time rating (IT) of the conductor (current current time). For a period of 5 seconds, the short-time rating is approximately 1A for every 42.25 circular mils area.
A 6 AWG copper conductor has a circular mil area of 26,240 (Table 8) divided by 42.25 = 621A. A 6 AWG copper conductor can safely carry 621A for 5 seconds. A lightning strike, which may carry thousands of amperes, is only present for a fraction of a second and will dissipate safely in 5 seconds or less. Based on the resistance of the ground rod, the 6 AWG copper conductor will carry all of the current the ground rod can dissipate into the earth in a given time frame. An underground metal water pipe has almost unlimited resistance to ground and can safely carry large amounts of fault current. For that reason, larger grounding electrode conductors are used.
Tamper-resistant receptacles in public restrooms
Does the NEC require tamper-resistant receptacles to be used in a public restroom if using ground-fault circuit interrupter (GFCI) receptacles? The contractor installed GFCI receptacles that are not tamper-resistant, and they are below 5 feet. This public restroom is stand-alone and next to a playground.
The NEC lists requirements relating to tamper-resistant receptacles in sections 406.12, 406.13 and 406.14. Tamper-resistant receptacles are not required in public restroom areas. The 5-foot measurement you reference is a part of the requirement for receptacles to be tamper-resistant in dwelling units.
Using GFCI circuit in bathroom
Can you settle the debate about whether the fan light and the vanity light can be fed from the 20A GFCI circuit as long as No. 12 wire is used?
NEC 210.11(C)(3) requires at least one 20A branch circuit to supply dwelling-unit bathroom receptacle outlets. Such circuits shall have no other outlets. These circuits can include more than one bathroom. However, there is an exception that permits that, where the 20A circuit supplies a single bathroom, outlets for other equipment within the same bathroom are permitted to be supplied in accordance with 210.23(A)(1) and (A)(2). Section 210.23(A)(2) requires that the rating of the fastened-in-place utilization equipment shall not exceed 50 percent of the branch-circuit rating. The fan and vanity lights do not exceed 50 percent of the branch-circuit rating and may be used on the 20A circuit supplying the bathroom. Section 210.23(A)(1) references that, where cord-and-plug connected equipment are used, the rating of any one utilization not fastened in place shall not exceed 80 percent of the branch-circuit ampere rating.
Identifying arc-fault types
What is the difference between a series arc fault and a parallel arc fault?
Series arc faults occur when the conductor in series with the load breaks and arcing occurs between the ends of the broken conductor or when a conductor termination becomes loose. A parallel arc fault occurs as a ground fault where an energized phase conductor contacts an opposing phase conductor or a short circuit occurs between a phase conductor and a grounded surface. A combination arc-fault device detects both series and parallel arc faults.
Connecting the grounding electrode conductor
Does the grounding electrode conductor have to be connected to the neutral, or can it be connected to the ground bus?
According to NEC 250.24(A)(4), where the main bonding jumper is a wire and is installed from the grounded conductor terminal bar to the equipment grounding terminal bar, the grounding electrode conductor is permitted to be connected to the equipment grounding bar to which the main bonding jumper is connected. The NEC does not permit the enclosure to be used as a part of the main bonding jumper.
Calculating copper bus bars
I can’t find the Code section that gives the ampacity of 4-inch-by--inch copper bus bars. Can you help me locate it?
The NEC doesn’t show the ampacity of bus bars. However, a 1-inch-square copper bus bar has an ampacity of approximately 1,250A. The 1 inch can be in the form of a 1/4-inch-by-4-inch ( 1/4 x 4 = 1) bus bar. A 1/2-inch-by-4-inch bus bar would be 2,500A. A 1-inch-square aluminum bus bar has an ampacity of approximately 1,000A. A 1/2-inch-by-4-inch aluminum bus bar would be approximately 2,000A.
GFCI protection for hand dryer
Is GFCI protection required for a hard-wired 230-volt (V) hand dryer in a nonresidential bathroom? If so, would you direct me to the proper Code reference section?
There is no requirement in the NEC for GFCI protection for a hard-wired 230V hand dryer. The only requirement for GFCI protection in an other than a dwelling unit bathroom is in 210.8(B)(1) where all 15- and 20A, 125V receptacles installed must be GFCI-protected.
Protecting service-entrance conductors
How are the service-entrance conductors protected from short-circuit, ground-fault or overload current?
The only protection from short-circuit or ground-fault overcurrent for the service--entrance conductors is the utility transformer primary protection. The NEC does not require overcurrent protection for service-entrance conductors other than overload protection, which NEC 230.90 requires where each ungrounded service conductor is required to have overload protection. Article 100 defines service-entrance conductors as service conductors. They are protected from overload by the overcurrent devices located as an integral part of the service-disconnecting means or located immediately adjacent thereto (230.91).
Motor branch-circuit conductors
Are motor branch-circuit conductors sized at 125 percent of motor full-load current able to handle the motor-starting current?
NEC 430.22(A) requires conductors supplying a single motor have an ampacity of not less than 125 percent of the motor full-load current rating as determined by 430.6(A)(1), which refers to the motor tables for full-load currents. The reason for this requirement is the permitted setting of the motor overload protection in 430.32(A)(1) is generally 125 percent, which would allow a motor to run on at a 125 percent overload for an indefinite period and could damage the motor conductors if they were not sized to accommodate.
Wiring a campsite
We are about to wire a recreational vehicle campsite. The designing electrical engineer set up the loads using 2 AWG direct-burial aluminum wire fed from a 90A breaker, which the electrical inspector specified. The pedestal has a panel and either 50 or 30A receptacles, including the proper breakers. The pedestal panels are fed in one and then out to the next, daisy-chained. A typical layout may feed into a 50-30 combination then out to a 30-30 combination, using 2 AWG aluminum. For the feed to the last 30-30 pedestal, the engineer used 4 AWG aluminum wire. The voltage-drop calculations work out fine using 4 AWG aluminum wire. The electrical inspector said we have to use 2 AWG aluminum wire throughout because we are feeding the runs from a 90A breaker. I believe feeder conductors have to be big enough to service the calculated load and not be sized to the feeder breaker. I believe the 4 AWG aluminum should be allowed. Am I wrong? Additionally, does the feeder breaker have to be 90A, or is a 100A breaker OK?
The circuit conductors must be sized to carry the load, and the overcurrent device (circuit breaker) must be sized to protect the conductors. Based on the information provided in the question, 2 AWG aluminum direct-buried conductors must be protected at not more than 90A (using the 75°C column). If 4 AWG direct-buried aluminum conductors are used, they must be protected at not more than 65A (75°C column).
TROUT answers the Code Question of the Day on the NECA-NEIS website. He can be reached at firstname.lastname@example.org.