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 a parking lot light
I keep hearing that you shouldn’t use a ground rod at a standard metal parking lot light and that you should use an equipment-grounding conductor run with the circuits. The specifications for the project I’m working on now require both grounding methods. Is there a problem with this, and will I have to install two rods if I don’t have 25 ohms to ground?
The NEC requires the equipment-grounding conductor to be run with the circuit conductors, but it does not prohibit the use of auxiliary grounding electrodes, such as the ground rods. Section 250.54 repeats the requirement shown in 250.4(A)(5), where it states that the earth shall not be used as the sole equipment-grounding conductor. These auxiliary grounding electrodes do not have to be bonded to the grounding-electrode system as shown in 250.50 and do not have to meet the resistance requirements of 250.56. There are many pros and cons concerning the value of the auxiliary grounding-electrode ground rods for metal poles. They are of no value in providing a low-impedance ground-fault path to de-energize the circuit, but they may be of some value in providing a path for lightning strikes. Apparently, the specifiers think so.
Feeder overcurrent protection size
Is the feeder or the feeder’s overcurrent protection size any different if the controller is an adjustable-speed drive rather than a NEMA-type starter?
NEC 430.122(A) requires the circuit conductors supplying power--conversion equipment included as part of an adjustable--speed drive system to have an ampacity of not less than 125 percent of the rated input to the power--conversion equipment. The conductors must be sized to the rated current on the conversion unit nameplate and not the motor full-load current. NEC 430.122(B) requires that if the adjustable drive system uses a bypass device, the conductors supplying the power-conversion equipment shall be the larger of 125 percent of the rated input to the power-conversion equipment or 125 percent of the motor full-load current rating as determined by 430.6. NEC 430.124 requires overload protection of the motor. However, (A) states that, if the adjustable-speed drive unit is marked that it includes overload protection, additional overload protection is not required.
Burial depth for UF cable
I am running UF cable from a house to a metal post light at the end of the driveway. What is the minimum burial depth for the UF cable? I am running the cable inside the metal post up to the post fixture. Should I ground the post?
Table 300.5 in Column 4 shows a minimum cover requirement of 12 inches for residential branch circuits rated 120 volts (V) or less with ground-fault circuit interrupter (GFCI) protection and maximum overcurrent protection of 20 amperes (A). Yes, although the post is grounded by its burial into the ground, 250.4(A)(5) requires an effective ground-fault current path and tells us the earth shall not be considered as an effective ground-fault current path. An equipment--grounding conductor must be run with the circuit conductors for grounding the pole.
Current transformer jumpers
I received some CTs for the current transformer fitting I installed, and the secondary terminals had jumpers across them. The service is energized, so I was going to take the jumpers off but was told not to. I thought shorting them out would burn up the transformers. Can you explain this?
When the primary is energized and the secondary is open (jumpers removed and no secondary load), there is no opposing magnetic force to limit the core flux. A small primary current will produce a very high voltage on the secondary winding. This voltage under these conditions can reach a value that may damage the insulation and be dangerous. For example, if the line voltage is 120V and the turns ratio is 120:1, the winding voltage is 120 120 or 14,400V. There are many other applications for current transformers besides transformer metering. Measuring meters, such as ammeters relaying equipment and others use current transformers. Most of these instruments are equipped with a shorting switch when the meter is not connected. Section 110.23, “Current Transformers,” reads, “Unused current transformers, associated with potentially energized circuits, shall be short-circuited.”
What is meant by the term “dead-front,” and where are dead-front panels required in the NEC?
Dead-front is defined in Article 100 of the NEC as being “without live parts exposed to a person on the operating side of the equipment.” Section 408.38 requires that panelboards be mounted in cabinets, cutout boxes, or enclosures designed for the purpose and shall be dead-front. The term is used in other places in the NEC, but basically the Code requires that distribution panels, panelboards (load centers), and switchboards (stage and theater 520.21) be constructed so that switches, circuit breakers and other electrical components can be operated without the user being exposed to live parts.
What is GTO cable, and where is it mentioned in the NEC?
Gas tube and oil burner ignition cable does not appear in the NEC other than where Type GTO is mentioned in Section 600.32(E).
To the best of my knowledge, GTO cable is used principally for neon sign installations. This cable has been evaluated by Underwriters’ Laboratories and is listed as GTO-5, GTO-10 and GTO-15 to indicate the kilovolt designations of different cable subtypes. The cable generally consists of conductors that use thermoplastic insulation, enclosed in an outer thermoplastic jacket. It is a special-use cable construction and, thus, is not listed under NEC Article 310, which lists conductors for general wiring.
20A receptacle on 15A circuit?
In a recent Code Question of the Day, you said that if you used an individual branch circuit for the refrigeration equipment in a dwelling-unit kitchen, you could use a 20A receptacle on a 15A circuit. Table 210.21(B)(3) shows only a 15A receptacle can be used on a 15A circuit. Please explain.
Table 210.21(B)(3) relates to branch circuits supplying two or more receptacles or receptacle outlets. This requirement prevents someone from looking at the receptacle and mistakenly thinking that, if it’s a 20A receptacle, it must be a 20A circuit and attempting to use loads greater than permitted for a 15A circuit. An individual branch circuit using a 20A single receptacle on 15A circuit would not permit adding other loads.
Conductors on a water heater
I am wiring a residential water heater. It is rated 4,500 watts (W) at 240V. What size copper conductors must be used to wire this water heater?
The nameplate rating of the water heater is 4,500W at 240V or 18.7A. Assuming the water heater has a capacity of 120 gallons or less, 422.13 requires the rating of the branch circuit supplying the water heater to be not less than 125 percent of the nameplate rating of the water heater. Multiply 18.75A by 1.25 = 23.4A. This rating will require a 25A fuse or circuit breaker, which is the next standard size larger than 23.4A as shown in 240.6. The next higher size is permitted by 240.3(B). In Table 310.16, we find the allowable ampacity of conductors in the 60°C column as required by 110.14(C)(1)(a). A 12 AWG conductor would be suitable for the load, but 240.4(D) permits a maximum overcurrent device of 20A for a 12 AWG conductor. The conductor would have to be a minimum of 10 AWG, which can be properly protected by a 30A overcurrent device.
Cold cathode vs. fluorescent
The designer of a house specified “cold cathode” lighting in the valances. What’s the difference between cold cathode and fluorescent?
Cold cathode is a form of neon lighting. Cold cathode tubes can be formed into various shapes. The tubes are larger than fluorescent lamps, usually 18 millimeters (mm), 20 mm and 25 mm. The life expectancy of cold cathodes is about 10,000 hours to 20,000 hours. The coves must be larger to handle cold cathode without the lamp showing. If cold cathode is installed in dwelling units, 410.140(B) and 600.32(I) do not permit open circuit voltage exceeding 1,000V, which eliminates the use of cold cathode transformers. A one- or two-lamp cold cathode ballast with an output under 1,000V must be used.
Load center info
Where can I find Code information regarding load centers?
The NEC doesn’t define or recognize the term “load center.” Load center is simply a manufacturer’s name for a class of panelboards. Load centers must meet the same requirements as panelboards in NEC Article 408. Load centers are shipped with the interior mounted in an enclosure. Panelboards are actually the interiors (guts) and are designed to be placed in a cabinet or cutout box at the job site and be accessible only from the front.
TROUT answers the Code Question of the Day on the NECA website. He can be reached at firstname.lastname@example.org.