If you have a National Electrical Code (NEC)-related problem, are experiencing difficulty in understanding an NEC requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Send your questions to email@example.com.
I have a grounding question. I am working on a project in Anchorage, Alaska. We are adding a heating, ventilating and air conditioning (HVAC) chiller to an existing building. The chiller will be pad-mounted on a new concrete slab next to the existing building. It will be fed with a branch circuit from the existing main distribution panel and grounded with an appropriate equipment-grounding conductor. Does the Code require that the new concrete slab rebars be considered a grounding electrode and be connected to the building’s existing grounding-electrode system?
Installing a chiller in Anchorage? What will they think of next? The last sentence of NEC 250.52(A)(3) states, “Where multiple concrete-encased electrodes are present at a building or structure, it shall be permissible to bond only one into the grounding electrode system.” All segments of a footing or foundation that may individually qualify as concrete-encased electrodes are not required to be bonded, but they are permitted to be used and will enhance the grounding capabilities. The following will qualify as a concrete-encased electrode: single concrete-encased electrode consisting of not less than 20 feet of one or more bare or electrically conductive coated steel-reinforcing bars not less than ½ inch in diameter or consisting of at least 20 feet of bare copper conductor not smaller than 4 AWG.
The rebars installed in the footing at the building I am working on have an insulated coating. Do I scrape off or remove the coating where I attach the grounding-electrode conductor?
The rebars installed in your building do not qualify as a concrete-encased electrode. Rebars must be bare or have an electrically conductive coating. The whole purpose is to establish an effective ground-fault path to earth. You must use the alternate method permitted. You are required to install at least 20 feet of bare copper conductor not smaller than 4 AWG in the footing.
Where a concrete-encased electrode and a metal water pipe are used as the grounding electrodes for the grounding-electrode system, is a supplemental ground rod required for the metal water pipe?
The supplemental-electrode requirement for a metal underground water pipe does not specifically require a ground rod type of electrode. A supplemental electrode of a type specified in 250.52(A)(2) through (8) must be used. The concrete-encased grounding electrode already being used can act as the supplemental electrode required for the water-pipe electrode.
When used as part of a grounding-electrode system, is a ground rod required to meet the “25 ohm or less” requirement?
A grounding-electrode system or any individual part of the system is not required to meet the resistance requirements of 250.56. A single rod-type electrode installed where not a part of an electrode system must meet the requirement of 25 ohms or less. A single rod electrode that does not meet the resistance requirements must be augmented by one additional electrode of any type specified by 250.52(A)(4) through (A)(8).
Does Section 300-5(D) apply only to direct-burial conductors, or does a warning ribbon have to be used for underground installations using nonmetallic raceways?
First, I presume you’re talking about service laterals, since 300.5(D) only requires warning tape for service laterals. The warning ribbon is required if the service lateral is not concrete-encased and is buried 18 inches or more below grade. The warning ribbon must be at least 12 inches above the underground installation. The rule could have been more specific, but since it is referring to laterals not encased in concrete, it is referring to raceways.
Is a 10 AWG copper conductor required for an HVAC unit that is marked with a minimum circuit ampacity of 24 amperes or can I use a 12 AWG copper conductor?
Based on the 75° column of Table 310.16, a 12 AWG copper conductor has an ampere-carrying capacity of 25 amps. The asterisk refers you to 240.4(D), which refers you to Table 240.4(G) where it shows that this ampacity can be used for air conditioning circuit conductors.
Does the primary disconnect for a transformer have to be within sight of the transformer?
Currently, there is no NEC requirement for a disconnecting means to be within sight of the transformer. This does not mean you cannot install a disconnecting means within sight of the transformer. The NEC contains minimum requirements, and I believe a good design would include a disconnecting means.
Does it make a difference whether the line or load conductors are connected to the top or bottom of a main breaker in a separate enclosure?
According to the UL Marking Guide, a circuit breaker may or may not have its terminals marked as “line” and “load.” If it doesn’t have these markings, then it is acceptable to connect the line and load conductors to either set of terminals. According to NEC 240.81, circuit breakers must clearly indicate whether they are in the open “off” or closed “on” position. When operated vertically, the up position of the handle shall be the “on” position.
I have a 3-inch conduit with 3-350 kcmil conductors entering the back of a 12-foot-by-12-foot junction box and going out the top. What is the required depth of the junction box?
You will be required to comply with 314.28(A)(2) Exception, which refers you to Table 312.6(A). For one wire per terminal, a minimum depth of 5 inches is required for the junction box.
Does the NEC still permit cartridge fuses with renewable links?
NEC 240.60(D) permits Class H fuses of the renewable type to be used only for replacement in existing installations where there is no evidence of overfusing or tampering. Some manufacturer’s instructions do not permit the use of renewable fuses.
What is meant by the term “series rated”?
A circuit breaker can be used on a circuit with an available fault current higher than its marked interrupting rating if it is connected on the load side of an acceptable overcurrent device with the higher rating. As an example, if a service panel had 14,000 amperes (A) of available fault current at its terminals, circuit breakers marked 10,000A could be used if the main breaker was marked 22,000A. If a fault occurs on the load side of a branch breaker, then the main breaker and the branch breaker act “in series,” and both trip “off.” Section 240.86 has additional information relating to series ratings.
Are bushings or grommets required where electrical nonmetallic tubing (ENT) passes through holes or slots in metal framing members?
No. Section 300-4(b)(1) requires this for nonmetallic-sheathed cable (Type NM), but it is not a requirement for ENT.
I have two 120V, 1,000W baseboard heaters. What should the branch-circuit rating be?
The baseboards are rated at 1,000 watts each, which, when divided by the 120 volts, gives you 8.33 amperes. NEC 220.5(B) permits that where calculations result in a fraction of an ampere that is less than 0.5, such fractions are permitted to be dropped. So you have 8A for each of two baseboards resulting in a total of 16 amperes. Section 424-3(B) requires fixed electric space-heating to be considered continuous load. The rating of the branch-circuit overcurrent protective device must be not less than 125 percent of the total computed load of the heaters. The rating of the branch circuit must be 16A times 1.25, which requires 20A for the rating of the branch circuit.
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