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.
In the November issue, you state that “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.” The length of the loop is not limited; however, there is a restriction. “Each loop or coil of unbroken conductor not less than twice the minimum length required for free conductors in 300.14 shall be counted twice.” This allows for the conductor to be spliced at some future point without violating the box-fill requirements or adding an extension ring.
Yes, thanks for your additional clarification. The conductor fill must be calculated using Table 314.16(B)(1). Additionally, a conductor that has no part leaving the box shall not be counted. This refers to conductors connected to devices that are attached to the box.
Conductors for underground feeders
When sizing conductors for the underground feeders of a substation, do the tables provide enough data to properly size the conductors, or should the calculations be used to ensure the proper sizing? NEC 310.60(B) appears to say either, but not one over the other. My interpretation is that the tables and derating factors cover everything and then some for proper sizing. When I reference 310.60(D), I understand that section to be an avenue to actually downsize the conductors as shown in the tables, maybe by as much as two conductor sizes. If you size according to the tables, do you need to verify through the use of the calculation and/or thermal imaging?
The NEC is not intended to be a design manual (90.1C). Verification of design through calculations is certainly a minimum requirement for any designer.
PV disconnect location
Regarding the inquiry about determining the location of the photovoltaic (PV) disconnect, while not an NEC requirement per se, utilities seem to universally insist on a location by the meter so that, if a meter has to be changed out or removed, the service person performing this change can shut down power from the PV system to ensure there is no chance of a back-feed from the PV system to the meter. We have performed scores of PV power installations and find this required nearly every time.
NEC 690.14(C)(1) shows the requirements for the location of the PV disconnecting means. It must be installed at a readily accessible location outside of a building or inside nearest the point of entrance of the system conductors. The utility or the authority having jurisdiction can require definite locations of the PV disconnecting means.
Grounding path continuity
In a recent Code question, you stated that, no matter which side of the water meter you bonded to, you had to bond around it. In our area, the meters are located near the street. There is more than 10 feet of pipe in the ground before it enters the house. Why is the bond required?
The continuity of the grounding path shall not rely on water meters. This information may be found in NEC 250.53. To be permitted for use as a grounding electrode, the grounding path requires 10 feet or more of underground metal water pipe in direct contact with the earth. So, in your area, they don’t have water meters in the house. Well, you don’t have to bond around something that’s not there. NEC 250.50 describes the grounding--electrode system. In your question, the metal underground water pipe qualifies as a grounding electrode and must be supplemented by an additional electrode, which generally satisfies the required concrete-encased electrode.
The NEC has no minimum height for receptacles. The Americans With Disabilities Act requires commercial occupancies to have all receptacles at 18 inches or higher above the floor and all switches 48 inches or lower. These requirements extend to handicapped designated occupancies in residential buildings.
The NEC has an exception to 210.52(C)(5) that references countertop receptacle outlet locations in construction for the physically impaired.
Jumpers on or off?
I received CTs for the current transformer fitting I installed, and the secondary terminals had jumpers across them. The service is energized. I was going to take the jumpers off, but I 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 to life. For example, if the line voltage is 120 volts (V) and the turns ratio is 120-to-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. NEC 110.23 “Current Transformers” tells us “Unused current transformers, associated with potentially energized circuits, shall be short-circuited.”
75°C versus 90°C
If we connect to equipment that specifies 75°C rated terminals and internal wiring, can we use 90°C rated conductors at comparable amperage rating? An example would be 500 MCM wire at 75°C is 380 amperes (A) versus 400 MCM wire at 90°C is 380A, or do we use 500 MCM wired 90°C derated to the 75°C rate? Why or why not? What is the logic behind this?
If you have equipment rated at 75°C, you are permitted to use 90°C rated conductors provided the ampacity of the conductors is based on the 75°C ampacity of the conductor size used. The logic is that the temperature rating associated with the ampacity of a conductor shall be selected so as not to exceed the temperature rating of the connected termination. This information may be found in 110.14(C)(1)(a)(3) or 110.14(C)(1)(b)(2).
Compressor ground-fault protection
I am bidding a remodel job, and the customer is adding air conditioning. In the Code, Article 440.22(A), rating or setting for individual motor compressor, states “The motor-compressor branch-circuit short-circuit and ground fault protective device shall be capable of carrying the starting current of the motor.” Can you tell me when we had to start putting ground-fault protection on compressors? This even has my inspectors puzzled.
Don’t confuse the branch-circuit short-circuit and ground-fault protective device with ground-fault circuit interrupter (GFCI) protection for personnel. The branch-circuit, short-circuit and ground-fault protection that 440.22(A) is referring to is the protection for the motor compressor branch-circuit -conductors—that is, the fuses or circuit breaker in the switch or panel that serves the motor compressor.
Are motor-control circuits feeding remote-control devices required to have overcurrent protection?
Overcurrent protection for motor--control circuits is covered in 430.72(B). The requirements for conductors, which extend beyond the enclosure (remote) can be found in column C of Table 430.72(B). For example, if your motor branch-circuit protective device is rated at 60A and you are using copper control-circuit conductors, then you find 60 in the copper column and move to the left to control circuit conductor size where you find 12. As a result, you need to install control-circuit conductors not smaller than 12 AWG copper. If you use smaller conductors, they would require supplemental overcurrent protection.
20A receptacle on a 15A circuit
Can receptacles rated at 20A be used on 15A circuits, or can 15A-rated receptacles be used on 20A circuits? I know what the Code says in Tables 210.21(b)(2) and 210.21(b)(3), but why can’t you put a 20A receptacle on a 15A circuit?
A 125V, 20A rated receptacle has a different configuration than a 15A rated receptacle. The grounded-conductor slot is a T-slot on the 20A receptacle. If this receptacle were used on a 15A circuit, it would appear to the user as a 20A rated circuit, and the user may assume loads exceeding 12A may be used.
Double pole/single throw switch
I have had an ongoing discussion on the job about the proper use of double pole/single throw switches to switch both a 277V circuit and a 120V circuit simultaneously. Some say Section 404.8(B)—which states that a divider is needed between adjacent devices if they exceed 300V—will not allow it. I don’t think it applies because there are not two devices. Others claim that 210.4(B) would negate the use. It states, where a multiwire branch circuit supplies more than one device or equipment on one yoke, all circuits shall be disconnected simultaneously. I don’t think it applies as it is not a multiwire branch circuit but rather two circuits. I feel that the practice may not be up to Code but cannot find where it disallows this practice.
Check out 404.8(C). A multipole snap switch is not permitted to be fed from more than a single circuit unless the switch is marked as a two-circuit switch or unless its voltage rating is not less than the nominal line-to-line voltage of the system supplying the switch.
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