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.
While doing a house inspection, I came upon a problem that I find all too frequently. The circuit breaker panel was marked, but it just said “receptacles” or “lights.” Except for some appliances, there was no definite indication of what area each circuit breaker took care of. Does the Code require better marking than that?
Yes, NEC 408.4(A) Circuit Directory requires all circuits to be legibly identified as to purpose or use on a circuit directory located on the face or inside of the panel door. Sufficient detail must be provided to allow each circuit to be distinguished from all others. Clearly, enough space is not provided in most residential panelboards, but the Code is clear; it is the installer’s responsibility to ensure the panel marking is in accordance with the Code requirements.
Connecting the grounding electrode conductor
Does the grounding electrode conductor have to be connected to the neutral bus, or can it be connected to the ground bus?
NEC 250.24(4) permits the grounding electrode conductor to be connected to the ground bus in the panelboard if there is a wire from the ground bus to the neutral bus.
Which has a higher short-circuit rating, a breaker feeding lighting or a breaker feeding a motor?
A circuit breaker is assigned an interrupting rating based on its construction. For example, it could be 22,000-ampere (A) 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 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. 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 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 difference 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(C) is about 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, which is five to six times the full-load amperes (FLA) of the motor, is considered to be the motor’s contribution to a fault. For example, if a 100A FLA motor is running at the time of a fault, the motor contributes approximately 500A to the fault.
Is there any specific Code requirement that does not allow aluminum conductors? Please provide pros and cons of using aluminum specifically for large feeders in sizes 500 kcmil to 750 kcmil.
There are many references to aluminum conductors in the NEC. NEC 110.14 shows the requirements for aluminum electrical connections, and 110.14(A) requires that terminals used to connect aluminum shall be so identified. Generally, aluminum conductors may be used anywhere copper is used; however, bare aluminum conductors cannot be in contact with the earth. Aluminum conductors are not permitted to be terminated within 18 inches of the earth. As far as using aluminum conductors in larger sizes is concerned, follow the manufacturers’ recommendations according to 110.3(B), and your installations will be trouble-free. I don’t believe aluminum cable manufacturers’ instructions mandate the use of corrosion inhibitors, but the termination-device manufacturers generally require the use of inhibitors or compounds when using aluminum conductors.
Vending machine receptacles
Are receptacles installed for vending machines in malls or other public areas required to be ground-fault circuit -interrrupter (GFCI) protected?
Yes, NEC 422.51 requires cord-and-plug-connected vending machines manufactured after Jan. 1, 2005, to include a GFCI as a part of the attachment plug or be located within 12 inches of the attachment plug. Vending machines manufactured prior to Jan. 1, 2005, shall be connected to a GFCI-protected outlet.
Neutrals and ground wires on the same terminal bar
What’s wrong with terminating the neutrals and the ground wires on the same terminal bar in a panelboard? The neutral bar and the ground bar are connected together by a bonding wire already.
The neutral bar and ground bar are only connected together in panelboards used as service equipment. In panelboards that have this bonding wire installed, it serves as the main bonding jumper required by 250.28. In many areas where metal raceways are used, a grounding terminal bar is not furnished in the panelboard. Where the panelboard is used with a nonmetallic raceway or cable or where separate equipment grounding (bonding) conductors are used, a terminal bar for the equipment grounding (bonding) conductors must be secured inside the cabinet. NEC 408.41 does not permit equipment grounding (bonding) conductors to be connected to a terminal bar provided for grounded (neutral) conductors. NEC 250.142(B) does not permit a connection of the grounded (neutral) conductor and the equipment grounding (bonding) conductors on the load side of the main service disconnecting means. A connection between the grounded (neutral) conductor and the equipment grounding (bonding) conductors would allow unbalanced (neutral) current to flow on the equipment grounding (bonding) conductors.
Counting the neutrals
Some job specs call for a neutral conductor to be installed for every circuit conductor, even though the conduit has three phases of a 4-wire wye-connected system. Do we count the neutrals when figuring adjustment factors, or does Code Section 310.15(B)(5)(b) apply?
When each circuit has a separate neutral for each phase conductor, there is no unbalanced current. The full current on each phase conductor also appears on its neutral conductor. When applying the provisions of 310.15(B)(3)(a), these neutral conductors must be counted as current-carrying conductors in accordance with 310.15(B)(5)(b).
Transformer disconnecting means
I have a 277/480V meter and main fusible service disconnect located in an electric room at a shopping mall. From the service disconnect, the 277/480V feeder leaves the electric room and goes to a dry-type transformer located in a store in the mall, which, in turn, feeds a 120/208V main breaker distribution panel for the store. Am I required to have disconnecting means for the transformer located in the same room with the transformer?
The 2011 NEC introduced a new Section 450.14 Disconnecting Means. This disconnecting means can be located either in sight of the transformer or in a remote location. Where located remotely, the disconnecting means must be lockable, and the location must be field-marked on the transformer.
How many screws?
When wiring receptacles, is it permitted to use both screws on the neutral side for feed-in and feed-out, or should you pigtail the neutral and just use one terminal screw?
The answer can be found in Section 300.13(b) Device Removal. In multiwire branch circuits, the continuity of the grounded (neutral) conductor shall not depend on device connections. If you are installing a multiwire circuit, the grounded (neutral) conductor serves more than one circuit. If you use both screws for connecting the grounded conductor, removal of the receptacle would open the neutral path and would affect the continuity of the other circuits using that neutral conductor. This would not be a grounding problem, but there would be no path for unbalanced current flow, resulting in a division of voltages across loads in a manner where both over--voltage and under-voltage could occur. In single circuits (not multiwire), there is no problem in connecting to both screws on a neutral terminal of a receptacle.
TROUT answers the Code Question of the Day on the NECA-NEIS website. He can be reached at firstname.lastname@example.org.