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.
Dry-type transformer protection
I have some questions regarding overcurrent protection for a dry-type transformer. What are the NEC requirements for a disconnect switch on a transformer 50 kilovolt-amperes (kVA) or less?
Overcurrent protection for transformers rated 600 volts (V) or less can be found in 450.3(B), which includes Table 450.3(B). Section 450.14 is new in the 2011 Code and requires a disconnecting means located either in sight of the transformer or in a remote location. Where located in a remote location, the disconnecting means must be lockable, and the location of the disconnecting means must be field-marked on the transformer. NEC 450.13(B) permits dry-type transformers not exceeding 50 kilovolt-amperes to be installed in hollow spaces of buildings not permanently closed in by structure provided they meet the ventilation requirements of 450.9.
Baseboard heaters on a branch circuit
I have two 120V baseboard heaters on a branch circuit. Each one draws 8.3 amperes (A). What should the branch-circuit rating be?
Baseboard heaters do not have amperage ratings on their nameplates. The only way you can get that amperage is by making a computation. The baseboards are obviously rated at 1,000 watts (W), which, when divided by the 120V, gives you 8.33A. Section 220.5(B) permits fractions of an ampere to be dropped when computations result in fractions of an ampere less than 0.5. So you have 8A for each of two baseboards, resulting in a total of 16A. Section 424.3(b) requires the rating of the branch-circuit overcurrent protective device to be not less than 125 percent of the total computed load of the heaters. Sixteen amperes times 1.25 yields 20A for the rating of the branch circuit.
In your January 2012 answer to “How much free conductor?” you stated that 314.16(B)(1) permits unbroken conductors passing through the box to be counted as one, and it doesn’t restrict the length. What about the line in that same section, “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?”
Each loop or coil of unbroken conductor whose length is twice the minimum length for free conductors shall be counted twice. This is done because, if the loop or coil is twice the length for free conductors, it could be cut, and each conductor could then be used to connect to a device.
Arc-fault breaker requirements
After 2011, arc-fault breakers are required on all the lighting circuits throughout a residence with few exceptions. I found a problem with small electronic arcing loads, especially the motion sensors required for Title 24 in California. My Code says that the arc-fault breaker is supposed to tell the difference between an electronic arc such as a dimmer, ballast or transformer and the type that can cause a fire. They trip frequently or all the time with required motion sensors and, also, I believe other electronic loads the breaker is required to protect. What do you do?
The NEC arc-fault circuit interrupter protection requirements for dwelling units (residences) are shown in 210.12. Where an arc-fault breaker is installed in an existing home, a common cause of tripping will be that the neutral of the circuit is mixed somewhere with the neutral of another circuit. The two common places this mixing of neutrals would occur are at a junction box where both circuits are present or in a three-way-switch system where the neutral for the light(s) has been used from another circuit. What should you do? Title 24 is the energy law in California, and you are required to follow the law. The NEC is not a law and only becomes a law if a municipality adopts it by reference. However, I have to say there was a big outcry when arc fault was required, but to me, it appears that most of the problems were caused by improper installation. Following the requirements of 110.3(B) is essential.
OCPD for a water heater
When sizing the overcurrent protection device (OCPD) for a water heater, are the OCPD and the branch-circuit conductors required to be sized at 125 percent? From what I’ve read in Article 422, it states that any water heater that is less than a 120 gallons needs to follow the 125 percent rule.
NEC 422.13 requires that a fixed storage-type water heater that has capacity of 120 gallons or less be considered a continuous load for sizing branch circuits. The water heater nameplate rating is used for the purpose of determining the branch-circuit rating.
There was a question in the February 2012 Electrical Contractor about supporting a drop of LFMC about 12 feet from a tray. You implied that support is required every 4 feet. I believe, however, Exception No.1 applies to the question. The Code says: “(A) Securely Fastened. LFMC shall be securely fastened in place by an approved means within 300 mm (12 in.) of each box, cabinet, conduit body, or other conduit termination and shall be supported and secured at intervals not to exceed 1.4 m (4 ft). Exception No. 1: Where LFMC is fished between access points through concealed spaces in finished buildings or structures and supporting is impractical.” I read that as no support may be needed.
The question asked for specific information regarding the Code requirement for supporting liquidtight flexible metal conduit (LFMC) hanging 12 feet from a cable tray to production equipment. NEC 350.30(A) requires a means of support at intervals not to exceed 4 feet. The exception has nothing to do with the question but provides information for other types of installations. For instance, if LFMC were “fished in” through a concealed space in a building or other type of structure, support would be impractical and is not required.
Copper bus ampacity
What Code section gives the ampacity of copper bus bars size, inch by 4 inches?
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. This can be in the form of a -inch-by-4-inch bus bar. A -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 -inch-by-4-inch aluminum bus bar would be approximately 2,000A.
Up, down, parallel?
What is meant by a parallel arc fault and a series arc fault, and upstream and downstream?
A parallel arc fault is an arc caused by a short circuit between a phase conductor to ground or to a grounded or neutral conductor or to another phase conductor of opposite polarity. Series arc faults are caused by a current-carrying conductor that is broken and making intermittent contact at the break, by a loose connection at a conductor termination point, or a loose connection at a splice point. References to upstream or downstream relate to whether the arc-fault protective device will protect the circuit in front of the device, after the device or both.
TROUT answers the Code Question of the Day on the NECA-NEIS website. He can be reached at firstname.lastname@example.org.