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 to questions are based on the 2011 NEC.
Panelboard overcurrent devices
What is the 2008 NEC standard for the number of overcurrent devices on one panelboard? I’ve noticed that 408.35 (2005 NEC) is not even in the 2008 NEC.
In the 2005 NEC, Section 408.35—Number of Overcurrent Devices on One Panelboard, limited the number permitted to 42 overcurrent devices. This section was deleted in the 2008 NEC, and a new Section 408.54 was added to the 2008 NEC, limiting the number of overcurrent devices to that for which the panelboard was designed, rated and listed. For the 2008 NEC, 408.36 was revised, and a new Exception No. 2 was added, limiting the number of overcurrent devices to 42 where the panelboard is protected by two main circuit breakers or two sets of fuses having a combined rating not greater than that of the panelboard. No changes were made to 406.36 or 408.54 for the 2011 NEC.
Garage receptacle height
What is the required height from the floor for receptacles in a residential garage?
NEC 210.52(G) requires at least one receptacle, in addition to those for specific equipment, to be installed in each attached garage and in each detached garage with electric power, but the NEC does not have a height requirement for a receptacle installed in a residential garage.
Conductors and lugs
Can more than one conductor be put under the lug on a circuit breaker?
NEC 110.14(A) requires that terminals for more than one conductor must be so identified.
How many receptacles?
I have a question regarding the interpretation of 517.18 (B): “a minimum of four receptacles.” Would two duplex receptacles meet the Code, or are four different locations needed whether they are single or duplex? Going to the extreme, would a two-gang box with two duplex receptacles meet the Code?
The receptacles are permitted by 517.18(B) to be single, duplex, quadruplex or any combination of the three. A two-gang box with two duplex receptacles would satisfy the Code requirement.
Drive or motor match
Should the branch circuit feeding a variable-frequency drive be sized to match the drive or the motor?
NEC 430.122(A) requires that the circuit conductors supplying power-conversion equipment shall have an ampacity not less than 125 percent of the rated input current to the power-conversion equipment. For equipment using a bypass device, 430.122(B) requires the larger of either of the following: 125 percent of the rated input current to the power-conversion equipment or 125 percent of the motor full-load current rating as determined by 430.6.
How do you size the conductors from a variable-speed drive to the motor?
The conductors supplying the motor are sized in accordance with 430.6(A)(1) and 430.22. Refer to the tables in accordance with 430.6, and use 125 percent of the full-load current as required by 430.22.
‘Suitable Only for Use as Service Equipment’
Why are some panelboards marked as “Suitable Only for Use as Service Equipment”?
Panelboards marked as such have the neutral bar factory-bonded to the frame or enclosure. NEC 250.142(B) does not permit panelboards bonded in this manner to be used on the load side of the service-disconnecting means.
Rubber mat voltage
What voltage is required for a rubber mat in front of switchgear?
The NEC does not have a requirement for a rubber mat in front of switchgear. Electrical safety matting has a minimum required thickness in order to provide protection adequate for the voltage involved. Check out NFPA 70E for safe work practices and protective equipment.
Dividing a conductor
Can a stranded neutral conductor be divided into two parts and be terminated in two adjacent holes in the neutral bus?
NEC 408.41 requires that each grounded conductor be terminated in an individual terminal.
Ground up on receptacles
Is ground up on receptacles a preference or a Code requirement? If the ground is up and the cord end starts to pull out and hang down, the ground is the first thing to be disconnected. I’m told it is to protect if an object was to fall against the cord end when it is pulled away from the receptacle.
There is no Code requirement for the orientation of the ground prong on a grounded receptacle. Every Code cycle, proposals are submitted to require the ground up or ground down with the submitters substantiation as to the importance of installing it that way. These proposals have always been rejected, leaving the orientation up to the installer. The ground prong on a plug is longer than the other prongs and is always the last to become disengaged.
Minimum size conductor
We have a situation where a 3,000-ampere (A) service is been fed underground from a metering cabinet approximately 50 feet from a main switchgear. For this we are using 8 parallel runs of 600 kcmil copper conductors in 4-inch conduit. What would be the size of the equipment-grounding conductor to be run in each conduit run with the phase and neutral conductors?
The minimum size equipment--grounding copper conductor required for a 3,000A overcurrent device is 400 kcmil as shown in Table 250.122. Where conductors are run in parallel in multiple raceways, the equipment--grounding conductors are required to be installed in parallel in each raceway. Each equipment-grounding conductor shall be sized according to 250.122. This means that a 400 kcmil equipment-grounding conductor must be run in each conduit.
Working space requirements
Does the Code contain working space requirements for equipment operating at 30-volts (V) or less?
NEC 110.26(A)(1)(b) permits, by special permission, smaller working spaces where all exposed live parts operate at not greater than 30V rms, 42V peak, or 60V direct current.
Service-entrance conductor distance
What is the maximum distance that service-entrance conductors can be run in a building to the service-disconnecting means?
The NEC does not specify a maximum distance that service-entrance conductors can be run in a building to the main service-disconnecting means. The requirement in 230.70(A)(1) is that the service-disconnecting means be located nearest the point of entrance of the service conductors. It is important that these conductors be kept to a minimum because they are protected only by the supplying utility and have limited protection.
Interpretation of the 250.53 electrode listings
Recently, a city electrical inspector turned me down on a service change I did on an old house. The existing service was on the back of the house, and the grounding electrode used was one ground rod. I drove another ground rod at the required 6-foot distance and bonded them together per 250.53(A)(2). The inspector required the water pipe as a grounding electrode. My interpretation is that any of the 250.53 electrode listings are acceptable as long as they meet the resistance requirements. Can you clarify where the metal water pipe entrance is always required and if it is the only acceptable choice as the inspector insisted?
NEC 250.50 requires that if a metal underground water pipe, in direct contact with the earth for 10 feet or more [250.52(A)(1)], is present at the building, it must be bonded to the grounding--electrode system. NEC 250.53(D)(2) requires that a metal underground water pipe be supplemented by an additional electrode, which can be the ground rod. Note that the requirement specifies “present,” not “available,” which means you must do what is necessary to bond to the water pipe.
Three sets of conductors
Can I run three sets of 500 kcmil copper conductors to feed a 1,200A switch?
Table 310.15(B)(16) shows a 500 kcmil copper conductor with a 75-degree rating as having an ampacity of 380A. Three sets in parallel is 1,140A. NEC 240.4(B) permits the next higher standard overcurrent device rating above the ampacity of the conductor to be used where the next higher standard rating does not exceed 800A. NEC 240.4(C) requires that, where the overcurrent device is rated over 800A, the ampacity of the conductors it protects shall be equal to or greater than the rating of the overcurrent device. Therefore, you cannot run three sets of 500 kcmil copper conductors for a 1,200A system.
TROUT answers the Code Question of the Day on the NECA website. He can be reached at firstname.lastname@example.org.