Send to a Friend View / Add Comments  
NEC Questions 017

By Mike Holt for EC&M Magazine

Q1 Where should the high-leg conductor from a 120/240V, three-phase, 4-wire delta-connected system be landed? My utility requires it to be in the right hand or "C" phase position at the meter.

A1 I'm not sure of electric utility requirements, but it's my understanding that The ANSI standard for meter equipment requires the high-leg conductor to terminate on the "C" (right) phase of the meter enclosure. The NEC requirements as follows:

Identification. On a 4-wire three-phase delta-connected system, where the midpoint of one phase winding is grounded, the conductor with the higher phase voltage-to-ground must be durably and permanently marked by an outer finish that is orange in color or other effective means. Such identification must be placed at each point on the system where a connection is made if the grounded neutral conductor is present [110.15]. Figure 110-33

Author's Comment: Similar language is contained in 230.56 for service conductors.

Panelboards. Panelboards supplied by a 4-wire three-phase delta-connected system must have the high-leg conductor connected to the "B" (center) phase of the panelboard [408.3(E)]. Figure 408-2


WARNING: When replacing equipment in existing facilities that contain a high-leg conductor, care must be taken to ensure that the high-leg conductor is replaced in the original location. Prior to 1975, the high-leg conductor was required to terminate on the "C" phase. Failure to re-terminate the high-leg in accordance with the existing installation can result in 120V circuits inadvertently connected to the 208V high-leg, with disastrous results.

There is an exception to 408.3(E) that permits the high leg to remain in the same position as required for the metering equipment when the meter is in same single section or multisection switchboard or panelboard.

Note: The high leg is sometimes called a 'wild leg' or 'bastard leg.' Oh yea, my wife feels that maybe the term 'orphan-leg' would be more appropriate.

Q2 How many equipment grounding conductors are permitted to terminate under a single screw of a grounding bar located in a panel?

A2 The NEC only permits one conductor to terminate on a terminal, unless the terminal is identified for more than one conductor [110.14(A)]. Generally the label located on the cabinet identifies that 2 or 3 grounding conductors are permitted on a single grounding terminal. Careful, 408.21 restricts termination on the grounded neutral bar to a single conductor per terminal.

Q3 Can a 15A, 125V duplex receptacle be installed on a 20A, 120V circuit?

A3 Yes, as per Table 210.21(B)(3). The reason a 15A duplex receptacle is permitted on a 20A circuit is because internally it's rated 20A. However, a single 15A, 125V receptacle is not permitted on a 20A branch circuit [210.21(B)(2)].

Q4 Can a 15A or 20A, 125V receptacle on a dwelling unit porch serve as the required outdoor receptacle outlet?

A4 No, 210.52(E) requires one 15A or 20A, 125V receptacle outlet at the front and one at the back of dwelling unit. Such receptacles must be accessible "at grade level." Receptacles installed above a porch or deck that is elevated are not accessible "at grade level," they are accessible "from grade level" and do not meet the requirements of this section. Personally, I think this is ridicules.

Q5 A new school in my area has the classroom light switch installed on the wall across the room from the entrance. Does the NEC permit this?

A5 Yes, the NEC does not specify the location or placement of wall switches.

Q6 Does the code permit a circuit breaker to be installed horizontally?

A6 Enclosures containing overcurrent protection devices must be mounted in a vertical position unless this isn't practical [240.33]. However, where circuit breaker handles are operated vertically, the "up" position of the handle must be in the "on" position [240.81]. So, in effect, an enclosure that contains one circuit breaker can be mounted horizontally, but an enclosure that contains a panelboard/loadcenter with multiple circuit breakers would have to be mounted vertically. Figure 240-36

Q7 Can the optional method load calculation be used to size the feeder conductors for individual dwelling units of a multifamily building? The service is 120/208V, three-phase, 4-wire from a why-connected system. The individual feeders are 120/208V, single-phase, 3-wire.

A7 Yes the optional calculation method is permitted for the individual dwelling unit feeders [220.82(A)], but the conductors must be sized in accordance with Table 310.16, not Table 310.15(B)(6) [310.15(B)(6)].

For example, lets say the calculated load according to the optional method is 100A: Table 310.15(B)(6) would permit 4 AWG (for 120/240V 3-wire feeder), but Table 310.16 requires the conductor to be a minimum 3 AWG.

Q8 Is it permitted to install covers or guards over mushroom emergency stop buttons to prevent accidental operation of the emergency stop button?

A8 The NEC does not prohibit this, but the general practice in industrial occupancies is to provide a guard surrounding the button in such a way as to allow the button to be depressed by pressing straight in without requiring that a cover be opened. This method will allow rapid operation of the button when needed, but will prevent inadvertent operation by someone or something leaning on the control station.

Q9 Can service drop conductors be supported by trees?

A9 It all depends on who installed the conductors. If the installation is under the exclusive control of the utility, then this is not a violation of the NEC (but it might be a violation of the National Electric Safety Code).

If the service conductors are not under the exclusive control of the utility, then the installation violates 230.10. In addition, trees or other vegetation cannot be used for the support of overhead conductor spans of branch circuits and feeders [225.26], including temporary installations [590.4(J)].

Q10 Can nonplenum rated data cable be installed in the under floor area of an information technology room?

A10 Yes, if the cables are associated with the data processing equipment as per 645.5(D)(5)(c).

Q11 How many load-side receptacles can be installed on a "feed-through" GFCI receptacle in a dwelling unit?

A11 The code does not place a restriction on the number of receptacles, but there are practical limit based on the expected load.

Q12 Where do I find the requirements for locating smoke detector in single-family dwelling unit?

A12 The requirements are found in NFPA 72, National Fire Alarm Code, not the NEC.

A13 Can a single-pole AFCI breaker be installed on a multiwire branch circuit?

A13 NO, a single-pole AFCI breaker is not permitted on a multiwire circuit because it will not work. You'll need to use a two-pole AFCI for this purpose, just like you would if the circuit was to be GFCI protected.

  Send to a Friend View / Add Comments  

  [ Back to Top ]

Copyright © 2004 Mike Holt Enterprises,Inc.
1-888-NEC-CODE (1-888-632-2633)