Code Quandaries

Jul 1, 2009 12:00 PM, By Mike Holt, NEC Consultant

Stumped By the Code?

Q. I am having a hard time understanding the rule on how to size the circuit breaker/fuse for a motor that has separate overload protection. Can you please explain this rule?

Conductor sizing guidelines for a 2-hp, 230V, single-phase motor.

A. Sure. The motor branch circuit, short circuit, and ground-fault protective device must comply with the following:

• A motor branch circuit, short circuit, and ground-fault protective device must be capable of carrying the motor's starting current. [430.52(B)]

• Each motor branch circuit must be protected against short circuit and ground faults by a protective device sized no greater than the percentages listed in Table 430.52. [430.52(C)]

Example: What size conductor and inverse time circuit breaker are required for a 2-hp, 230V, single-phase motor?

Step 1: Determine the branch-circuit conductor [Table 310.16, 430.22(A), and Table 430.248]: 12A x 1.25 = 15A

A 14 AWG copper conductor is rated 20A at 75°C [Table 310.16]

Step 2: Determine the branch circuit protection [240.6(A), 430.52(C)(1), and Table 430.248]: 12A x 2.50 = 30A

Q. We drove a ½-in. ground rod with a tested resistance of less than 10 ohms. We are now being told that this rod needs to be 5/8 in. in size because it isn't listed. If the rod yields a reading of less than 25 ohms, do I really have to remove it and install a larger rod?

A. Well, unlisted ground rods made of stainless steel, copper coated steel, or zinc coated steel must have a diameter of at least 5/8 in. Listed ground rods must have a diameter of at least ½ in. Therefore, your inspector is correct. [250.52(A)(5)(b)]

Q. We have been written up for installing 15A receptacles on the 20A circuits serving a residential kitchen. Have we been doing this wrong for all of these years, or is this a new rule?

A. Where connected to a branch circuit that supplies two or more receptacles, receptacles must have an ampere rating in accordance with the values listed in Table 210.21(B)(3). Remember that a duplex receptacle has two contact devices on the same yoke [Art. 100], which means even one duplex receptacle on a circuit makes that circuit a multi-outlet branch circuit. That said, a 15A duplex receptacle is allowed to be installed on a 20A branch circuit.

Q. Does the Code tell us the largest size conductor we can splice with a twist-on type wire connector?

A. Not really. The closest thing you are going to find is 110.14, which tells us that conductor splicing devices must be identified for the conductor material, and they must be properly installed. The answer to how many conductors and what size can be installed in a wire connector is simply "whatever the connector is listed for."

Q. We installed a 2-wire, 20A circuit plus a twisted pair of 18 AWG THHN/THWN conductors for data signal inside of a raceway serving some gasoline dispensers. Now we are being told that we have four current-carrying conductors inside of the raceway and need to de-rate the conductor ampacity as per 310.15(B)(2)(a). Do we really have to do this?

A. No, the ampacity doesn't have to be adjusted because the twisted pair cables are not considered current-carrying conductors. See 725.51(B).

Q. Do I need to provide the same working space requirements discussed in 110.26 for a fire alarm control panel?

A. Yes, you do. For the purpose of safe operation and maintenance of equipment, sufficient access and working space in accordance with 110.26 must be provided. Remember that Art. 110 contains the general requirements of the Code, which are applicable to all installations covered in Chapters one through seven (such as fire alarm systems noted in Art. 760). Because nothing in Art. 760 supplements or modifies [90.3] the general rule of 110.26, compliance is mandatory.

Note: Where special permission is granted in accordance with 90.4, working space for equipment that operates at not more than 30VAC or 60VDC can be less than the distance in Table 110.26(A)(1) [110.26(A)(1)(b)].

Q. We cut our wire too short in a few outlet boxes so I spliced on a couple inches of conductor to make sure I had the required 6 in. of free conductor at each location. The inspector says the free conductor must be "unspliced." Is he right?

A. No, he is not. He is correct that the NEC requires at least 6 in. of free conductor be available from the point in the box where the conductors enter the enclosure [300.14]. However, nowhere in this rule does it require that the free length of conductor be unspliced.

Q. In a one-family dwelling, I have a hot tub disconnect located 15 ft from the hot tub. The disconnect is plainly visible from the tub. Someone told me the disconnect is required to be visible from the motor as well. Is this true?

A. No. The maintenance disconnecting means for hot tubs must be readily accessible and located within sight and at least 5 ft from the hot tub unless separated from the open water by a permanently installed barrier that provides a 5-ft reach path or greater. This horizontal distance is measured from the water’s edge along the shortest path required to reach the disconnecting means [680.12].

Note: For other than one-family dwellings, a clearly labeled emergency shutoff or control switch to interrupt power to the circulation pump is required. This emergency shutoff must be located within sight and within 5 ft of a spa or hot tub [680.41].

Q. We paralleled two sets of 500kcmil phase conductors on the secondary side of a customer-owned transformer. How do I size the equipment grounding conductor for each raceway, if the raceway is flexible metal conduit?

A. An equipment bonding jumper must be run to the secondary system disconnecting means. Where the secondary equipment grounding conductor is of the wire type, it must be sized in accordance with Table 250.66, based on the area of the largest ungrounded secondary conductor in each raceway or cable [250.30(A)(2) and 250.102(C)].

Example: What size equipment bonding jumper is required for each flexible metal conduit containing 500kcmil secondary conductors? As per Table 250.66, the answer is 1/0 AWG.

CORRECTION:

A reader recently wrote in with a question concerning the derating of conductor in a wireway, as discussed in the April 2009 edition of EC&M magazine in “Code Quandaries.” In the answer to the first question on page 38, the text stated that the derating of 310.15(B)(2)(a) is only required when the number of conductors exceeds 30. Although that is correct for metal wireway, as discussed in 376.22(B), the reader suggests this is not the case for non-metal wireway. He notes that 378.22 says, “the derating factors specified in 310.15(B)(2)(a) shall be applicable to the current-carrying conductors up to an including the 20 percent fill specified above.” In short, conductors in non-metal wireways should be treated the same as conductors in a conduit, and 310.15(B)(2)(a) applies. Mike Holt, NEC expert and author of “Code Quandaries,” confirms this analysis is correct. We apologize for any confusion this inadvertent error may have caused.