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Stumped by the Code?

Apr 1, 2007 12:00 PM, By Mike Holt, NEC Consultant

All questions and answers are based on the 2005 NEC.

Q. Is it legal to make a mechanical transition from electrical metallic tubing (EMT) to flexible metal conduit with a fitting that joins these two raceways instead of installing a box?

A. Yes, if the fitting is identified for the use, and the conductors are not spliced or terminated within the fitting [300.15(F)]. Otherwise, a box is required (Figure).

Q. Is the electrician required to leave us a way to bond the satellite dish to the electrical system?

A. Yes, an accessible bonding point must be provided at service equipment — and at the disconnecting means of separate buildings or structures for connecting intersystem bonding and grounding electrode conductors [810.21(F)(1)(c)]. According to 250.94, that point can be any one of the following:

• An exposed, nonflexible metallic raceway.

• An exposed grounding electrode conductor.

• An external connection approved by the authority having jurisdiction (AHJ).

Note: The 2008 NEC contains some new rules for this application.

Q. Can I use a single receptacle without GFCI protection to supply a refrigerator in a kitchen?

A. If this is a dwelling unit, the answer is yes. You can use either a single or duplex receptacle to supply the refrigerator as long as it is not installed to serve the countertop, according to 210.8(A)(6). However, all 15A and 20A, 125V receptacles installed in commercial kitchens (even those that do not supply the countertop surface) must be GFCI protected. In other than dwelling units, a kitchen is defined as an area with a sink and permanent facilities for food preparation and cooking [210.8(B)(2)].

Q. The NEC has a requirement that the grounding conductor for satellite systems be as short as possible. Does this mean the satellite dish must be located as close as possible to the electrical service ground?

A. No, it does not. Actually, 810.21(F)(1) requires that the grounding conductor be connected to the nearest accessible location of the following:

• Building or structure grounding electrode system [250.50].

• Interior metal water piping system, within 5 feet from its point of entrance [250.52(A)(1)].

• Accessible service bonding means [250.94].

• Metallic service raceway.

• Service equipment enclosure.

• Grounding electrode conductor or the grounding electrode conductor metal enclosure.

If a ground rod is installed to serve as the ground for the dish, it must be connected to the building's power grounding electrode system with a minimum 6 AWG conductor [810.21(J)].

The bonding of separate system electrodes (building and radio and television equipment electrode) reduces voltages that may develop between the building's power and the radio and television equipment grounding electrode system during lightning events.

Q. I have a 480V to 120V, single-phase 1.5kVA transformer. The engineer tells me that no overcurrent protection is required on the secondary side, because it's provided by the primary protection device. Is this true?

A. Yes, as long as the primary overcurrent protection does not exceed 5A. The rule is as follows: “The primary overcurrent protection device sized in accordance with 450.3(B) can protect the secondary conductors of a 2-wire system, provided the primary protection device does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio [240.21(C)(1)].

Let's review an example to better illustrate this concept.

Step 1. Calculate the primary current for a 1.5kVA, 480V single-phase transformer.

I = VA ÷ E

I = 1,500VA ÷ 480V = 3.13A

Step 2. Size the primary overcurrent protection device in accordance with 450.3(B). The primary overcurrent protection must not exceed 167% of the 3.13A primary current rating.

Primary protection (max) = 3.13A × 167% = 5.2A

Now, confirm that the 5A primary overcurrent protection is suitable to protect the secondary conductors.

Step 1. Calculate the secondary current rating.

I = VA ÷ E

I = 1,500VA ÷ 120V = 12.5A

Step 2. Size the secondary conductors.

According to Table 310.16 and 110.14(C), you must use a minimum of 14 AWG, which has an ampacity of 20A in the 60°C column of Table 310.16.

Step 3. Determine that the primary overcurrent protection device of 5A does not exceed the value determined by multiplying the secondary conductor ampacity (20A) by the secondary-to-primary transformer voltage ratio (120V/480V) [240.21(C)(1)].

Primary protection (max) = 20A × [120V ÷ 480V]

Primary protection (max) = 20A × 0.25 = 5A

Q. Does the NEC require metal manhole covers to be grounded to an effective ground-fault current path as per 250.4(A)(3)?

A. No, not specifically. Metal handhole covers are required to be bonded in accordance with 250.96(A) by 314.30(D). A similar reference dealing with manhole covers in Sec. 110.75(D) does not include the bonding requirement. A 2008 NEC proposal to require manhole covers to be connected to an equipment grounding conductor was rejected. Code Making Panel 1 states “The construction requirements of manholes… virtually exclude the covers from contacting energized parts.”

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