NEC Questions and Answers Based on the 2008 NEC
            May – Part 1

 

By Mike Holt for EC&M Magazine

Here’s the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent yesterday so you can see how you did.

Q1. We are designing a hotel that will use wall mounted A/C units in each room. Is there a demand factor that we can use for these to allow a smaller feeder, or do we have to use 100 percent of the load for each?
A1. The calculated load for a feeder or service must not be less than the sum of the branch-circuit loads, as determined by Part II article 220, as adjusted for the demand factors contained in Parts III, IV, or V [220.40]. Nothing in Parts III, IV or V provides a demand factor hotel air conditioning, so they must be taken at 100 percent.

Q2. We hired someone to install receptacles in the soffit of our house for Christmas lights. The contractor installed a cable through the rain gutter downspout to get to the roof. Is this really allowed?
A2. Surprisingly enough, nothing in the Code says you can’t, so by default it is allowed. Using UF cable as a substitute for NM or NMC cable for the wet location [340.10(3) and 340.10(4)], securing and supporting seems to be the only issue…but 334.30(B)(1) allows unsupported cables where fished between concealed access points in finished buildings or structures.

Q3. We have a very large structure that is fed from multiple utility transformers. How do we go about grounding the services?
A3. The only specific requirement is that when separate services, feeders, or branch circuits supply a building, the same grounding electrode must be used [250.58].

Q4. I am remodeling a log cabin and have to splice the feeder circuit. A junction box would look absolutely hideous in a log cabin, so I am hoping I can conceal the splice without a box. Can I?
A4. No, you can’t. A box must be installed at each splice or termination point, except as permitted in 300.15 (A) through (M) [300.15]. Nothing in (A) through (M) waives the requirement for aesthetic reasons.

Q5.  I have a 600A service disconnect that supplies four 200A panels downstream. I left the service disconnect with parallel 350 kcmil, then went to a wireway and used 3/0 to each of the panels.  The panels are main lug, and I am being told they must be main breaker, even though the taps are less than 25 feet. Can you help me understand this rule please?
A5. You really have two issues here: overload protection for the conductors, and overcurrent protection for the panels themselves. 240.21(B) allows for feeder taps without overload protection at the feeder overcurrent device, but the tap conductors must terminate in a single circuit breaker, or set of fuses rated no more than the tap conductor ampacity in accordance with 310.15 [240.21(B)(2)(2)].
In addition, each panelboard must also be provided with overcurrent protection located within, or at any point on the supply side of, the panelboard. The overcurrent device must have a rating not greater than that of the panelboard, and it can be located within, or on the supply side of, the panelboard [408.36].

Q6. Is there a maximum number of duplex outlets on a 20A Circuit?
A6. For dwelling units there is no maximum number [220.14(J)]. For nondwelling applications, each 15A or 20A, 125V general-use receptacle outlet is considered as a 180 VA per mounting strap. With that said, the maximum number of 15A or 20A, 125V receptacles on a 20A circuit is 13.
Circuit VA = Volts x Amperes
Circuit VA = 120V x 20A
Circuit VA = 2,400 VA
Number of Receptacles = 2,400 VA/180 VA
Number of Receptacles = 13

Q7. My satellite dish is on the opposite side of the house from the electrical service. I am being told that I need to drive a ground rod, then run twenty feet of wire, then drive another ground rod, then go another twenty feet and drive another until I reach the service. Please tell me this isn’t true!
A7. No, this isn’t true. For one- and two-family dwellings, the grounding conductor must not exceed 20 ft [820.100(A)(4)]. Where it’s not practicable to limit the coaxial grounding conductor to 20 ft for one- and two-family dwellings, a separate ground rod not less than 8 ft long, with fittings suitable for the application [250.70 and 820.100(C)] must be installed [820.100(A)(4) Ex]. Notice that the exception refers to “a separate ground rod”, not multiple ground rods.

Q8. In the dental office we are wiring, there are many hand wash sinks near the patient chairs. Do the receptacles near these sinks need to be GFCI protected?
A8. For nondwelling units, all 15A and 20A, 125V receptacles installed within 6 ft of the outside edge of a sink must be GFCI protected [210.8(B)(5)], but Ex No. 2 allows receptacles in patient care areas (other than bathrooms) to be unprotected.

Q9. What is the required burial depth for a grounding electrode conductor?
A9. There is no depth requirement for grounding electrode conductors, so, you can bury them as shallow as you like.

Q10. For a duct heater that has a built in on/off switch, do I still a disconnect?
A10. No, a unit switch with a marked “off” position that is an integral part of the equipment can serve as the heater disconnecting means, if it disconnects all ungrounded conductors of the circuit [424.19(C)].

Q11. Can we install a three-wire feeder (no equipment ground) to a pool house and bond the neutral and ground at the new panel?
A11. No. To quickly clear a ground fault and remove dangerous voltage from metal parts, the building or structure disconnecting means must be connected to the circuit equipment grounding conductor of a type described in 250.118. The supply circuit equipment grounding conductor must be sized in accordance with 250.122, based on the rating of the overcurrent device [250.32(B)].

Q12. For a wall mounted emergency light that contains an internal wiring compartment, do I need a box in the wall or can I simply run my cable to the back of the light with a connector and make up the wires inside the light?
A12. An integral junction box or wiring compartment is allowed in lieu of a box [300.15(B)], so you are okay.