NEC Questions Based on the 2008 NEC

August

 

 

Here is 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.

 

By Mike Holt for EC&M Magazine

 

Q1. What are the rules for installing boxes in a plenum ceiling?

A1. Only electrical equipment constructed with a metal enclosure is permitted in a space used for environmental air [300.22(C)].

 

Q2. Can two breakers be used to provide secondary protection for a transformer?

A2. Transformers are typically protected by the primary overcurrent device only, sized no higher than specified in 450.3(B). When this method is used, the breakers on the secondary side have no role in protecting the transformer. If the value for primary protection exceeds 125% of the primary current rating, then the secondary overcurrent device shall be permitted to consist of not more than six circuit breakers or six sets of fuses grouped in one location. The total of all the device ratings shall not exceed the allowed value of a single overcurrent device [Table 450.3(B), Note 2].

 

Q3. How do you size the ground wire in a cable tray used for parallel conductors?

A3. When circuit conductors are run in parallel [310.4], an equipment grounding conductor must be sized in accordance with Table 250.122, based on the rating of the circuit overcurrent device, but this conductor is not required to be larger than the circuit conductors [250.112(A)]. But the equipment grounding conductor in the cable tray must not be smaller than 4 AWG [392.3(B)(1)(c)].

 

Q4. What are the rules for derating multiconductor cables in a cable tray?

A4. The conductor ampacity adjustment factors of 310.15(B)(2)(a) only apply to the number of current-carrying conductors in the cable and not to the number of conductors in the cable tray [392.11(A)(1)].

 

Q5. I am told that all receptacles in a dwelling need to be child proof in 2008. Is this true?

A5. Pretty much. The rule is all 15A and 20A, 125V receptacles required in the following areas of a dwelling unit [210.52] must be listed as tamper resistant.

•    Wall Space—210.52(A)

•    Small-Appliance Circuit—210.52(B)

•    Countertop Space—210.52(C)

•    Bathroom Area—210.52(D)

•    Outdoors—210.52(E)

•    Laundry Area—210.52(F)

•    Garage and Outbuildings—210.52(G)

•    Hallways—210.52(H)

 

This rule as currently written applies to 15A and 20A, 125V receptacles installed behind appliances, above countertops and other locations out of the reach of children!

Note: From 1991 through 2001 over 24,000 children less than 10 years of age were admitted to the emergency rooms of study participating hospitals for injuries involving electrical receptacles. Most of the victims were male and used a hairpin or other metal object, in the home, and sustained first or second degree burns.

 

Q6. For a panel fed from a transformer, does the panel need a main breaker if it is in the same room as the transformer?

A6. When a panelboard is supplied from a transformer, the overcurrent protection for the panelboard must be on the secondary side of the transformer. The required overcurrent protection can be in a separate enclosure ahead of the panelboard, or it can be in the panelboard [408.36(B)].

 

Q7. Can I use parallel 4/0 XHHW aluminum for a 400A breaker?

A7. Yes is most cases. The general requirement is that conductors must be protected against overcurrent in accordance with their ampacity after ampacity adjustment, as specified in 310.15 [240.4]. However, the next higher standard rating of overcurrent device (above the ampacity of the ungrounded conductors being protected) is permitted, provided all of the following conditions are met:

(1) The conductors don’t supply multioutlet receptacle branch circuits.

(2) The ampacity of a conductor, after ampacity adjustment and/or correction, doesn’t correspond with the standard rating of a fuse or circuit breaker in 240.6(A).

(3) The overcurrent device rating doesn’t exceed 800A.

 

Q8. Can I use shared neutrals on an AFCI breaker?

A8. Only if a 2-pole AFCI is used. Single pole AFCI breakers will not operate on a multiwire branch circuit.

 

Q9. What is the difference between a plenum and other space used for environmental air?

A9. A plenum is a physically constructed chamber to which ducts are connected and form part of the air distribution system [Article 100 Plenum]. The space above a dropped ceiling used for other spaces used for environmental air-handling purposes is not a plenum, it’s an “other space used for environmental air” [300.22(C)].

 

Q10. Is a Ufer ground required if there is a vapor barrier under the concrete of the footing?
A10. If a moisture/vapor barrier is installed under a concrete footer, then the steel rebar can’t be part of a concrete-encased electrode. The concrete must be in contact with the earth in order to qualify as an electrode [250.52(A)(3)].

Q11. Do swimming pool lights require GFCI protection?

A11. Branch circuits that supply underwater luminaires operating at more than 15V must be GFCI protected [680.23(A)(3)].

Q12. Does a water cooler require GFCI protection?
A12. Electric drinking fountains must be GFCI protected [422.52], but bottled water coolers do not.

 

Q13. Can an outside transformer supply two buildings?

A13. Yes.

 

Q14. Is a disconnect required for a hot tub?

A14. Yes, a maintenance disconnecting means is required and it must be readily accessible, located within sight, and at least 5 ft from the hot tub, unless separated from the open water by a permanently installed barrier. This distance is measured from the water’s edge along the shortest path required to reach the disconnecting means [680.12].