This article was posted 06/05/2006 and is most likely outdated.

NEC Questions and Answers - June
 

 
Topic - NEC Questions
Subject - NEC Questions and Answers - June

June 5, 2006  

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NEC Questions and Answers

 

By Mike Holt for EC&M Magazine

 

Q1. What are the NEC requirements on grounding gas piping?

A1. Gas piping can’t be used as a grounding electrode [250.52(B)(1)], however, it must be bonded if it is likely to be become energized [250.104(B)]. The equipment grounding (bonding) conductor for the circuit that may energize the piping can serve as the bonding means. So effectively, this means that no action is actually required by the electrical installer!

 

Q2. Is all electrical equipment required to be listed?

A2. Not all electrical items required or permitted by the NEC are required to be listed, however if the equipment you are using is listed, it must be installed and used in accordance with any instructions included in the listing or labeling requirements [110.3(B)]. There are many products that the NEC does require to be listed such as receptacles [406.2(A)], low voltage lighting systems [411.3], EMT fittings [358.6], grounding and bonding fittings [250.8] and many, many others. Because of this, it may be worth your time to use all listed products, since the NEC requires so many items to be listed.  Also, there are some states, such as Washington and Minnesota, that require that all items be listed., so be sure to check any local amendments that your jurisdiction might have.

 

Q3. I have been told that when conductor ampacity adjustment is required (bundling or ambient temperature), we are to determine the conductor ampacity based on the temperature rating of the terminals in accordance with Table 310.16. Is this true?

A3. No, conductor ampacity adjustment is based on the ampacity of the conductors in accordance with the temperature rating of the conductors. Typically conductors used today are THHN in a dry location, for these circuits, the ampacity is based on the 90°C rating of Table 310.16. [110.14(C)(1)(a)(2) and 110.14(C)(2)(b)(2)].

 

For Example: If we bundle three 12/2 and one 12/3 cable (nine current-carrying 12 THHN conductors), the ampacity for each conductor (30A at 90°C, Table 310.16) is adjusted by a 70 percent adjustment factor [Table 310.15(B)(2)(a)].

Adjusted Conductor Ampacity = 30A x 0.70

Adjusted Conductor Ampacity = 21A

 

Q4. How do I size the ground wire for a circuit when the hot wires have been increased in size because of voltage drop?

A4. When ungrounded circuit conductors (hot wires) are increased in size for any reason, the equipment grounding (bonding) conductor must be proportionately increased in size [250.122(B)]. For example, if the ungrounded conductors for a 40A circuit are increased in size from 8 AWG to 6 AWG, the equipment grounding (bonding) conductor must be increased in size from 10 AWG to 8 AWG. This is because the circular mil area of 6 AWG is 59 percent greater than 8 AWG (26,240 cmil/16,510 cmil) [Chapter 9, Table 8]. According to Table 250.122, a 40A circit protection device would require a 10 AWG equipment grounding (bonding) conductor, but it must also be increased the equipment grounding (bonding) conductor in size by 59 percent (10380 cmil X 59% (1.59) =  16,504 cmil), which results in a 8 AWG.

 

Q5. Can ‘line’ and ‘load’ conductors be installed in the same raceway?

A5. Service conductors cannot be installed in the same raceway with feeder or branch-circuit conductors [230.7], but line and load conductors for feeders and branch circuits can be in the same raceway or enclosure. For example, it is common to use the ungrounded (hot) conductor and the switch leg in the same raceway for a single pole switch, which would be line and load in the same raceway.

 

Q6. Q6.What type of precautions must I take when splicing aluminum conductors to copper?

A6. Copper and aluminum conductors must not make contact with each other in a device unless the device is listed and identified for this purpose [110.14]. Few terminations are listed for the mixing of aluminum wire and copper, but if they are, they will be marked on the product package or terminal device.

 

The reason copper and aluminum should not be in contact with each other is because corrosion develops between the two different metals due to galvanic action, resulting in increased contact resistance at the splicing device. This increased resistance can cause overheating of the splice and cause a fire.

 

Q7. Can I put two wires under a single terminal?

A7. The answer is generally no. However, two conductors can be placed under the same terminal, if the terminal is identified for this purpose, either within the equipment instructions or on the terminal itself [110.14(A)]. Careful, each grounded (neutral) conductor within a panelboard, must terminate to an individual terminal [408.41].

 

Q8. Am I required to install the secondary circuit conductors of a low-voltage lighting system (less than 30V) in a raceway or cable like power conductors?

A8. If the low-voltage conductors will be concealed or extended through a building wall, they must be installed using any Chapter 3 wiring method such within a raceway, cable, or enclosure, just like power conductors [411.4(A)(1)]. In addition, if the low-voltage wiring is supplied by a Class 2 power source, it can be installed using the requirements of 725.52. This means you can use 150V cable [725.82(G)], boxes are not required, and the Class 2 conductors cannot be mixed with power conductors in raceways or enclosure unless a separated by a barrier [725.55].

 

Q9. Must the offsets at outlet boxes be counted towards the total amount of bends between pull boxes for EMT?

A9. Yes. To reduce the stress and friction on the conductor insulation, the total amount of bends between pull points must not exceed 360° [358.26]. This includes all bends, even the small ones at the outlet boxes.

 

Q10. Can 277V circuit conductors be within the same raceway with 120V circuit conductors?

A10. Yes, power conductors of different systems can occupy the same raceway, cable, or enclosure if all conductors have an insulation voltage rating not less than the maximum circuit voltage [300.3(C)(A)].

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Comments
  • It must also be stated that Emergency circuits can not occupy the same raceway as Power and Bracnh circuits.

    Hank Henrie
    Reply to this comment

  • In the NEC Q&A (June) your answer A3 can be missleading. Although, I agree with the content of your answer A3, I think that the question Q3 might have been asking for something different.

    If you are using THHN wire rated at 90'C and connecting to a device that is only rated at 60'C, I would say that you must size the THHN AWG conductor according to the current carrying requirements of a 60'C conductor. (assuming there is no other derating requirement. EXAMPLE: A 60'C device requiring a 100A conductor (#1) can not use a 90'C THHN #3 conductor (100A) but must use a #1 THHN sized conductor.

    Would you agree with this?

    ROBERT HOSLEY
    Reply to this comment
  • Reply from: Fred Lucas   
    Iam referring to Q 3 .The code section 110.14C 1 a 1 2 3 4.Would you please give me your interpretation of both .
    Reply to Fred Lucas


  • I have a question related to question#3. When sizing multiconductor flexible cable for ampacity,are conductors intended for control circuits considered to be current carrying conductors?

    Mark
    Reply to this comment

  • Question #3; I believe should be clarified for the 3 wire cable that all 3 conductors are current carrying for a 3 phase circuit. It can be understood that the white conductor is grounded and carrying only the unbalanced current, not requiring to be counted when applying the provisions of 310.15(B)(2)(a). That is how I initially understood it. Further, I always basically understood the 90C column for a 90C conductor may be used for a 90C condition for adjusting for bundling as long as the final ampacity requirement did not exceed the 75C column for the 75C terminations, or 60C for 60C terminations.

    David Engelhart
    Reply to this comment

  • all info. on article 517&725 past present or future ! thanks gary

    gary
    Reply to this comment

  • Guy... the question is simply asking what the ampacity is for a given size conductor. In this example, the ampacity for 12 THHN is 30A with no ampacity adjustment and 21A after adjustment based on the factors.

    When sizing a conductor for a circuit the first step it to determine the conductor required based on terminal ratings. Then once we know the conductor size, we need to determine if the conductor can still be used in the application after ampacity adjustment for Temperature and/or bundling. In this example, we would still be able to use 12 THHN on a 20A circuit.

    Hopes this clarifies the issues. The question as written and answer is correct.

    Mike Holt
    Reply to this comment

  • The Line and Load question has nothing to do with emergency circuits, since emergency circuits are neither considered line or load.

    Mike Holt
    Reply to this comment

  • Regarding A4. [310.15(B)(4)(c)] requires one to consider the neutral as a current carrying conductor when serving nonlinear loads. Is it required to increase the size of the grounding/bonding conductor when a current carrying neutral requires larger ungrounded conductors than a non-current carrying neutral?

    If increasing the size of the grounding/bonding conductor is required above, consider the following scenario(using THHN conductors):

    1. A 50 amp, 3-phase, 4-wire circuit (non-current carrying neutral) would be 4-#8, 1-#10G.

    2. A 60 amp, 3-phase, 4-wire circuit (non-current carrying neutral) would be 4-#6, 1-#10G.

    3. A 50 amp, 3-phase, 4-wire circuit (current carrying neutral) would be 4-#6, 1-#8G. NOTE: Grounding/bonding conductor increased as ungrounded conductors increased from #8 to #6.

    4. A 60 amp, 3-phase, 4-wire circuit (current carrying eutral) would be 4-#6, 1-#10G. NOTE: No increase in grounding/bonding conductor since no increase in ungrounded conductors.

    So, applying [310.15(B)(4)(c)] and [250.122(B)] means that we use a #8 grounding/bonding conductor for a 50 amp circuit, but only a #10 grounding/bonding conductor for a 60 amp circuit.

    Am I missing something here? Comments?

    John Bravard
    Reply to this comment
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    Reply to diannafevaips


  • One thing that MUST not be done is to install a bonding jumper around a gas meter. Gas meters almost always have a dielectric union on the custormer side of the meter. Metal gas pipes have a negative direct current voltage on them for corrosion protection. The reason why underground gas pipes are insulated is to conserve corrosion protection current and to concentrate the voltage gradient at the suface of the pipe so that it does the most good.

    Similarly, plastic underground gas pipes have an insulated tracer wire. This wire also has a corrosion protection voltage on it.

    In both cases of metallic pipes and tracer wires the ability to place and audio tone current over the metal needs to be possible so that gas lines can be located. A few months ago a gas company technician was having a devil of a time across the street from us because someone who had patched some metal gas pipeline with plastic pipe had NOT hooked up the tracer wires so that the connection was electrically sound and chemically durable.

    Mike Cole, mc5w at earthlink dot net

    Michael R.Cole
    Reply to this comment


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