NEC Questions and Answers – Based on the 2011 NEC
June 2011

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, so you can see how you did.

Q 1. What are the requirements for antenna grounding?

A 1. Article 810 contains the installation requirements for the wiring of television and radio receiving equipment, such as digital satellite receiving equipment for television signals and amateur/citizen band radio equipment antennas.  

The antenna mast [810.15] and antenna discharge unit [810.20(C)] must be grounded as follows [810.21].

Grounding the lead-in antenna cables and the mast helps prevent voltage surges caused by static discharge or nearby lightning strikes from reaching the center conductor of the lead-in coaxial cable. Because the satellite dish sits outdoors, wind creates a static charge on the antenna as well as on the cable attached to it. This charge can build up on both the antenna and the cable until it jumps across an air space, often passing through the electronics inside the low noise block down converter feedhorn (LNBF) or receiver. Connecting the coaxial cable and dish to the building grounding electrode system (grounding) helps to dissipate this static charge.

Nothing can prevent damage from a direct lightning strike, but grounding with proper surge protection can help reduce damage to the satellite dish and other equipment from nearby lightning strikes.

The bonding conductor or grounding electrode conductor to the electrode [810.21(F)] must be copper or other corrosion-resistant conductive material. It can be stranded or solid, insulated, covered or bare [810.21(A) and (B)].

The bonding conductor or grounding electrode conductor must be securely fastened in place and must be mechanically protected where subject to physical damage. Where installed in a metal raceway both ends of the raceway must be bonded to the bonding conductor or grounding electrode conductor, although installing it in PVC conduit is a better practice. [810.21(C) and (D)].

Lightning doesn’t like to travel around corners or through loops, which is why the bonding conductor or grounding electrode conductor must be run as straight as practicable [810.21(E)].

The bonding conductor for the antenna mast and antenna discharge unit must terminate to the intersystem bonding termination as required by 250.94 if there is one available. Bonding all communication systems to the intersystem bonding termination helps reduce induced potential (voltage) differences between the power and the radio and television systems during lightning events. [Article 100, 250.94 and 810.21(F)].

In a building or structure without an intersystem bonding termination, the bonding conductor or grounding electrode conductor for the antenna mast and antenna discharge unit must terminate to the nearest accessible location on the following [810.21(F)(2)]:

• Building/structure grounding electrode system [250.50].
• Interior metal water piping system, within 5 ft from its point of entrance [250.52(A)(1)].
• Accessible means external to the building, as covered in 250.94.
• Nonflexible metallic service raceway.
• Service equipment enclosure.
• Grounding electrode conductor or the grounding electrode conductor metal enclosure.

If a building or structure does not have any grounding means, the grounding electrode conductor for the antenna mast and antenna discharge unit must be connected to any grounding electrode as described in 250.52.

The bonding conductor or grounding electrode conductor can be installed either inside or outside the building and must not be smaller than 10 AWG copper or 17 AWG copper-clad steel or bronze. Copper-clad steel or bronze wire (17 AWG) is often molded into the jacket of the coaxial cable to simplify the grounding of the lead-in conductor from an outdoor antenna to the discharge unit [810.21(F)].

If a ground rod is installed to serve as the grounding electrode for the radio and television equipment, it must be connected to the building’s power grounding electrode system with a minimum 6 AWG conductor.

Termination of the bonding conductor or grounding electrode conductor must be by exothermic welding, listed lugs, listed pressure connectors, or listed clamps. Grounding fittings that are concrete-encased or buried in the earth must be listed for direct burial [250.70].

 

Q 2. What are the pull box calculations requirements and can you give me an example?

A 2.  For raceways containing conductors 4 AWG and larger, the minimum dimensions of boxes and conduit bodies must comply with the following [314.28(A)]:

(1) Straight Pulls. The minimum distance from where the conductors enter the box or conduit body to the opposite wall must not be less than eight times the trade size of the largest raceway.

(2) Angle Pulls, U Pulls, or Splices.

• Angle Pulls. The distance from the raceway entry of the box or conduit body to the opposite wall must not be less than six times the trade size of the largest raceway, plus the sum of the trade sizes of the remaining raceways on the same wall and row.
• U Pulls. When a conductor enters and leaves from the same wall of the box, the distance from where the raceways enter to the opposite wall must not be less than six times the trade size of the largest raceway, plus the sum of the trade sizes of the remaining raceways on the same wall and row.
• Splices. When conductors are spliced, the distance from where the raceways enter to the opposite wall must not be less than six times the trade size of the largest raceway, plus the sum of the trade sizes of the remaining raceways on the same wall and row.
• Rows. If there are multiple rows of raceway entries, each row is calculated individually and the row with the largest distance must be used.
• Distance Between Raceways. The distance between raceways enclosing the same conductor must not be less than six times the trade size of the largest raceway, measured from the raceways’ nearest edge-to-nearest edge.

 

Q 3. How do I size the conductors for an arc welder?

A 3.  The supply conductors for arc welders must have an ampacity not less than the welder nameplate rating. If the nameplate rating isn’t available, the supply conductors must have an ampacity not less than the rated primary current as adjusted by the multiplier in Table 630.11(A), based on the duty cycle of the welder.

 

Table 630.11(A) Duty Cycle Multiplication Factors for Arc Welders
Duty Cycle	Nonmotor Generator	Motor Generator
100	1.00	1.00
90	0.95	0.96
80	0.89	0.91
70	0.84	0.86
60	0.78	0.81
50	0.71	0.75
40	0.63	0.69
30	0.55	0.62
20 or less	0.45	0.55

 

Individual Welder Example

Question: A nonmotor-generator arc welder has a primary current rating of 40A with a duty cycle of 50 percent. The branch-circuit conductor for the welder must not be sized less than:

(a) 15A               (b) 20A              (c) 25A            (d) 30A

Answer: (d) 30A

Calculated Load = Primary Rating x Multiplier [Table 630.11(A)]

Calculated Load = 40A x 0.71

Calculated Load = 28.40A

Conductor: 10 AWG has an ampacity of 35A at 75ºC [110.14(C) and Table 310.15(B)(16)].

 

Q 4. What are the isolated grounding conductor requirements for 15A or 20A, 125V Isolated Ground receptacles?

 

A 4.  If installed for the reduction of electrical noise, the grounding terminal of an isolated ground receptacle must be connected to an insulated equipment grounding conductor run with the circuit conductors [250.146(D)].

The circuit equipment grounding conductor is permitted to pass through panelboards [408.40 Ex], boxes, wireways, or other enclosures [250.148 Ex] without a connection to the enclosure as long as it terminates at an equipment grounding conductor terminal of the derived system or service.

CAUTION: Type AC Cable—Type AC cable containing an insulated equipment grounding conductor of the wire type can be used to supply receptacles having insulated grounding terminals because the metal armor of the cable is listed as an equipment grounding conductor [250.118(8)].

Type MC Cable—The metal armor sheath of interlocked Type MC cable containing an insulated equipment grounding conductor isn’t listed as an equipment grounding conductor. Therefore, this wiring method with a single equipment grounding conductor can’t supply an isolated ground receptacle installed in a metal box (because the box isn’t connected to an equipment grounding conductor). However, Type MC cable with two insulated equipment grounding conductors is acceptable, since one equipment grounding conductor connects to the metal box and the other to the isolated ground receptacle.

The armor assembly of interlocked Type MCAP^® cable with a 10 AWG bare aluminum grounding/bonding conductor running just below the metal armor is listed to serve as an equipment grounding conductor in accordance with 250.118(10)(b).

Nonmetallic Boxes—Because the grounding terminal of an isolated ground receptacle is insulated from the metal mounting yoke, a metal faceplate must not be used when an isolated ground receptacle is installed in a nonmetallic box. The reason is that the metal faceplate isn’t connected to an equipment grounding conductor [406.3(D)(2)].

When should an isolated ground receptacle be installed and how should the isolated ground system be designed? These questions are design issues and must not be answered based on the NEC alone [90.1(C)]. In most cases, using isolated ground receptacles is a waste of money. For example, IEEE 1100—Powering and Grounding Electronic Equipment (Emerald Book) states: “The results from the use of the isolated ground method range from no observable effects, the desired effects, or worse noise conditions than when standard equipment bonding configurations are used to serve electronic load equipment [8.5.3.2].”

In reality, few electrical installations truly require an isolated ground system. For those systems that can benefit from an isolated ground system, engineering opinions differ as to what’s a proper design. Making matters worse—of those properly designed, few are correctly installed and even fewer are properly maintained. For more information on how to properly ground electronic equipment, go to: www.MikeHolt.com, click on the “Technical” link, and then visit the “Power Quality” page.

 

Q 5. What is the Code requirement for bonding of corrugated stainless steel tubing (CSST) gas piping systems?

A 5. According to 205.104(B), metal-piping systems such as sprinkler, gas, or air that are likely to become energized must be bonded. The equipment grounding conductor for the circuit that’s likely to energize the piping can serve as the bonding means [250.104(B)]. An Informational Note to this section advises that bonding all piping and metal air ducts within the premises will provide additional safety.

Another new Informational Note refers to The National Fuel Gas Code, NFPA 54, Section 7.13 for further information about bonding gas piping, which includes the bonding of CSST. Informational Notes in the NEC are for information purposes only and aren’t enforceable as a requirement of the Code [90.5(C)].

 

Q 6. Please explain the 10 foot feeder tap rule and provide an example.

 

A 6. Feeder tap conductors up to 10 ft long are permitted without overcurrent protection at the tap location if the tap conductors comply with the following [240.21(B)(1)]:

• The ampacity of the tap conductor must not be less than the calculated load in accordance with Article 220, and the rating of the device or overcurrent device supplied by the tap conductors.
• The tap conductors must not extend beyond the equipment they supply.
• The tap conductors are installed in a raceway if they leave the enclosure.
• If the tap conductors leave the enclosure or vault in which the tap is made, the tap conductors must have an ampacity not less than 1/10th of the rating of the overcurrent device that protects the feeder.

Example: A 400A breaker protects a set of 500 kcmil feeder conductors. There are three taps fed from the 500 kcmil feeders that supply disconnects with 200A, 150A, and 30A overcurrent devices. What are the minimum size conductors for these taps?

• 200A: 3/0 AWG is rated 200A at 75°, and is greater than 10 percent of the ampacity of 500 kcmil, which is rated 380A at 75°.
• 150A: 1/0 AWG is rated 150A at 75°, and is greater than 10 percent of the ampacity of 500 kcmil, which is rated 380A at 75°.
• 30A: 8 AWG is rated 50A at 75°, and is greater than 10 percent of the ampacity of 500 kcmil, which is rated 380A at 75°. Anything smaller than 8 AWG can’t be used, as it will have an ampacity of less than 10 percent of 380A (38A) in the 75° column of 310.15(B)(16).

Q 7. Please explain the 25 foot feeder tap rule and provide an example.

 

A 7. Feeder tap conductors up to 25 ft long are permitted without overcurrent protection at the tap location if the tap conductors comply with the following [240.21(B)(2)]:

• The ampacity of the tap conductors must not be less than one-third the rating of the overcurrent device that protects the feeder.
• The tap conductors terminate in a single circuit breaker, or set of fuses rated no more than the tap conductor ampacity in accordance with 310.15 [Table 310.15(B)(16)].
• The tap conductors are protected from physical damage by being enclosed in a manner approved by the authority having jurisdiction, such as within a raceway.

Example: A 400A breaker protects a set of 500 kcmil feeder conductors. There are three taps fed from the 500 kcmil feeders that supply disconnects with 200A, 150A, and 100A overcurrent devices.

• 200A: 3/0 AWG is rated 200A at 75°C, and is greater than one-third of the rating of the 400A overcurrent device [Table 310.15(B)(16)].
• 150A: 1/0 AWG has an ampacity of 150A at 75°C, and is greater than one-third of the rating of the 400A overcurrent device [Table 310.15(B)(16)].
• 100A: 1/0 AWG has an ampacity of 150A at 75°C, and is greater than one-third of the rating of the 400A overcurrent device [Table 310.15(B)(16)]. Anything smaller than 1/0 AWG can’t be used, as it will have an ampacity of less than one-third of 400A in the 75°C column.

 

Q 8. What are the disconnecting requirements for a transformer?

 

A 8. A disconnect is required to disconnect all transformer ungrounded primary conductors. The disconnect must be located within sight of the transformer, unless the location of the disconnect is field-marked on the transformer and the disconnect is lockable [450.14]. This new Code requirement applies to transformers of any voltage except Class 2 or Class 3 transformers. Within sight means that it’s visible and not more than 50 ft from one to the other [Article 100].