If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty in understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions can be sent to email@example.com. Answers are based on the 2011 NEC.
Sharing neutral or yoke
Your July column said, “Simultaneous disconnection would only be required for those circuits sharing the same neutral and those required by 210.7 for any multiple circuits supplying devices on the same yoke.” What is the difference between the two?
The NEC does not define multiple circuits. However, relating to 210.7, it appears to mean more than one circuit, on separate phases and each with its own neutral. If the circuits were on the same phase, simultaneous disconnection in the panelboard would not be possible. If they shared a neutral, they would be multiwire circuits covered by 210.4(B).
NM under deck?
For an open patio under a deck, is it permitted to run Type NM cable for lighting and receptacle outlets?
Type NM cable is not permitted by NEC 334.12(B)(4) to be used in wet or damp locations. An open patio under a deck would be classified as a damp location subject to a moderate degree of moisture. Check out “Location Damp” in Article 100—Definitions.
Type NM physical protection
Where running Type NM cable through a floor, physical protection is required. Can Schedule 40 PVC conduit be used for physical protection?
Schedule 40 PVC is not permitted for physical protection in this instance.
NEC 334.15, Protection from Physical Damage, permits the use of Schedule 80 PVC conduit. The 2011 NEC now permits Type RTRC—reinforced thermosetting resin conduit—marked with the suffix XW to be used.
Stainless steel vs. copper
Is there a reason to use a stainless steel ground rod instead of the copper ones?
Copper-bonded steel ground rods are subject to corrosion and deteriorate over time. Stainless steel rods last longer but cost more. An increase in diameter increases life expectancy. Length increases grounding capabilities.
Ground rod for primary ground
Should ground rods be the primary ground for a house that is 5,200 square feet with a lot of electronics?
The NEC does not use the term “primary ground.” The NEC requires a grounding-electrode system, which is shown in 250.50. All of the grounding electrodes that are present must be bonded together to form the grounding-electrode system. A concrete-encased electrode is not required to be a part of the grounding-electrode system on existing buildings where not available for use.
GFCIs vs. grounded systems
Why do we need ground-fault circuit interrupters (GFCIs) when we have grounded systems?
Grounded systems provide a path of low impedance for ground faults. To interrupt the circuit, the ground-fault current must reach the trip level of the overcurrent device. A person who is touching an energized object and is in contact with a grounded surface will be subjected to electric shock. The seriousness of the shock is dependent on the length of time it takes to open the circuit and the voltage level of the circuit. With a 15-ampere (A) overcurrent-protective device, the current may never reach that trip level, and electrocution may occur. With GFCI protection, the sensing coil will detect a leakage of current not returning to the source through the return conductor (the current flows through a person’s body), and when it reaches a level of 4 to 6 milliamps (mA), the GFCI will open the circuit. The 4 to 6 mA through the body of a person to ground may still result in a severe shock, but will not cause electrocution.
Full voltage-starting applications
Please explain the requirements of the NEC for full voltage motor-starting applications and how the short-circuit and ground-fault protection is sized based on Table 430.52. What about reduced voltage-starting applications, and what about adjustable speed drive applications?
For full voltage-starting applications, the NEC allows the motor branch-circuit short-circuit and ground-fault protective devices to be sized in accordance with Table 430.52. However, a motor short-circuit protector is permitted in lieu of the devices listed in Table 430.52 if the motor short-circuit protector complies with 430.52(C)(7), which requires the motor short-circuit protector to be a part of a listed combination motor controller having overload protection and short-circuit and ground-fault protection in each conductor. A new informational note tells us that the motor short-circuit protector in this section is a fused device and not an instantaneous-trip circuit breaker.
For reduced voltage-starting applications, the branch-circuit short-circuit and ground-fault protective devices are selected from 430.52 in the same manner as full voltage-starting applications. For adjustable speed drive applications, the conductors must have an ampacity of not less than 125 percent of the rated input current of the power-conversion equipment (430.122). If the power-conversion equipment is marked to indicate that overload protection is included in the power-conversion equipment, additional overload protection is not required [430.124(A)]. If the power-conversion equipment is not marked as having overload protection included, a separate overload protection device must be provided. This device should be rated at 115 percent of the rated input to the power-conversion equipment [430.32(A)(1)]. The disconnecting means shall have a rating not less than 115 percent of the rated input current of the conversion unit (430.128).
Bonding metal well casing
Is a metal well casing required to be bonded to the other grounding electrodes on the premises?
The metal well casing is not generally a part of the grounding-electrode system and is not required to be bonded to the water pipe. However, if it is effectively bonded to the water pipe, it becomes a part of the metal underground water-pipe electrode, which is required to be bonded to any other electrodes on the premises. Where a submersible pump is used in a metal well casing, the well casing must be bonded to the pump circuit equipment-grounding conductor [250.52(A)(1) and 250.112(M)].
Fundamentals of overcurrent protection
It’s my understanding that the purpose of overcurrent protection (fuse, circuit breaker, etc.) is to protect the wire. Where the current rating of a conductor does not match the standard rating of a circuit breaker, the Code allows you to choose the next higher circuit breaker up to 800A.
It seems to me that it would be possible in some circumstances for the circuit breaker to have a higher rating than the wire it is intended to protect. What’s the logic behind that?
Short-circuit and ground-fault currents are generally of such magnitude that the overcurrent device will open before the conductors take any damage. For this reason, NEC 240.4(B) permits using the next higher standard overcurrent device rating (above the ampacity of the conductors being protected), but only when all of the three of the following conditions are met:
1. The conductors being protected are not of a branch-circuit supplying more than one receptacle for cord-and-plug connected portable loads
2. The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or circuit breaker
3. The next higher standard rating selected does not exceed 800A
Condition 1 prevents the circuit from having cord-and-plug connected loads attached to the circuit that would exceed the ampacity of the conductors but not the overcurrent device. Condition 2 prevents using this section where overcurrent devices that match the ampacity of the conductors are available. Condition 3 prevents using this section where the next higher rating exceeds 800A. The next higher standard rating is 1,000A, which is a giant step upward.
Overcurrent devices protect the circuit conductors in two ways. They protect the conductors from overload conditions and from short-circuit or ground-fault conditions. If you put too high a load on the circuit, the overcurrent device will open. If the circuit has equipment, such as a motor, and it becomes overloaded, the motor overload relays will open.
Running NM outdoors
Can NM cable be used outdoors if run in a raceway?
The interiors of raceways installed outdoors are considered to be a wet location (300.9). Type NM cable cannot be used in a wet or damp location [334.12(B)(4)].
Overcurrent-protection requirement for remote control devices
Are motor control circuits feeding remote control devices required to have overcurrent protection?
Overcurrent protection for motor control circuits is covered in 430.72(B). The requirements for conductors, which extend beyond the enclosure (remote) can be found in Table 430.72(B) in Column C. For example, if your motor branch-circuit protective device is rated at 60A and you are using copper control circuit conductors, you find 60 in the copper column and move to the left to control-circuit conductor size where you will find 12. Therefore, you need to install control-circuit conductors not smaller than 12 AWG copper. Smaller conductors may also be used, but they require supplemental overcurrent protection.
TROUT answers the Code Question of the Day on the NECA website. He can be reached at firstname.lastname@example.org.