If you have a problem related to the National Electrical Code (NEC), are experiencing difficulty understanding a Code requirement, or are wondering why or if such a requirement exists, ask Charlie, and he will let the Code decide. Questions and comments can be sent to email@example.com.
Communications and electric cables
Can communication cables be run in the same conduit with power/light conductors (4-wire, 208-volt, three-phase feeder)? We installed a 1-inch piece of nonmetallic tubing in the conduit and pulled in a Category 5 shielded cable with a 300-volt (V) rating. The local inspector insists we cannot do this, as 800.133 requires a minimum 2-inch separation. We believe the exception removes the requirement if there is a permanent barrier installed and our listed nonmetallic tubing meets this requirement. The conduit is large enough that conduit fill is not an issue.
NEC 800.133(A)(c) Exception No. 1 permits conductors of electric power and light to be run in the same raceway as communications conductors where the communications circuits are separated by a permanent barrier. The nonmetallic tubing provides the required separation.
A necessary nuisance
In the April 2010 column, you addressed a question about ground-fault circuit interrupters (GFCIs) for block heaters: “The use of GFCI-protected receptacles in outdoor areas is considered necessary for the safety of personnel and should be followed. I really don’t see a problem of nuisance tripping in this application and an alarm system could be incorporated to detect a tripped device. The NEC requirements are the minimum, [90.1(B)] and the designer can expand on them.”
This is one of those places where contractors get put into an awful quandary. My customer base includes several municipalities. For several of them, I installed GFCIs in their truck garages and adjacent outdoor parking spaces for block heaters. The problems were more severe than “nuisance” tripping; tripping was constant.
As much as I appreciate and support the Code, this is one of those places where the very restrictive rule just doesn’t work in the real world. In the interest of safety, we compromised, and they installed welding clamps with the leads permanently secured to building steel. They use them to ground the truck chassis before plugging in the block heater. They mounted signs at the receptacles, limiting their use to block heaters and only on positively grounded vehicles. The situation is not perfect. But they are safer than they were, and it was the best answer we could find by which they would abide. Several other towns adopted their approach, not having been able to make the GFCIs function. I wish the folks who write the Code could come up with a real-world solution, so that we could make a legal installation that our customers could safely live with.
Although it may be a nuisance to you, the ground-fault detector is just doing what it was designed and installed to do—detect ground faults within its range and de-energize the circuit. The GFCI will not open unless there is a ground fault. The ground fault is either in the engine block heater or in the cord that you are using to energize the block heater. The truck frame is not grounded, and when energized by a ground fault in the block heater or heater cord, it poses a serious threat to anyone in contact with ground and the engine frame. Grounding the truck frame to the building ground, as you have done, will, in the event of a ground fault, open the branch-circuit overcurrent device, depending on the magnitude of the ground fault, which of course results in de-energizing the block heater. So one way, the GFCI opens the circuit, and the other way, the branch-circuit overcurrent device opens the circuit.
It appears that you have an equipment and maintenance problem, which can be either in the cord feeding the heater or in the heater itself. In any case, we don’t want the frame of the vehicle to become energized where it will pose potential shock or electrocution hazard to anyone touching the energized frame while standing on the ground. I would suggest that you make the necessary repairs to your equipment and follow the safety provisions of the Code.
Ufer ground and ground ring
If a new steel building has a ground ring around the perimeter and bonding to the steel structure at four corners, does the building also need a Ufer footing ground?
NEC 250.50 requires that if a concrete-encased electrode (sometimes called a Ufer ground, after the man who developed the system) is present at the building, it must be bonded to the grounding-electrode system. Note that “present” does not mean available. If present but not available, it must be made available. The ground ring, metal frame of the building, underground metal water pipe, and the concrete-encased electrode must all be bonded together to form the grounding-electrode system.
Automatic transfer of loads
We are installing a backup generator rated 20 kilowatts (kW) 120/240V, single-phase, using a 200-amp (A) service entrance-rated automatic-transfer switch in a residence. The service size on the residence is 200A. The load analysis is 141A. The authority having jurisdiction (AHJ) is telling me that, per NEC 702.5(B)(2), I need to size the generator for the 141A. This would be a 35-kW generator. The homeowner understands that the 20 kW already purchased will not hold everything electrically if used at one time. They just want the air conditioner, kitchen, laundry and a couple of general circuits for use on the generator. I explained to the AHJ, if that was the case, 99 percent of generators installed are incorrect. I think 702.5(B)(2) is referring to the ATS switch and not the generator unit itself. The generator unit has a 100A breaker for OCP.
You have an 83A source (20 kW divided by 240) feeding a possible load of 141A. NEC 702.5(B)(2)(a) requires that where automatic-transfer equipment is used, the standby equipment must be capable of supplying the full load that can be transferred by the automatic-transfer equipment, or 702.5(B)(2)(b) permits the use of a load-management system that will automatically manage the connected load so that the standby source shall have a capacity to supply the maximum load that will be connected by the load-management system. The 100A circuit breaker overcurrent protection is not there for load management.
If a load center is mounted in a horizontal position, would it be a violation of 404.7, which requires that the up position of the circuit breaker handle shall be the on position? Most circuit breakers in load centers are installed so that moving the circuit-breaker handle horizontally to the center is the on or closed position. If the load center is mounted horizontally, wouldn’t the up position of the top row of breakers be the off or open position?
Yes, you are correct. For this reason, load centers (panelboards) are not permitted to be mounted in a horizontal position. The term “load center” is not used in the NEC. It is a manufacturer’s designation. The NEC requirements for panelboards covers load centers.
Capacity of on-site generators
If I have a 100-kilovolt-amperes (kVA) fire pump, is the size of the generator 100 kVA or 100 kVA 600 percent? Does the generator have to provide locked-rotor current for the fire pump motor?
In accordance with 695.3(B)(1), an on-site generator must have sufficient capacity to allow normal starting and running of the fire pump motor. It is not required to size for the locked-rotor current of the fire pump motor.
Identifying floating neutrals
What exactly is a floating neutral? Where is it?
A floating neutral is a neutral that is not bonded to ground in accordance with 250.20(B), which requires alternating current systems of 50V to 1,000V to be grounded in accordance with 250.26 where the conductor to be grounded on alternating systems is specified. Not bonding the neutral to ground can cause voltages that will subject loads to a dangerous overvoltage condition.
Sizes of motor load assets
Could you help me in to determine the sizes of the conductor protector, conductor and disconnecting means of the motor load?
The requirement for sizing motor branch-circuit conductors can be found in 430.22, which requires the conductors to have an ampacity of not less than 125 percent of the motor’s full-load current as shown in Tables 430.247, .248, .249, or .250. The motor branch-circuit short circuit and ground-fault protection for the motor branch circuit conductors is shown on Table 430.52. The required rating of the motor-disconnecting means is shown in 430.110.
TROUT answers the Code Question of the Day on the NECA Web site. He can be reached at firstname.lastname@example.org.