The Purpose Of Overcurrent Protection, Grounding Well Casings And More

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 Answers are based on the 2011 NEC.

The purpose of overcurrent protection

It’s my understanding that the purpose of overcurrent protection (fuse, circuit breaker, etc.) is to protect the wire. Since the Code allows you to choose the next higher circuit breaker, up to 800 amperes (A), 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?

NEC 240.4(B) permits using the next higher standard overcurrent protective device rating (above the ampacity of the conductors being protected) only when all of the three conditions are met: the conductors being protected are not of a multioutlet branch-circuit supplying receptacles for cord-and-plug-connected portable loads, the ampacity of the conductors does not correspond with the standard ampere rating of a fuse or circuit breaker, and the next higher standard rating selected does not exceed 800A. Condition one prevents the circuit from having cord-and-plug-connected loads connected to the circuit that would exceed the ampacity of the conductors but not the overcurrent protective device. Condition two prevents using this section where overcurrent devices that match the ampacity of the conductors are available. Condition three prevents using this section where the next higher rating exceeds 800A (this is because the next higher standard rating is 1,000A, a giant step upward). Overcurrent protective 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 much of a load on the circuit, the overcurrent device will open. If the circuit supplies equipment, such as a motor, and it becomes overloaded, the motor overload relays will open. Short-circuit and ground-fault currents are generally of such magnitude that the overcurrent device will open before any damage is done to the conductors.

Grounding the well casing

I was required to ground the well casing to the grounding conductor. It seems to me that the casing is pretty well grounded already. Am I right?

Section 250.112(M) requires that, “where a submersible pump is used in a metal well casing, the well casing shall be bonded to the pump circuit equipment grounding conductor.” If the well casing were not bonded to the equipment grounding conductor and the ungrounded pump circuit conductor were to accidentally energize the well casing, the only path for the ground-fault current would be through the earth to either or both the building grounding electrode conductor and the grounding electrode conductor at the utility transformer location. Both 250.4(A)(5) and 250.54 state, “the earth shall not be used as the sole equipment grounding conductor.” The impedance of the earth is usually too great to permit a sufficient current to open the circuit overcurrent protective device. Please note that, while there is not sufficient current to open the circuit overcurrent protective device, there is sufficient current to cause severe electric shock or electrocution to a person who may become part of the load by touching the energized well casing and the earth simultaneously.

Being flexible

If I use electrical metallic tubing (EMT) as the raceway for a motor branch-circuit and flexible metal conduit (FMC) for the connection to the motor, do I need to install an equipment grounding conductor?

NEC 348.60 requires that, when FMC is used to connect equipment where flexibility is required, a separate equipment grounding conductor must also be installed.

Protection in the cord?

I have a portable advertising sign that I use at my trailer park. I always plug it into a ground-fault-protected receptacle outlet. The inspector now tells me the protection must be in the cord. Why?

Because no matter where you put that sign, he wants to know it is protected. Section 600.10(C)(2) requires the ground-fault circuit interrupter to be a part of the attachment plug or located in the power supply cord within 12 inches of the attachment plug.→

Branch-circuit sizing for water heater

I have a 40-gallon water heater in a single-family dwelling. The nameplate rating is 4,500 watts (W) for each of two elements. What size branch circuit do I need for this heater?

The arrangement of the thermostat in the heater will only permit one 4,500W element to be connected at a time. Using a nominal 240-volt (V) circuit, 4,500 ÷ 240 = 18.75A. Section 422.13 requires a water heater with a capacity of less than 120 gallons to be supplied by a branch circuit that has a rating of not less than 125 percent of the nameplate rating of the water heater. Based on the previous computation, 18.75 × 1.25 = 23.4A. Although Table 310.16 shows a 12 AWG conductor as having an ampacity of 25A, the asterisk directs us to Section 240.3(D) where we find that a 12 AWG conductor is limited to 20A for our purposes. Therefore, the minimum conductor size is 10 AWG, and the overcurrent protection or branch circuit rating is either 25 or 30A. 

Marking service equipment

If a motor control center is used as service equipment, is it required to be marked “Suitable for Use as Service Equipment”?

No, but if it is used as service equipment and a grounded conductor (neutral) is provided, a main bonding jumper, sized in accordance with Section 250.28(d) shall be provided.

Motor duty rating

What does motor duty rating mean? How is continuous duty determined?

Duty rating is the length of time a motor can be operated without causing over-temperatures in the motor windings. A motor with a continuous duty rating can be run indefinitely at its rated load without overheating. Several factors determine the duty rating of a motor, such as type of motor enclosure and type of insulation.

Bonding a steel I-beam

Where a steel I-beam is installed in a wood frame residential dwelling, is it required to be bonded?

NEC 250.4(A)(4) tells us that normally noncurrent-carrying, electrically conductive materials that are likely to become energized shall be connected together and to the electric supply source in a manner that establishes an effective ground-fault current path. In my opinion, the steel I-beam is not likely to become energized and is not required to be bonded or connected to the equipment grounding conductor of a circuit. Section 250.104(C) also indicates that, where structural metal is interconnected for forming a building frame and is likely to become energized, the structural metal framing is required to be bonded. The information in the question appears to indicate there is a single steel beam in conventional wood construction of a dwelling unit. This I-beam is usually an isolated length of metal that is not the entire frame of the building, and it is unlikely to become energized. Generally, bonding is not required. Always check with your local inspection authority for their interpretation and ruling.

Three-pole vs. three single-pole

I am running a 4-wire multiwire circuit to feed fluorescent fixtures in a retail store. That is, it’s three hots and one neutral. Forget about working on a ballast; do you mean that I can’t use three single-pole breakers to feed these circuits unless I use a handle tie to make a three-pole breaker out of the three single-pole breakers so they open all three circuits at the same time? That’s ridiculous. The owner wants to be able to leave some fixtures on as night lights and wants them on a separate breaker.

The new text in 210.4(B) requires simultaneous opening of all ungrounded conductors of a multiwire circuit. There are a lot of ways to accomplish what you want, but none of them can be done with a 4-wire multiwire circuit in accordance with the new rule. You could use a 3-wire multiwire circuit and use the other circuit as a single circuit to feed the night lights, but I believe the best way is to quit sharing the neutral and run single-pole circuits, each with its own neutral. When you quit sharing neutrals, you eliminate a lot of harmonic problems and you eliminate the voltage-division problems that occur when a shared neutral is opened. I wouldn’t advise forgetting about working on ballasts; NEC 410.130(G)(2) is there for a good purpose.

About the Author

Charlie Trout

Code Contributor
Charlie Trout is most known for his work with the National Electrical Code (NEC). He helped write the NEC Since 1990; he was a member of NECA’s National Codes & Standards Committee and chairman of the National Fire Protection Association (NFPA)’s Cod...

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