Considering Voltage Drop, Counting The Neutral 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.

Considering voltage drop

We have a 120/208 panel from which we will run two 37/C, No. 12 cables used as home run cables to another junction box. I understand there will be an adjustment factor from Table 310.15(b)(2)(a). Will there be a voltage drop issue also?

From your statement, I believe you are referring to a 120/208-volt (V) service panel. From a junction box, you are running two 37 conductor, 12 AWG cables as home run conductors to the service panel. Yes, there will be an adjustment factor in accordance with 310.15(B)(2). Consideration of voltage drop is always an issue based on the installation criteria such as length of run and load. See Informational Note No. 1 to 310.15(A)(1), which tells us, “Ampacities provided by this section do not take voltage drop into consideration.”

Counting the neutral

Section 310.15(B)(5)(b) discusses two-phase conductors and a neutral, and it requires counting the neutral as a current-carrying conductor. However, in “Counting the Neutrals,” (ELECTRICAL CONTRACTOR, November 2012), they are calling for three hots, three neutrals and one ground. That means, for every ungrounded conductor, there is a dedicated neutral. That tells me that 310.15(B)(5)(a) states it shall not be counted. Am I interpreting this correctly?

When each circuit has a separate neutral for each phase conductor, there is no unbalanced current flow. The total circuit current is present on the neutral circuit conductor as well as the phase conductor. Each neutral conductor must be counted as a current-carrying conductor.

Shunt trip main breaker

While working in a rural area of the South, I found the fire department requires a main disconnect on the outside of the building so the firefighters can turn the power off before they enter the building. At one project, they did not have a main disconnect on the outside of the building. Instead, they had a glass-break pushbutton on the outside, and when the glass was broken, it tripped the shunt trip main breaker in the main distribution panel to kill the electricity in the building. I would like to know if there is any NEC requirement pertaining to this. Or, is this installation a local jurisdiction requirement?

There is no NEC provision relating to the installation of a main disconnect on the outside of a building for use by the fire department. The NEC is not intended as a design specification [90.1(C)]. This type of installation is a local municipal code requirement found in many municipalities.

Not your enemy

Is the type of conduit that is part of the service lateral from the electric utility company that enters a residence and terminates inside the exterior wall under the authority having jurisdiction (AHJ) or the utility company? NEC 90.2 (B)(5)(a) states that service laterals are not covered. Does the service lateral include the conduit? I ask because I had an electrical inspector refuse the type of conduit that was used even though approved by the utility company. I feel, per my understanding of the Code, he didn’t have the right to do so.

You didn’t mention the type of conduit you used. Did the municipality where you are working adopt the NEC verbatim, or is there a municipal electrical code? If you believe you are correct, talk to the AHJ. He is not your enemy.

Using 220.54

Please explain how to use 220.54 Electric Clothes Dryers—Dwelling Units to calculate the load for 32 dryer units supplied by a three-phase, 4-wire feeder.

Where two or more single-phase dryers are supplied by a three-phase, 4-wire feeder, 220.18 permits the total load to be computed on the basis of twice the maximum number of dryers connected between any two phases. We need to determine the maximum number of dryers that would be connected between two phases. We have 32 dryers. This means, to balance across the three phases, we would have 11 across A and B, 11 across B and C, and 10 across A and C. The maximum connected between any two phases would be 11. Twice that number is 22 dryers. Now we go to Table 220.54, and in the number of dryers column, we find 12–23 and the formula we use to find the demand factor percent. The demand factor percent is equal to 47 percent minus 1 percent for each dryer exceeding 11. The number of dryers (22) exceeding 11 equals 11. Subtracting 11 from 47 equals 36, which is the demand factor percent to use on the total dryer load. The total dryer load is 22 × 5,000 watts (W) = 110,000W, × 36 percent = 39,600W. This is the load across two phases, so we divide by 2 and get 19,800 per phase, and for three phases, we multiply 19,800 by three and get 59,400.

Burial depth

In the past, I have always thought that electrical metallic tubing (EMT) could not be used in underground applications. But according to Article 358.10(B) of the 2011 NEC, it is permitted. If that is the case, Table. 300.5 doesn’t provide any burial depths. Does that mean it is not permitted? Could you please clarify for me the depths and the corrosive protection it may be referring to and whether it is permitted?

Sections 358.10(B) and (C) permit the use of EMT in direct contact with the earth where protected by corrosion protection and approved as suitable for the condition. Check out exhibit 300.11 in the NEC 2011 Handbook for information relating to corrosion protection.

Identifying temporary power cable

What are the requirements (if any) for hanging tags on temporary power cables in a construction site? The question was raised where a 480V temporary power cable was strung across a hallway about 10 feet in the air and was clearly marked as 480V. Down the same hallway, in another room, the same cable again crossed the hallway with no tag.

There are no NEC requirements for tagging or marking voltage being used on temporary power cables on a construction site. This information can be found in the job specifications.

Multiwire circuits

Why does 605.8(D) not permit multiwire circuits in office furnishings partitions?

The receptacles used in office partitions are likely to be used for office equipment, such as computers, that include electronic switching devices. These “nonlinear loads” have the potential to create harmonic distortion that can result in overloading of the neutral conductor with consequent overheating.

Transformer overcurrent protection

Does a transformer installation require overcurrent protection on both the primary and secondary sides if the current is more than 9A?

For transformers rated 600V or less, Section 450.3(B) requires overcurrent protection to be provided in accordance with Table 450.3(B). Table 450.3(B) does not require overcurrent protection for the secondary of a transformer if the primary current is 9A or more and the primary overcurrent protection is limited to 125 percent of the transformer-rated current. Table 450.3(B) permits primary overcurrent protection to be 250 percent of the transformer-rated current if the secondary current is 9A or more and secondary overcurrent protection, limited to 125 percent, is provided. The requirement for overcurrent protection for the secondary winding of a transformer depends on the protection provided for the primary winding. The overcurrent protection of the feeder conductors for the transformer, if not larger than 250 percent of the transformer primary current, may serve as the overcurrent protection for the transformer primary winding. Protection of the transformer secondary conductors must be provided. Section 240.21(C)(1) through (6) provides the requirements for this protection. The requirements for overcurrent protection for a panelboard supplied by the secondary of a transformer are shown in 408.16(D).

The receptacles used in office partitions are likely to be used for office equipment, such as computers, that include 
electronic switching devices. These “nonlinear loads” 
have the potential to create 
harmonic distortion that can 
result in overloading of 
the neutral conductor with
consequent overheating.

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...

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