Bad Harmony: What estimators need to know in the NEC

Shutterstock / Enid Versfeld
Shutterstock / Enid Versfeld
Published On
May 14, 2021

I recently read a specification phrase I had never seen before. It said, “For the purpose of derating, all neutrals shall be considered to be current-carrying conductors.” Estimators need to pay close attention to phrases such as this.

A while back, I wrote about how much of the National Electrical Code estimators need to know. The specification phrase above told me I needed to understand more. This subject was beyond my knowledge, so I spoke with one of Electrical Contractor’s Code columnists, Jim Dollard. He sent me the following excerpt from the Code: “310.15(E) Neutral Conductor. Neutral conductors shall be considered current carrying in accordance with any of the following:

“(1) A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(C)(1). “(2) In a 3-wire circuit consisting of two phase conductors and the neutral conductor of a 4-wire, three-phase, wye-connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors and shall be counted when applying the provisions of 310.15(C)(1). “(3) On a 4-wire, three-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.”

I am most concerned about paragraph 3, because the lion’s share of my estimating work is on 4-wire, three-phase systems. However, Dollard taught me about single- and three-phase systems so I could understand all the concepts. Here are the examples he provided as to how much current is on a neutral conductor.

  • A single-phase 120/240V circuit has one phase conductor and one neutral conductor. The circuit is connected to a coffee maker, which draws 10A. This load is resistive and, therefore, linear. The neutral will also carry 10A.

  • A single-phase 120/240V circuit has two phase conductors and one neutral conductor. Each circuit is connected to a 10A linear load. Here is where it gets tricky. I thought there would be 20A carried on the neutral. I was wrong, because the phase conductors are 180 degrees out of phase with each other, resulting in the neutral theoretically carrying zero amps.

Now, let’s move to a three-phase, 4-wire circuit. The phases in this system are 120 degrees apart. For instance, consider a circuit with two phase conductors and one neutral conductor, with 10A on each phase conductor. The formula for the current on the neutral is the square root of (A2 + B2) – A × B, or the square root of (102 + 112) – 10 × 11, which is 10.48A.

Back to nonlinear loads, which are what I am most concerned about as an estimator. These loads induce harmonic currents on the neutral conductor of the circuit to which they are connected. Since the beginning of my estimating career, it was standard practice to run circuits with three-phase conductors and one neutral conductor. However, the computer power supply in almost every electronic item has become a problem. This power supply converts AC to DC and induces harmonic voltages on the neutral to which it is connected.

Why is this a problem? If you are wiring a three-phase, 4-wire system, then the harmonic feedback from all three-phase conductors is dumped onto one neutral, which can be too much current for the wire to carry.

This a big deal, primarily because LED fixtures are nonlinear loads, and almost every new fixture is an LED. Second, because many receptacles now have items, such as computers and chargers, plugged into them.

This problem was solved years ago by upsizing the neutral or by requiring separate neutral conductors for each phase conductor. However, I did not know that, according to the NEC , on a three-phase, 4-wire wye circuit where the major portion of the load consists of nonlinear loads, the neutral conductor shall therefore be considered a current-carrying conductor. My next question was the meaning of “major” in the previous sentence. It is just what it sounds like: 50% or more.

So back to the specification phrase that started this article. For the purpose of derating, neutrals shall be counted as current-carrying conductors. This makes it more difficult to use the practice of combining circuits because combining just two circuits could give you four current-carrying conductors, which will require you to derate the conductors by 20%. This is one more thing we have to watch out for while preparing estimates.

About the Author

Stephen Carr

Estimating Columnist

Stephen Carr has been in the electrical construction business since 1971. He started Carr Consulting Services—which provides electrical estimating and educational services—in 1994. Contact him at 805.523.1575 or steve@electrical-estimating.com, and...

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