Characteristics of the Neutral Conductor
In any electrical system, the neutral is a grounded conductor that you must size and treat differently from ungrounded phase conductors.
Do you know how to properly size a neutral conductor? Do you know the rules surrounding its proper application? If not, read on. This article discusses how to calculate the neutral current for various circuit configurations to meet the requirements set forth in the Code.
Sizing the neutral: Sec. 220-22. You must size the neutral conductor to carry the maximum unbalanced current in the circuit (i.e. the largest load between the neutral and any one ungrounded phase conductor). You calculate the first 200A of neutral current at 100%. For all resistive loads on the neutral exceeding 200A, you must apply a demand factor of 70%. Then, you add this value to the first 200A, which we calculated at 100%.
You calculate all inductive neutral current at 100% with no demand factor applied. When working with cooking equipment or a dryer load, the feeder neutral load shall also be 70% of the demand load. You must use a multiplier of 140% when calculating the neutral current for a 3-wire, 2-phase or 5-wire, 2-phase system. The neutral conductors do not become overloaded because 120V loads switch in and out on the circuits at different intervals of time.
Using the neutral: Sec. 310-15(b)(4). This section has three subdivisions explaining the loading conditions and use of the neutral conductor. Let's take a closer look at each of these sections to help you fully understand their application.
Part (a). The Code considers the neutral conductor a current-carrying conductor only when it carries the unbalanced current from other ungrounded phase conductors. When circuits are properly balanced, the neutral carries very little current. When sizing the load for a 2-wire circuit, the grounded neutral conductor carries the same amount of current as the ungrounded phase conductor. This type of installation has no unbalanced load; therefore, the neutral conductor carries full current.
Example: What is the neutral load for a single-phase, 120V, 2-wire circuit supplying a load of 14A?
Step 1: Find amperage per Sec. 220-22 and Sec. 310-15(b)(4)(a).
Ungrounded conductor = 14A
Grounded neutral conductor = 14A
Solution: Size the neutral conductor to carry a load of 14A.
When sizing the load for a 3-wire circuit, the grounded neutral conductor must carry the unbalanced load of the two ungrounded phase conductors. This type of installation has an unbalanced load - unless both ungrounded conductors pull the same amount of current on each ungrounded phase conductor.
Example: What is the unbalanced neutral load for a 3-wire circuit carrying 64A and 52A on the ungrounded phase conductors?
Step 1: Find amperage per Sec. 220-22 and Sec. 310-15(b)(4)(a).
Ungrounded phase conductor: Phase A = 64A
Ungrounded phase conductor: Phase B = 52A
Unbalanced load = 12A
Solution: The grounded neutral conductor load is 12A for the unbalanced condition.
You must use a specific formula to calculate the neutral current for 3-phase feeder-circuits. Where currents on Phases A, B, and C are of different values, you can compute the neutral current
Part (b). The Code requires the grounded neutral conductor of a 3-wire, 120/208V feeder-circuit to be the same size as the ungrounded phase conductors for a feeder-circuit derived from a 4-wire, 120/208V system.
This is because the grounded neutral of a 3-wire circuit (consisting of 2-phase conductors) carries approximately the same amount of current as the ungrounded phase conductor. Therefore, the Code does not allow a reduction in ampacity.
Example: What is the grounded neutral conductor load for a 120/208V, single-phase circuit taken from a 4-wire wye, 3-phase system with 190A on phase A, 170A on phase B, and 90A for the neutral?
Solution: You must size the grounded (neutral) conductor based on the largest ungrounded phase conductor. Therefore, you must size the grounded conductor to carry 190A.
Part (c). The grounded neutral conductor of a 4-wire, 3-phase system supplying nonlinear loads must be the same size as the ungrounded phase conductors. The Code considers the grounded neutral conductor a current-carrying conductor due to the harmonic currents generated by these loads.
A demand factor of 70% applies to neutral loads exceeding 200A for nonlinear loads. You shall calculate nonlinear related loads at 100%.
Example: What is the load for the neutral if it exceeds 200A and has more than 50% of its load affected by harmonics? The ungrounded phase conductors carry a total neutral load of 275A respectively.
Step 1: Find amperage per Sec. 310-15(b)(4)(c). Phases4275A
Step 2: Calculate amperage per Sec. 220-22.
First, 200A x 100% = 200A
Next, 75A x 100% = 75A
Therefore, the total = 275A
Solution: You must size the neutral conductor to carry 275A.
The Code considers the grounded neutral conductor a current-carrying conductor because of the harmonic currents generated by these loads. You must apply Sec. 310-15(b)(2)(a) for four or more current-carrying conductors in a conduit, cable, etc.
Example: What is the neutral load for 120V loads having harmonic currents of 400A per phase?
Step 1: Find amperage per Sec. 310-15(b)(4)(c). Ungrounded conductors = 400A
Step 2: Calculate amperage per Sec. 220-22. 400A2 x 100% = 400A
Solution: The neutral load is 400A.
Note: The Code does not permit reduction of ampacity due to harmonic currents.
You must determine the size of the neutral conductor (based on its use with ungrounded circuit conductors) carefully. For example, the manner in which it shares loads between the other conductors determines if you can reduce its ampacity rating. Likewise, you must consider the number of current-carrying conductors to see if you must derate the neutral's ampacity. The neutral conductor is special; therefore, you must size it accordingly.
