Neutral Conductor Size Requirements
The neutral conductor is not required to be larger than the largest (hot) phase conductor. There are situations where doing so should be considered. Specifically 4 wire wye connected systems consisting primarily of nonlinear loads.
When reducing the size of the neutral conductor it has to have sufficient ampacity for the maximum unbalanced load. Not every circuit has neutral load and there are several other permitted reductions. This will be discussed with the calculation section of this article.
There no longer is a requirement limiting the reduction to “two sizes down”. However, it can not be smaller than the grounding electrode conductor sized to table 250.66. This also applies to separately derived systems where the system bonding jumper is not located at the source.
This is because the neutral conductor is part of the fault current path. It has to be able to carry the fault current as well as the maximum unbalance. The minimum size is determined based on the circular mil area of the service or system conductors and/or equivalent area of parallel conductors.
In parallel installations both ungrounded (hot) and grounded (neutral) conductors must be at least size 1/0 AWG. The 1/0 AWG requirement does not apply to grounding conductors of parallel installations.
Neutral Conductor Calculations & Reductions
The neutral conductor’s load is the maximum calculated load between it and any ungrounded (hot) conductor. Line to line loads are not connected to the neutral conductor and therefore have no neutral load.
For dwelling unit feeders or services supplying cooking equipment and/or electric dryers. That portion of the load can be calculated at 70% of the load determined by tables 220.54 and 220.55. Even if you calculate the feeder/service demand using the optional method, you still use the standard method’s calculation for determining the load for neutral load purposes. This is shown in Annex D in example D2(a).
You are also permitted to calculate the neutral load in excess of 200 amps at 70%. The first 200 amps of neutral or unbalanced load is at 100%. So if you have a maximum unbalanced load of 400 amps. The first 200 amps will be taken at 100% and the next 200 amps with be at 70% totaling a calculated neutral load of 340 amps.
Prohibited Reductions for Wye-Connected Systems
No reduction can be applied to any portion of a 3 wire circuit consisting of 2 ungrounded (hot) conductors and the neutral conductor derived from a 4 wire wye connected system.
Lastly, no reduction is permitted to portions of the system supplying nonlinear loads.
This class covers neutral reductions for dwelling unit electric cooking equipment and dryers.
This class covers the reduction permitted in excess of 200 amperes.
This class discusses when the neutral conductor is considered current carrying.