Neutral Conductor Sizing

NEUTRAL CONDUCTOR SIZING & CALCULATIONS

The illustration above found on IAEI Magazine’s site show the neutral conductor in the fault current path when the main / system bonding jumper is installed in the service / system disconnect enclosure.

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 those systems consisting of a large amount 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 system. Also no reduction is permitted to portions of the system supplying nonlinear loads.

1. Let me try it this way: 310.15(B)(7) says that if you have a service rating of "X", then you can use the conductor size listed next to "X". What is a rating of a service? 230.79 leads me to believe that the rating of the service is the rating of the service disconnecting means. 230.80 tells me that when it consists of more than one disconnect, the combined rating of the handles is the "service rating." (Edit to add: 210.3 tells me that the rating of a branch circuit is the rating of the OCPD as well, but I'm still hunting for the definition of the rating of a feeder. I would call it the rating of the OCPD protecting the feeder.) So, if I want a 400A 120/240V residential service, then that is displayed by the presence of a 400A handle or a pair of 200A handles comprising the service disconnect. If I have this, then I can use the conductor size listed next to "400" on Table 310.15(B)(7). That is a "400A rated service." It is not an ampacity table, it is a table to accompany a rule. It is not telling me that a 400 kcmil CU conductor's ampacity changes when I connect it to a house; it is saying that I can blatantly ignore the ampacity if I want to use that table. The rating of the service didn't change when I went to install the neutral conductor, it is still a 400A service - and that is the row I have to use if I want to use that table for anything. There is not a 200A handle to be found in the picture, so the 200A row of Table (B)(7) is irrelevant to the picture. I can use 400 kcmil, or do some work to find a smaller conductor compliant with 310.15(B)(16). (Edit x 2 to add the picture that I was looking at as I wrote this post)

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