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Sizing Neutrals (Grounded) in a Parallel Service Explained

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Updated: Jun 27

We are often obsessed with electrical calculations that seem to say one thing but end up meaning something else altogether. One example of this may appear when sizing the grounded (neutral) conductor on a service with parallel ungrounded conductors.

In this article, we will examine the thought process for sizing the grounded (neutral) conductor for the service and assume copper-only conductors.

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Let's begin by looking at 220.61, which is the standard method calculation rule for sizing the grounded (neutral) conductor in terms of the actual calculated loads, which we will remember as the Golden Rule. We have to establish that when sizing the neutral (grounded) conductor, regardless of it being for the service, it has to be sized to handle the maximum unbalanced loads that it will be subjected to.

"220.61 Feeder or Service Neutral Load.

220.61(A) Basic Calculation.

The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.

Exception: For 3-wire, 2-phase or 5-wire, 2-phase systems, the maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor multiplied by 140 percent." - (Fair Use Law Extract)


As the above code language states, the neutral as to calculated to not only determine the maximum unbalanced load, it even tells you the maximum net calculated load between the natural conductor and any one ungrounded conductor. This is why we always have to start our sizing of the neutral by doing load calculations. This is a vital step most skip or circumvents due to the time involved in such tedious calculations.

Assuming you have done a neutral load calculation and have selected what you believe to be the properly sized neutral we now have to look at rules that establish a minimum size requirement for the neutral conductor. Yes, it must be sized to handle the maximum unbalanced load but it also cant be smaller than the NEC tells us it has to be so let's look at those rules as well.

Starting the minimum sizing in 250.24(C)

In the 2020 National Electrical Code, it states in 250.24(C) where an ac system that operates at 1000 volts or less and is grounded, meaning intentionally connected to earth back at the utility transformer, the grounded conductor shall be routed with the ungrounded conductors to each service disconnecting means. The last sentence of 250.24(C) says "The grounded conductor(s) shall be installed in accordance with 250.24(C)(1) through (C)(4)." So, in the first example below we will keep it simple, just a single raceway and (2) 500,000 circular mils (500 kcmil) THHN/THWN-2 copper conductors and a single neutral from a 120V/240V single-phase grounded system.

250.24(C)(1) says "Sizing for a Single Raceway or Cable - The grounded conductor shall not be smaller than specified in Table 250.102(C)(1)". In the direction of 250.24(C)(1) let's look at Table 250.102(C)(1).

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The extract used under the Fair Use Law*

As we see in the heading of this table, it states "Grounded Conductor" along with other bonding jumpers that also utilize this table. Looking at the far left heading it says "Size of Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors". This will become important when we examine parallel conductors but right now only the first part of that heading is what we need to consider.

Since our conductors are 500,000 circular mils or 500 kcmil, and of course copper as stated, we would go to the left side of the table, under copper, and move down until we get to the row that says "Over 350 through 600". The 500 kcmil in our example falls right in the middle of that span of sizes. Now, moving left to right we span our way over to the copper column under the heading "Size of Grounded Conductor or Bonding Jumper*" and notice that the conductor size given for our 500 kcmil is 1/0 AWG.

As a result, our single raceway with (2) 500 kcmil THHN/THWN-2 copper conductors for the ungrounded "hot" conductors would require a minimum 1/0 AWG copper Grounded Conductor. Pretty simple stuff right?

At this point, the next logical question is, what if the ungrounded conductors are larger than 1,100 kcmil copper or 1,750 kcmil aluminum? That is where Note 1 under Table 250.102(C)(1) comes into play.

Image

The extract used under the Fair Use law*

The chances of having a single set of ungrounded conductors in a single raceway that is larger than 1,100 kcmil copper or 1,750 kcmil are rare. However, it is very common to have multiple "parallel" sets of conductors in a single raceway, wireway, auxiliary gutter, and so forth. This is where Note 1 really comes into play. Let's assume (2) sets of 500,000 circular mils (500 kcmil) in a single raceway. What would be the minimum size, based on 250.102(C)(1) under this situation?

Looking again at Table 250.102(C)(1) above, now the phrase "Equivalent Area for Parallel Conductors" will be a factor. Here is where the "trouble train" starts for some electricians. They are not talking about adding ALL the ungrounded conductors together, it's simply one ungrounded conductor from each set. So, this would be 500 kcmil x 2 = 1000 kcmil, because we have (2) parallel sets of 500 kcmil conductors.

Take that 1000 kcmil and go to Table 250.102(C)(1) and follow the same steps we previously did and the minimum sized grounded conductor would be a 2/0 copper, since we are working with copper in our examples.

Now, what if it was (2) sets of 650,000 circular mils (650 kcmil) with a total combined circular mill of 1,300,000 (1300 kcmil) in the same raceway. Based on Note 1 shown above, we would have to take 12½ Percent of that 1,300,000 circular mils, which would be 162,500 circular mils.

Going to Chapter 9, Table 8 we see that this would require at least a 3/0 AWG copper since the 2/0 AWG below it is just 133,100 circular mils, the installer would move to the next largest size, which is again 3/0 AWG as our minimum sized grounded conductor.

What if my service conductors are parallel in two or more raceways?

While the first example in this article is rather simple, the confusion starts to begin when some electricians attempt to explain the minimum sizing of the grounded conductor when installed in parallel using two or more raceways.

Given: Our installation has (3) sets of 350,000 circular mils (300 kcmil) copper conductors, with THHN/THWN-2 insulation in EMT (Electrical Metallic Tubing). They consist of (3) ungrounded conductors, as this is a 3-phase system, and (1) grounded (neutral) conductors in each raceway. We need to size the grounded conductor.

By now I am sure you all remember the Golden Rule, the neutral (grounded) conductor has to be sized to handle the maximum unbalanced load per 220.61(A). However, since we are not given that value we need to establish a minimum size. Now, keep in mind this is only an example, you really do need to calculate the loads to be served by that neutral as your first step in all cases.

Reexamining 250.24(C), but now looking at 250.24(2).

"250.24(C)(2) Parallel Conductors in Two or More Raceways or Cables.

If the ungrounded service-entrance conductors are installed in parallel in two or more raceways or cables, the grounded conductor shall also be installed in parallel. The size of the grounded conductor in each raceway or cable shall be based on the total circular mil area of the parallel ungrounded conductors in the raceway or cable, as indicated in 250.24(C)⁠(1), but not smaller than 1/0 AWG.

Informational Note: See 310.10(G) for grounded conductors connected in parallel."


The first underlined segment of the except above is demanding that the grounded conductor be installed in parallel, with the ungrounded conductors. The informational note below the code language, while unenforceable, it does provide a reminder that the rules for paralleling found in 310.10(G) are in play.

The second underlined segment is critical to the first method many educators use for sizing the grounded conductor where installed in two or more raceways or cables. It basically demands you size the grounded conductor per each raceway, so in our example, each raceway has one phase of A, B, and C, we only use one of the phases, and since they're all 500 kcmil we size the grounded neutral per Table 250.102(C)(1) based on the 500 kcmil in the single raceway, which would be a 1/0 copper.

You would repeat this for each raceway resulting in all of the raceways having a 1/0 copper grounded (neutral) conductor.

Let's briefly examine that statement that says "not smaller than 1/0 AWG" in 250.24(C)(2). That statement is referring to the parallel rules in 310.10(G) that state, and do not consider any of the exceptions as they are not directly applicable to our installation:

"310.10(G)(1) General.

Aluminum, copper-clad aluminum, or copper conductors for each phase, polarity, neutral, or grounded circuit shall be permitted to be connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG and larger where installed in accordance with 310.10(G)(2) through (G)⁠(6)."- (Fair Use Law Extract)

Why is this important? Well, if you had done the calculation and the grounded conductor ended up being smaller than 1/0, let's say a 2 AWG if the conductors were actually 350 kcmil in each raceway, then someone may contemplate that it is ok to use a 2 AWG in each raceway, after all, that is what the Table 250.102(C)(1) appears to express.

However, since the rules in 250.24(C)(2) demands the installer to parallel the grounded conductor when the ungrounded conductors are also paralleled in two or more raceways, the electrician is forced to follow the parallel rules in 310.10(G) and bump the 2 AWG up to a minimum 1/0 AWG due to the paralleling rules.

As you can see, the above method is fairly simple and similar to what you do when sizing supply-side bonding jumpers to each separate raceway as expressed in 250.102(C)(2). The size of those individual supply-side jumpers is based directly on the largest ungrounded conductor in the individual raceway. So, it is basically the same concept.

So what is changing for the 2023 NEC when calculating the grounded (neutral) conductor?

The 2023 National Electrical Code development process has come to an end with the recent completion of the NITMAM (Notice of intent to make a motion) stage. The method to establish a base minimum size for the grounded (neutral) conductor hasn't really changed, spoiler alert, but at the least easier to understand.

Image

The extract used under Fair Use Law*

The reference is now 250.24(D) for sizing grounded conductors, and 250.24(D)(2) specifically for our Two or More Raceways or Cables installations. The (a) is how we did our calculation in this article and is established based on the largest ungrounded conductor (note it has no (s) on the end) in each raceway or cable.

The (b) in 250.24(D)(2) is establishing the possibility of multiple raceways but also multiple parallel sets in each of those separate raceways. You could have (2) parallel raceways with (4) sets of ungrounded conductors with two in each raceway.

For example, Phase A, B, C, and N of one set, and Phase A, B, C, and N of another set together in the same raceway, properly sized of course, and repeated in yet another raceway for (4) sets in (2) separate raceways. In that case, you would add (2) of the set's largest ungrounded conductors together, in the same manner, we did earlier as if you had multiple sets in a single raceway.

It's just that simple because basically, that's what you actually have. Now, the neutral will be larger in each raceway of course because you are using the parallel area when selecting from Table 250.102(C)(1).

At the end of the day, you should never forget the Golden Rule. You have to size the grounded (neutral) conductor to handle the maximum unbalanced calculated load as expressed in 220.61(A). All while keeping in mind the minimum sizing rules just in case the maximum unbalanced calculated load is rather small so as to still meet the minimum sizing rules discussed in this article.

For more information on Grounding and Bonding check out our 2020 NEC Grounding and Bonding Course linked below.
Best Regards,

Paul W. Abernathy, CMECP®
Fast Trax® System
Electrical Code Academy, Inc.
Paul.Abernathy@fasttraxsystem.com
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