How to Size an Auxiliary Gutter: A Quick Guide

When working with electrical installations, properly sizing components like auxiliary gutters is crucial for ensuring safety and functionality. Auxiliary gutters, often used in industrial and commercial electrical systems, help manage the distribution of conductors and protect them. This blog post will walk you through the process of sizing an auxiliary gutter, including key code references and quick calculation methods.

What is an Auxiliary Gutter?

An auxiliary gutter is essentially a raceway system used to house electrical conductors, such as wires or cables, within an enclosure. It provides protection and organization, especially in environments where conductors need to be safely routed and connected to other electrical components, like panels or switches.

Sizing the Auxiliary Gutter

Sizing an auxiliary gutter involves considering factors like the number of conductors, their size, and the allowable fill capacity. The National Electrical Code (NEC) provides guidelines to help professionals size raceways like auxiliary gutters. Let’s break it down into easy-to-follow steps:

Step 1: Consider the Conductors and Raceways

In this example, we are working with a feeder coming into the auxiliary gutter. The feeder conductors are large—specifically, three 250 kcmil THHN conductors. These conductors are then tapped with smaller conductors going to a switch or panel board. The NEC’s section 240.21(B)(5) is applied here, which deals with the tap conductors. A raceway enters the top of the auxiliary gutter because the length exceeds 24 inches, and there are nipples used in the taps.

Step 2: Determine the Gutter Size

For sizing the gutter, let’s look at a common example. If you use a 4×4 auxiliary gutter, the internal area of this gutter is 16 square inches. According to NEC guidelines, you are allowed to fill the gutter up to 20% of its total area. This means the gutter can hold 3.2 square inches of conductor space, which is the 20% fill allowed for this size.

Step 3: Quick Calculation Method 1

If you prefer a simple method to determine whether the 4×4 gutter is large enough, you can use Quick Count 1. Here’s how it works:

• The total area of the gutter is 16 square inches.
• You’re allowed 20% fill, which equals 3.2 square inches.
• The total area for conductors coming into the gutter is calculated at 2.71686 square inches.

Since 3.2 square inches is greater than 2.71686 square inches, the 4×4 auxiliary gutter can indeed accommodate the conductors.

Step 4: Quick Calculation Method 2

If you’d rather not use the percentage fill method, there’s an alternative approach. Using Quick Count 2, follow these steps:

• First, take 20% of the gutter’s total area (i.e., 20% of 16 square inches equals 3.2 square inches).
• Next, divide 100 by 20, which gives you 5.
• Multiply 5 by the cross-sectional area of all conductors coming into the gutter. For this example, let’s assume the cross-sectional area is 2.7186 square inches.

Multiplying 5 x 2.7186 square inches gives you 13.593 square inches.

Since the gutter’s area of 16 square inches is greater than 13.593 square inches, the 4×4 auxiliary gutter can be used for this application.

Conclusion: Sizing Your Auxiliary Gutter

Properly sizing your auxiliary gutter ensures that the electrical system remains safe and efficient. The 4×4 auxiliary gutter in this example can accommodate the conductors, following both Quick Count 1 and Quick Count 2 methods. The key is always ensuring the total conductor area does not exceed the gutter’s available space.

Final Thoughts

Whether you’re following NEC guidelines or performing quick calculations, understanding how to size an auxiliary gutter is a crucial skill for electrical professionals. Always make sure your gutter provides sufficient space for the conductors while adhering to safety codes.

For more detailed illustrations and examples, refer to Volume 1 and 2 of the Design Book for standard auxiliary gutter sizes and additional sizing guidance.

If you need to perform these calculations regularly, keeping a handy reference of NEC guidelines, such as 240.21(B)(5), will help you make quicker, more accurate decisions on the job.

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