Okay, let's break down the factors influencing the required size of your exterior trough (also known as a junction box or wireway) and how to determine the appropriate dimensions. This is based on the National Electrical Code (NEC) and good electrical practices.

Key NEC Articles and Considerations NEC 314.28(A)(1) Straight Pulls

This is the big one for your scenario with conduits entering and exiting on opposite walls (the bottom and back).

NEC 314.28(A)(2) Angle or U Pulls

If any conduit were entering at an angle, this would come into play, but it doesn't sound like that's the case here.

NEC 314.28(E) Minimum Wire-Bending Space at Terminals

While not directly related to the "pull" distances, you need to ensure you have enough space for bending the conductors inside the trough to connect them to the SER cables.

NEC 376 for Metal Wireways/Troughs

Specifically details on construction and use.

NEC 376.23 Number of Conductors

Limitations on fill.

NEC 314.16 Box Fill Calculations

While wireways don't directly use "box fill," it's a good principle to understand, and excessive fill is generally to be avoided.

Manufacturer's Instructions

Always follow the manufacturer's instructions for the trough you select. They may have specific requirements.

Calculations and Considerations1. Straight Pulls (NEC 314.28(A)(1))



Determine the largest conduit size

In your case, it's the 1 1/2" conduits.

Minimum length of the trough

The minimum length of the trough where the conduits enter on the bottom and the SER cables enter on the back must be at least 8 times the trade diameter of the largest raceway.

8 1.5" = 12"2.

Minimum Depth of the Trough

Conductor Size

You have #2 AWG conductors. Table 312.6(B) is what you need.

Determine Depth

Find the column for 0-300V conductors. The minimum depth for #2 AWG conductors is 4 inches3.

Minimum Width of the Trough

Conductor Size

You have #2 AWG conductors. Table 312.6(A) is what you need.

Determine Width

Find the column for 0-300V conductors. The minimum width for #2 AWG conductors is 4 inches4.

Fill Considerations (NEC 376.23)



Wireways (Troughs)

The NEC limits the number of conductors in a wireway to no more than 75% of the cross-sectional area of the wireway.

Number of conductors

You have 6 x 1.5" conduits (potentially carrying multiple conductors each) PLUS 6 x #2 SER cables (each SER will contain multiple conductors). This is a LOT of conductors.

SER Cable Considerations

SER cables are oval. Estimate the cross-sectional area of a #2 SER cable, including the outer jacket. You'll need to look up the diameter of #2 SER cable in manufacturer specifications.

Conduit Fill

Assuming your 1.5" conduits are full of conductors, it could increase the required size.

Calculation

Calculate the total cross-sectional area of ALL conductors (from conduits and SER) that will be inside the trough. This is not simple! You'll need to refer to NEC Chapter 9, Table 5A for conductor dimensions. You'll need to look up the dimensions of #2 SER cable from manufacturers spec sheets.

Compare

Ensure that total conductor cross-sectional area is <= 75% of the trough's internal cross-sectional area (width x depth). If it's over 75%, you need a larger trough.5.

Bending Space at Terminals (NEC 314.28(E))

You need sufficient bending space to make safe and code-compliant connections from the conduit conductors to the SER conductors inside the trough. This is where a slightly oversized trough can be very beneficial. #2 conductors can have a fairly large bending radius. Allowing ample space will make the installation easier and safer.6.

Termination

Make sure the trough has appropriate openings (knockouts or pre-cut holes) for your conduits. The trough needs to be rated and listed for exterior use (NEMA 3R or higher). Consider grounding and bonding requirements within the trough.

Recommended Steps


Detailed Conductor Count

Figure out exactly how many conductors are actually running in each of the 1 1/2" conduits. Knowing the circuit configurations is critical.2.

SER Cable Dimensions

Get the exact dimensions (including outer jacket) of the #2 SER cable you're using. Manufacturers' specs are essential.3.

Conductor Cross-Sectional Area

Calculate the total cross-sectional area of all conductors that will be inside the trough.4.

Trough Size Selection



Minimum Length

You know it must be at least 12".

Minimum depth

You know it must be at least 4".

Minimum width

You know it must be at least 4".

Choose a trough size that is LARGER than these minimums This can help with wire bending and future access.

Verify 75% Fill

Calculate the trough's internal cross-sectional area and ensure that the total conductor area is less than 75% of it.

Example

Let's say you have 3 conductors in each of the 6 conduits, and each #2 SER cable is roughly equivalent to 1 inch diameter circle.1. Conduit Conductors: 6 conduits 3 conductors = 18 conductors. Assuming #12 AWG, cross-sectional area = 0.0133 sq. in per conductor 18 = 0.2394 sq. in2. SER Cables: 6 cables 1 inch diameter = 4.71 sq. in3. Total conductor area: 0.2394 sq. in + 4.71 sq. in = 4.9494 sq. inLet's consider a trough that is 12" long, 6" deep, and 6" wideCross Sectional Area = 6" x 6" = 36 sq. in75% fill = .75 36 sq. in = 27 sq in. This passes the 75% fill test.

In this example, a 12" x 6" x 6" trough would be code compliant

Important Considerations



Local Codes

Always check with your local electrical inspector or authority having jurisdiction (AHJ). They may have additional or more stringent requirements.

Future Expansion

Consider slightly oversizing the trough if there's any possibility of adding more circuits in the future.

Ease of Installation

A larger trough generally makes the installation and maintenance easier. Tight spaces can lead to poor workmanship and unsafe conditions.

Professional Advice

If you are at all unsure, consult with a licensed electrician. Electrical work can be dangerous, and improper installations can lead to fires, injuries, or code violations.

In conclusion, to properly size your trough, you MUST:1. Know the exact number of conductors entering the trough from both the conduits and the SER cables.2. Determine the minimum dimensions based on the largest conduit size and the wire size.
3. Calculate the conductor fill to ensure it complies with NEC 376.23.This is not a simple calculation, and getting it wrong can have serious consequences. If you are not comfortable with these calculations, please consult a qualified electrician.* Flag for review