Okay, let's break down why troughs (also called wireways or auxiliary gutters) don't automatically mean you'll need to drastically oversized wires due to derating. It's a common misconception, and here's the ELI5 explanation

The Core Issue: Ampacity Derating
What it is

When you bunch conductors carrying current together, they heat up more than if they were isolated. This heat reduces the amount of current they can safely carry (their ampacity). Electrical codes (like the NEC in the US) have rules about "derating" - reducing the allowable ampacity based on how many current-carrying conductors are bundled together.

The 80% Rule (and others)

You've heard of the 80% derating rule, and it's real. This, and other rules, come into play when you have a large number of current-carrying conductors bundled in a conduit or wireway. The more conductors carrying current, the lower the allowable ampacity of each individual conductor.

Why Troughs/Wireways Aren't Always a Derating Nightmare


"Current-Carrying Conductor" Definition is Key

This is the most important thing to understand. Not every wire in a trough counts as a "current-carrying conductor" for derating purposes. Specifically:

Grounding conductors (Equipment Grounding Conductors - EGCs)

These are not considered current-carrying conductors for derating. They only carry current during a fault condition.

Neutral Conductors (in most situations)

In a balanced 3-phase system, the neutral conductor theoretically carries very little current. Therefore, the NEC (National Electrical Code) often doesn't require you to count the neutral as a current-carrying conductor for derating if the load is primarily non-linear. If the majority of the load being fed from the panel/trough is something like VFD's or LED lighting which generates a lot of harmonics, the neutral will need to be counted. Check with your local inspector for guidance.2.

Diversity of Circuits

Consider the example picture you provided. It's unlikely that every single circuit running through that trough is loaded to its maximum capacity at the same time. There may be 50 circuits in the trough but odds are only 15 of them will ever be under a load at the same time.

Real-World Load

Think about a building with many lighting circuits. Not all the lights are on at 100% brightness all the time. Or consider receptacle circuits. Not every outlet has a high-power device plugged in and running simultaneously.

Load Calculations

Electrical engineers and electricians perform load calculations to estimate the maximum demand that will be placed on a system. This calculation takes into account the diversity of loads. The goal is to run these load calculations accurately so that you don't have to size the system for every circuit at its maximum, which would be incredibly costly.3.

Physical Space in the Trough/Wireway

Troughs and wireways often provide more physical space and ventilation compared to conduits. This can help dissipate heat and reduce the need for severe derating.

Fill Percentage Rules

The NEC has rules about how full a wireway can be. Exceeding the fill percentage can trigger additional derating.4.

Short Runs

If the conductors are only inside the wireway for a relatively short distance, the heat buildup is less of a concern.

Taps

Wireways are often used for "tapping" off power. A circuit might enter the wireway, run a short distance, and then exit to feed a specific piece of equipment. Because the conductor is only briefly inside the wireway, derating may be less of an issue.

Example Scenario

Imagine a trough feeding power to several lighting circuits. Let's say each circuit uses a hot, neutral, and ground. Only the hot and sometimes the neutral conductor are counted as "current-carrying" for derating. Even if there are 20 circuits, there might only be 20-40 current-carrying conductors to consider. (20 hots, possibly 20 neutrals) The load calculation might show that, on average, only 50% of the lighting circuits will be heavily loaded at any given time. This allows for a more reasonable wire size, rather than oversizing everything for 80%+ derating on every conductor.

In Summary

Troughs are great for organization and flexibility, but derating is a real concern. Here's the simplified version:1.

Know the rules

Understand which conductors count as "current-carrying" and how the NEC derating tables work.2.

Accurate Load Calculations

Perform realistic load calculations that account for diversity.3.

Consider the Specific Installation

Think about the length of the run in the wireway, the physical spacing, and ventilation.4.

Consult an Expert

If you're unsure, always* consult with a qualified electrician or electrical engineer. They can help you make the right decisions to ensure a safe and code-compliant installation.

Therefore, while derating is a consideration when using troughs, it doesn't automatically mean you have to drastically oversize wires. Careful planning, accurate load calculations, and a thorough understanding of the electrical code are essential. Flag for review