Skip to content
John Livingston's

The Nuclear Electrical Engineer

An educational resource for electrical engineers in the nuclear power industry

Here you can find design tips on technical skills like cable and breaker sizing, short-circuit evaluations and more, PLUS deep-dive analysis of nuclear-specific topics like design basis, Tech Specs, UFSARs, and safety-related SSCs — built from primary source documents.

The Engineer's Life · Feature

Two Things You Learn Standing at a Panel

John Livingston
August 22, 2026 · 3 min read

From Tips of the Week, 7 and 14 April 2018, following a February outage. Some questions only ever come up with a panel open in front of you and somebody waiting on an answer. Here are two of them.

One ground conductor, or five?

Pop quiz.

You have one breaker panel feeding five different devices. Five devices, five breakers. The cables all take the same conduit away from the panel to a junction box, where they split out and go to the different devices.

Do you need to install five individual grounds at the panel and pull them all through to the junction box? Or can you get by with pulling only one?

Pulling only one would save a lot of hassle. But is it “legal”?

It is.

Article 250.122 covers the size of equipment grounding conductors. Generally, ground conductors are sized per Table 250.122 — but you must remember this: they are sized based on the breaker, the automatic overcurrent device, not on the load.

For example, per Table 250.122, for a 15-amp breaker the smallest permissible ground conductor is #14 AWG copper. They can be larger. They just aren’t allowed to be smaller.

So what about running a single ground conductor in parallel with multiple circuits? The answer is in 250.122(C):

Where a single equipment grounding conductor is run with multiple circuits in the same raceway, cable, or cable tray, it shall be sized for the largest overcurrent device protecting conductors in the raceway, cable, or cable tray. (NEC 2017)

So if you had three circuits on 15-amp breakers and two on 20-amp breakers, all run in parallel in the same conduit, you need only a single ground conductor sized for the biggest breaker — 20 amps. Which means #12 AWG, minimum.

There’s a lot of meat in Article 250. I recommend taking small bites and chewing often.

Two conductors on one terminal

It’s often advantageous to land two conductors on a single terminal. You just have to be aware of the pitfalls.

If the terminal blocks accept ring-tongue lugs, most plants have standard procedures that allow two lugs on a single terminal, along with instructions on how to do it properly.

Screw-type compression terminals have different rules. We don’t often specify these, preferring ring-tongue lugs for their seismic qualities — but vendors do.

Just remember that while they may allow multiple conductors in a single terminal, the conductors must be reduced in size. You can have one large wire, or multiple smaller ones. This seems obvious, but it can trip you up.

To prevent that, read the vendor documentation and find out exactly what kind of terminal blocks they installed and what their wire-range specifications are — especially for multiple conductors.

That last point is the general lesson of both halves: the Code tells you what is permitted, and the vendor documentation tells you what this particular piece of hardware will actually accept. You need both, and the second one is the one nobody has on hand when the question comes up. So go in prepared.