NEC 110.58 - Disconnecting Means

Or: "Why Your Transformer Needs a Kill Switch You Can Actually See"

Alright, listen up. You know that feeling when you're about to work on a transformer and you gotta walk three buildings over, down two flights of stairs, and through the boiler room just to find the disconnect? Yeah, the Code writers hate that too. That's why they gave us 110.58.

Here's the deal in plain English: Every transformer and motor needs its own disconnect switch or breaker that you can SEE from where you're standing at the equipment. Not "kinda nearby." Not "down the hall." Not "somewhere in that panel behind the boxes of Christmas decorations." WITHIN SIGHT.

And this disconnect can't be some half-ass switch that only opens two out of three hot legs like your brother-in-law's wiring job. It's gotta simultaneously open ALL the ungrounded conductors. That means if you've got three hots coming in, all three hots gotta open at the same time. No lingering phantom voltage to bite you in the ass.

The Transformer Side of Things

For transformers, your disconnect switch or breaker needs to be rated at least as high as the ampacity of the supply conductors feeding it. Think of it this way: if you're feeding a transformer with 100-amp conductors, don't slap a 60-amp disconnect on there and call it good. The disconnect has to match or exceed what those conductors can handle. This isn't a "close enough for government work" situation.

The Motor Side of Things

Now motors? They're special snowflakes with their own rules in Article 430 Part IX. Motors get special treatment because of their unique starting characteristics (you know, that "holy crap the lights just dimmed" moment when a big motor kicks on). So for motor disconnects, you gotta follow Article 430's specific requirements. We're not getting into all that here—that's a whole different rabbit hole—but just know motors don't play by the same rules as transformers.

🔑 KEY TAKEAWAYS (The Stuff That Actually Matters)

1. Within Sight = Within Sight: If you can't see the transformer or motor from the disconnect location, you're doing it wrong. "Within sight" means visible and not more than 50 feet away (per Article 100 definition).

2. Opens ALL Ungrounded Conductors: Your disconnect must simultaneously open every hot leg. Two-pole switch for single-phase, three-pole for three-phase. No exceptions, no shortcuts.

3. Transformer Disconnect Rating: Must be rated at least equal to the ampacity of the transformer's supply conductors. If 90-amp conductors feed it, minimum 90-amp disconnect.

4. Motor Disconnects: Follow Article 430 Part IX—motors have their own special rules due to starting currents and other motor-specific considerations.

5. Can Be a Switch OR Circuit Breaker: Either works, as long as it meets the requirements. Fused disconnect? Great. Breaker? Also great.

🏗️ REAL-WORLD JOBSITE SCENARIOS

Scenario 1: The Hidden Transformer Mystery

You're doing service work on a 75 kVA transformer in a mechanical room. The prints show a disconnect, but it's located in an electrical closet around the corner and through a door. The maintenance guy says "it's only 30 feet away!"

Problem: Can't see it = not within sight = Code violation. You could be standing there verifying the transformer is "de-energized" while someone else walks by and flips that switch back on because they can't see you're working on the equipment. That's a "meet your maker" moment.

Solution: Relocate the disconnect or install a new one that's actually visible from the transformer location. Yeah, it costs money. Know what else costs money? Funerals.

Scenario 2: The "Two Outta Three Ain't Bad" Special

You're inspecting a new installation where someone installed a 2-pole disconnect on a 3-phase transformer. When you ask about it, the installer says "Well, opening two legs kills the transformer anyway, right?"

Problem: WRONG. Dead wrong. Literally could-be-dead wrong. That third leg is still energized. Article 110.58 says ALL ungrounded conductors must open simultaneously. If you're working on the line side and assume everything's dead because the disconnect is open, that third leg will teach you a lesson you'll never forget (assuming you survive the lesson).

Solution: Replace with a proper 3-pole disconnect. This isn't a suggestion—it's a requirement.

Scenario 3: The Undersized Disconnect Debacle

You're roughing in a transformer installation. The supply conductors are sized at 125 amps based on your calculations. Your apprentice grabs a 100-amp disconnect from the truck because "that's what we've got and the transformer is only 45 kVA."

Problem: The disconnect must be rated at least equal to the supply conductor ampacity (125A), not the transformer load. Your apprentice is thinking about what's downstream—the Code is concerned with what the conductors can deliver and what can safely interrupt that.

Solution: Go get a 125-amp (or larger) disconnect. Tell your apprentice to study 110.58 tonight instead of scrolling TikTok.

📚 WHAT TO STUDY (Exam Hot Topics)

If you're prepping for your journeyman or master's exam, the testing gods LOVE this section. Here's what they'll nail you on:

1. "Within Sight" Definition: Know Article 100's definition cold. Visible AND not more than 50 feet. They'll give you a scenario where it's 55 feet away but visible—that's a fail.

2. All Ungrounded Conductors: Expect a question about whether a 2-pole disconnect is acceptable for 3-phase equipment. (It's not, unless there's literally only 2 ungrounded conductors, but that's not what we're dealing with here.)

3. Transformer Disconnect Sizing: They'll give you the supply conductor ampacity and ask for minimum disconnect rating. It's a gimme question if you read the section—don't overthink it.

4. Motor vs. Transformer Requirements: Be ready to identify that motors have different rules and point to Article 430 Part IX. They might ask why motors are treated differently (starting currents, locked rotor current, etc.).

5. Switch OR Breaker: Know that either is acceptable. They might try to trick you with "must be a circuit breaker" or "must be a switch"—nope, either works.

6. Combination Questions: Expect them to combine this with other sections, like asking about transformer disconnect requirements while also testing your knowledge of 450.14 (disconnecting means for transformers over 1000V). Know where 110.58 ends and other sections begin.

The Bottom Line

This rule exists because someone, somewhere, got hurt or killed working on equipment they thought was de-energized because the disconnect was in another zip code. The "within sight" requirement isn't about convenience—it's about making sure when you're standing at that transformer or motor ready to work, you can verify with your own eyeballs that the disconnect is open and maybe even lock it out yourself.

Size your disconnects properly, make sure they open all the hots, and keep them visible from the equipment. It's not rocket surgery, but it'll keep you alive and keep the inspector from writing you up.

Now quit reading and go make sure all your transformer and motor disconnects are actually within sight. I'll wait.