110.39 & 110.40: The "Can You See It From Here?" Rule and Medium-Voltage Connection Reality Check

The Plain-Language Breakdown

110.39 - In Sight From (Within Sight From, Within Sight)

Alright, listen up. The Code loves to throw around phrases like "in sight from," "within sight from," and "within sight of" like they're directions to the nearest coffee truck. But what does it actually mean?

Simple version: If the Code says something needs to be "in sight from" something else, you better be able to stand at one piece of equipment, look over at the other piece of equipment, and actually see the damn thing. No binoculars, no walking around corners, no playing hide-and-seek. And it can't be more than 50 feet away.

Think of it like this: If you can't yell "HEY, SHUT THAT OFF!" to your buddy standing at the disconnect and have him hear you without a radio, it's probably too far.

The two requirements:

1. Visible - You can actually see it (not behind a wall, not around a corner, not blocked by a stack of drywall that "the GC promised would be moved yesterday")
2. Not more than 50 feet - Even if you can see the Sears Tower from your rooftop disco, it ain't "in sight from" your location for Code purposes

This comes up all the time with disconnects for HVAC equipment, motors, and when you're installing a controller that needs to be within sight of the equipment it controls.

110.40 - Electrical Connections (The Medium-Voltage Version)

Now we're talking about the big boy stuff - medium voltage. This is where you graduate from "ouch" to "vaporized." We're talking over 1000 volts AC or 1500 volts DC.

Here's the deal: If you're working with equipment rated 1001V to 2000V and you're not using Type MV cable, just follow the regular 110.14 termination rules you already know. Easy day.

But if you're using Type MV conductors (medium voltage cable), forget 110.14 - now you play by 110.40's special rules. Why? Because at these voltages, a loose connection doesn't just arc - it creates a light show that'll get you on the evening news.

(A) General - Don't Mix Your Metals (Or Your Moods)

The rules:

• Use termination devices that are identified for your conductor material (copper lugs on copper wire, aluminum lugs on aluminum - this isn't Tinder, don't try to mix and match)
• Don't let dissimilar metals touch unless the connector is specifically rated for it (galvanic corrosion is real, and it doesn't care about your deadline)
• Any goop, paste, compound, or snake oil you use must be identified for the job
• Multi-conductor connectors must be marked as such
• Terminals must be suitable for the class of conductor (stranded vs. solid, etc.)

Here's the kicker: If you're using a listed Type MV cable joint or termination that comes with its own connector, you MUST use that manufacturer's connector. No substitutions, no "but the supply house only had these," no creative problem-solving. The manufacturer's connector or nothing.

Why? Because some engineer who makes more than both of us combined spent months testing that specific combination, and your field improvisation skills - while impressive - aren't covered by their UL listing.

(B) Terminals - Pressure is Your Friend

All connections to terminals must be:

1. Mechanically secure - It shouldn't wiggle, jiggle, or move when you tug on it
2. Made with pressure connectors - We're not soldering medium-voltage terminations, chief

Think of it like a good handshake at the union hall - firm, solid, no slippage.

(C) Splices - Cover Your Assets (And Your Splices)

When you splice MV conductors:

• Use splicing devices identified for the application
• Cover every splice and every free conductor end with insulation equal to the original conductor insulation (or an identified insulating device)
• If it's going underground (direct burial), the splice connector must be listed for direct burial

You can't just slap some regular tape on a medium-voltage splice and call it good. That's not a repair - that's a future insurance claim.

(D) Temperature Limitations - The Weakest Link Rule

This is the "your system is only as strong as its dumbest component" rule.

The concept: Look at every part of your connection - the conductor, the terminal, the device. Find the one with the lowest temperature rating. That's your speed limit. You can't run hotter than that, period.

However - and this is slick - you CAN use conductors with higher temperature ratings than your terminals need. Why would you do this? Because when you're doing ampacity adjustments (like derating for conduit fill or high ambient temps), you can use the higher rating of the conductor to do your math, then throttle back to match the terminal rating.

Example: Your terminal is rated for 90°C conductors, but you install 105°C rated MV-105 conductors. You size them using the 90°C column, but if you need to derate, you can start from the 105°C rating and derate down - usually ending up with more capacity than if you'd started at 90°C.

(D)(1) Equipment Provisions

Unless your equipment and connector assembly are specifically identified for 105°C operation, you size your conductors based on the MV-90 (90°C) ampacity tables (Tables 315.60(C)(1) through (C)(20)).

But here's the loophole: You're permitted to install MV-105 conductors sized using the MV-90 ampacity values. Translation: Better conductors, same sizing math. It's like buying a Ferrari but only driving it at the posted speed limit (at least officially).

(D)(2) Separate Connector Provisions

If you're installing pressure connectors separately (not as part of listed equipment), use conductors at amperages that don't exceed the connector's listed temperature rating.

Don't try to push 105°C loads through 90°C connectors just because you have 105°C wire. The connector will become the victim.

(E) Terminal Connection Torque - Tighten It Right

This might be the most ignored section in all of medium-voltage work, and it's also one of the most important.

The rule: Use the torque values indicated on the equipment or in the manufacturer's installation instructions. And you must use an approved means to achieve that torque.

"Approved means" includes:

• Torque wrenches (calibrated ones, not the one you've had since the Nixon administration)
• Shear bolts or breakaway devices (they break off when proper torque is reached)
• Visual torque indicators

What this is NOT: Your calibrated elbow and the "feels tight enough" method.

Why this matters: At medium voltage, resistance is the enemy. A loose connection creates resistance, which creates heat, which creates more resistance, which creates more heat, until eventually you get what we in the business call "an unscheduled plasma event." Those are bad for business.

Too loose? High resistance, heat, failure.
Too tight? Damaged threads, crushed conductors, stress cracks, future failure.
Just right? Goldilocks would be proud, and your connection might outlast your career.

Pro tip: The Informational Notes reference NFPA 70B for more guidance. That's the maintenance standard. Read it. Because the inspector sure has.

Key Takeaways

110.39 (In Sight From):

• ✅ Must be VISIBLE - no obstructions, corners, or walls between
• ✅ Must be within 50 FEET maximum
• ✅ Both conditions must be met - not visible OR 50 feet, but visible AND 50 feet
• ✅ Applies to disconnects, controllers, and other equipment when specified elsewhere in the Code

110.40 (MV Connections):

• ✅ Equipment 1001V-2000V without Type MV cable = follow 110.14 rules
• ✅ Equipment with Type MV cable = follow 110.40 rules (these override 110.14)
• ✅ Use terminals identified for the conductor material
• ✅ Don't mix dissimilar metals unless the connector is identified for it
• ✅ Listed MV cable joints/terminations = use ONLY the manufacturer's supplied connector
• ✅ All connections must be mechanically secure pressure connections
• ✅ All splices must be insulated equivalent to conductor insulation
• ✅ Direct burial splices must be LISTED for direct burial
• ✅ System temperature rating = lowest-rated component temperature
• ✅ MV-105 conductors can be used but sized at MV-90 values (unless equipment is identified for 105°C)
• ✅ Torque all connections per manufacturer specs using approved torque tools
• ✅ Document your torque values (the inspector might ask, the lawyer definitely will)

Real-World Scenarios

Scenario 1: The HVAC Disconnect Debate

You're installing a rooftop HVAC unit. The equipment is on the roof. You want to put the required disconnect down on the wall, around the corner from the ladder.

Apprentice thinking: "It's only 30 feet away, we're good!"

Journeyman thinking: "Can I see the unit from the disconnect?"

Master thinking: "Can I see the unit from the disconnect, and is it 50 feet or less?"

In this case, you're around a corner. Can't see it = not in sight from = Code violation. You need to either relocate the disconnect or use the exception that allows the disconnect to be out of sight IF it's lockable in the open position (with the lock remaining in place).

The fix: Put the disconnect where you can see the unit, or use a lockable disconnect and train the customer on lockout/tagout procedures.

Scenario 2: The Motor Controller Mystery

You're installing a large motor in a mechanical room. The controller is going in a separate electrical room 75 feet away. The engineer's plans don't show a local disconnect at the motor.

Problem: Article 430.102 requires a disconnecting means "in sight from" the motor location (or with an exception for lockable disconnects). Your controller is 75 feet away - even if you could somehow see through the wall, you're over the 50-foot limit.

The fix: Install a local disconnect within sight (visible and within 50 feet) of the motor, or install a lockable disconnect at the controller and placard it clearly.

Scenario 3: The Substation Termination Catastrophe (That Didn't Happen)

You're terminating 13.8kV cable (Type MV-105) at a transformer. Your wholesaler sends you "compatible" lugs that are rated for 90°C.

Apprentice thinking: "They're rated for the voltage, let's rock!"

The problem:

1. The MV cable joint came with manufacturer-supplied connectors, but you substituted different ones
2. You didn't verify the connector was identified for your conductor material
3. You hand-tightened the bolts until they "felt good"
4. You used MV-105 ampacity values even though your connector is only 90°C rated

The correct approach:

1. Use the manufacturer-supplied connectors from the cable joint kit (110.40(A))
2. Verify they're rated for your conductor material
3. Size based on MV-90 ampacity values (110.40(D)(1))
4. Torque to manufacturer specifications using a calibrated torque wrench (110.40(E))
5. Document everything

What you prevented: A 13.8kV fault that would've turned that termination compartment into a temporary sun, probably injured someone, definitely vaporized thousands of dollars in equipment, and potentially killed someone's career (or worse).

Scenario 4: The "Creative" Splice

You're splicing 15kV cable in a vault for a data center. The supply house is out of the specified splicing kit, but they have one "that's basically the same thing."

Red flags everywhere:

• Not identified for the specific conductor
• Not rated for the voltage
• Not listed for the environment (damp location)
• Probably not rated for the current
• Definitely not going to pass inspection

The right move: Order the correct splicing kit. Wait for it. The two-day delay is way cheaper than the lawsuit or the rebuild after a fault. "Close enough" is not a voltage rating.

Scenario 5: The Torque Wrench "Situation"

You're terminating a 480V feeder at a switchboard (okay, this is below MV levels, but the torque principle applies everywhere). The gear says "35 lb-ft" on the terminal lugs. Your apprentice says, "I've been doing this for two years, I know what 35 lb-ft feels like."

No. Just... no.

You pull out the calibrated torque wrench. You torque to spec. You make the apprentice do the next one under supervision with the torque wrench.

Why? Because:

• His "calibrated elbow" might be 25 lb-ft or 45 lb-ft
• Inconsistent torque = inconsistent connections = future service calls
• On MV gear, this isn't optional - it's Code (110.40(E))
• When the switchboard faults in 3 months due to a loose connection, there's a record that YOU didn't do it (you documented your torque values, right?)

What to Study (For Exams)

High-probability exam questions:

1. The "in sight from" definition

   • Visible AND within 50 feet (both conditions required)
   • Expect scenario questions about disconnect placement
   • Know the exceptions (lockable disconnects)

2. Voltage thresholds for 110.40

   • Over 1000V AC or 1500V DC, up to 2000V nominal
   • Type MV cable = use 110.40 rules
   • Non-Type MV cable = use 110.14 rules

3. Dissimilar metals rule

   • Cannot mix where physical contact occurs unless identified for it
   • Know common examples: copper to aluminum

4. Manufacturer-supplied connectors

   • Listed MV cable joints/terminations must use ONLY the supplied connector
   • No field substitutions allowed

5. Temperature rating coordination

   • System limited by LOWEST rated component
   • MV-105 conductors can be installed but sized at MV-90 values (unless equipment identified for 105°C)
   • Understand the ampacity adjustment exception

6. Splice insulation requirements

   • Must be equivalent to conductor insulation
   • Direct burial = must be LISTED for direct burial
   • Cover ALL splices and free ends

7. Torque requirements

   • Per manufacturer specifications
   • Must use approved means (torque tools)
   • Know the examples: torque wrench, shear bolts, breakaway devices

8. Pressure connector requirements

   • All MV terminal connections must be pressure-type
   • Mechanically secure
   • No soldered connections at MV

Memorize these numbers:

• 50 feet (in sight from distance)
• 1000V AC / 1500V DC (threshold for 110.40)
• 2000V (upper limit for this section)
• 90°C vs 105°C (MV conductor ratings)

Watch for trick questions:

• "Within sight" does NOT mean "in the same room" - must be visible
• "Close enough to see it" is not 50 feet - must meet BOTH requirements
• Using higher-temp conductors doesn't mean you can load them higher if terminals are lower rated
• "Compatible" connectors are not the same as "manufacturer-supplied" connectors

Code references to cross-reference:

• Article 430.102 (motor disconnect in sight from requirement)
• Article 440.14 (A/C disconnect in sight from requirement)
• 110.14 (general termination requirements - compare to 110.40)
• Tables 315.60(C)(1) through (C)(20) (MV conductor ampacity tables)
• NFPA 70B (maintenance standard - informational only, but good to know)

The Bottom Line

110.39 is about safety through visibility - you can't disconnect something you can't see. Fifty feet is the limit because beyond that, you're not really supervising the equipment, you're just hoping for the best.

110.40 is about not dying from medium-voltage connections, and not vaporizing expensive equipment. At these voltages, every connection is critical. Use the right stuff, torque it right, insulate it right, or face the consequences (career-ending at minimum, life-ending