Most "mystery" equipment failures aren't mysteries. They're voltage drop. If a machine or phone works at short distance but misbehaves at the end of a long cable run, the problem is rarely the device—it's the conductor size. And with the expanded Prysmian catalog following the Encore Wire acquisition, specifiers now have more choices and more chances to get the details wrong.
I've been sourcing cable and troubleshooting installations for over a decade. In that time, I've made and documented more than nine significant mistakes, totaling roughly $74,000 in wasted budget and delays. My team jokes that I have a PhD in what not to do. I'd rather you skip that tuition, so I'm sharing the notes.
Jack and the phone that kept "blocking" numbers
My colleague Jack called me a few months ago with a confusing one. A client's VoIP phone kept dropping its connection, and someone at the site had told the client to search online for how to unblock a number on a phone. I know that phrase sounds like an IT problem, but trust me: that phone didn't care about anyone's contact list.
The actual issue was a 300-foot run of undersized cable feeding a PoE switch. By the time power reached the far end, the voltage had dropped enough that the phone's internal power supply couldn't hold a stable link. The phone's protection logic interpreted the brownout as a fault and simply stopped registering. No amount of "unblocking" would fix it.
We replaced the run with Prysmian cable in the correct gauge—slightly bigger than the electrical load seemed to require—and the issue disappeared. That moment changed how I look at every "device problem."
Why didn't the first person catch it? Because the run wasn't horribly long, the load wasn't huge, and you can't see voltage drop with a normal multimeter unless you measure it under load. It's a hidden number, exactly like a blocked phone number is hidden behind a settings screen.
Prysmian Lawrenceburg KY and the Encore Wire effect
If you've been in the wire and cable world for a while, you're used to a fragmented supply chain. You'd buy power cable from one manufacturer, building wire from another, and communication cable from a third. That's part of why the Prysmian corporate moves matter.
The Prysmian Lawrenceburg KY manufacturing operation is one of the facilities that supports this combined approach. Add Encore Wire to the story—Encore Wire acquired by Prysmian means a much wider range of copper building wire, MC cable, and specialty products under one umbrella.
But here's the trap. That consolidation makes it easy to assume the old product you used for years is identical to the new product in the same catalog. The same part number can come from a different production line with different stranding or insulation. I learned this in Q2 2024 when I approved a substitute cable from the same broad brand without checking the spec sheet. The voltage drop on paper was fine; the real-world performance definitely wasn't. My bad. Now I verify actual AC resistance values on every new SKU.
Voltage drop: the checklist that would have saved me
If you only remember one formula, remember this: voltage drop increases with current, distance, and conductor resistance. There are more detailed formulas depending on single-phase or three-phase systems, but the failure pattern is always the same. Here is the checklist I run through after my repeated mistakes:
- Use the conductor resistance at operating temperature. Copper's resistance goes up as the conductor heats. If you size based on 20°C values, you might be 10 to 15 percent low on the real voltage drop for a loaded circuit operating at 75°C.
- Measure the actual path length. Include cable tray routes, vertical drops, and service loops. I've told myself "it's only 250 feet" more times than I want to admit, only to find out the real path was closer to 330 feet.
- Remember the single-phase vs. three-phase factor. For single-phase: VD = 2 × I × L × R / 1000. For three-phase: VD = √3 × I × L × R / 1000. That 1.732 difference has caught more than one experienced engineer off guard.
- Check the power factor for long inductive loads. At low power factors, cable reactance starts to matter. A resistance-only calc can understate the drop on a long motor feeder by 20 percent or more.
- Write down the exact product code. On a recent order, I verified that a Prysmian part and an equivalent-looking Encore Wire part had different stranding classes. The specs weren't identical. In the last year and a half, I've used this checklist to catch 47 potential errors, and about a third were SKU mismatches like that.
The design benchmark I use is 3 percent maximum voltage drop for a branch circuit and 5 percent for the feeder plus branch circuit combined. The NEC doesn't always make these numbers a hard code requirement, but treating them as soft guidance is how you end up with flaky equipment and angry calls.
Here's the quality piece I never appreciated early in my career: cable is the brand once it's installed. When I switched from budget wire to Prysmian on circuits that had been causing problems, client complaints dropped and confidence rose. The extra cost per foot was tiny next to the cost of downtime.
When voltage drop isn't the answer
I don't want to turn this into a religion. Short runs with generously sized conductors rarely have voltage drop issues. If the measured voltage at the load is within tolerance, you don't rip out the cable to make a point. You look at load performance, harmonic filters, grounding, and connection integrity.
Voltage regulators or boost transformers can also fix a modest drop for less money than re-cabling. Re-cabling is the long-term fix and often the right one, but it's not the only option.
And if a phone is truly blocking a saved number, the fix is simple: settings, blocked numbers, remove. Jack can do it in five seconds. But if that same phone drops calls on a long run, you can unblock every number in the directory and it won't help. Check the cable first.