Short answer: the standard exists. Inside a machine control enclosure, NFPA 79 requires conductors from 22 AWG through 4/0 AWG, and 250 through 1000 kcmil, to be stranded soft-annealed copper. Every cabinet you build or rebuild therefore gets stranded MTW rated 90 C [194 F], which also satisfies the UL 508A listing standard. The solid conductors in conduit between starters and motors are plant wiring under the NEC, which sets no stranding rule for fixed raceway runs, so they stay as installed. Replace solid wire only where a circuit lands inside the enclosure, and leave the rest until it comes out for its own reasons.
The rule inside a machine panel
NFPA 79 is the electrical standard for industrial machinery, and it settles the stranding question in one sentence: conductors 22 AWG through 4/0 AWG and 250 through 1000 kcmil "shall be only of stranded soft-annealed copper." That covers the machine's own wiring, including the control enclosure bolted to it. A neatly dressed run of solid 14 AWG THHN to a contactor still fails the rule, however good it looks.
When a panel goes to a shop for a UL 508A listing, the conductor clause there — 29.2.1 in the editions in circulation — sets the insulation side: internal power wiring rated 90 C [194 F] minimum, built as machine tool wire to UL 1063, thermoset to UL 44, thermoplastic to UL 83, appliance wiring material to UL 758, or welding cable. Those five names describe insulation systems, not stranding, so you apply both rules at once. A control-panel design guide built on that standard adds two practical floors: copper only, and power-circuit conductors no smaller than 14 AWG.
For how NFPA 79, UL 508A and the NEC split the work on one machine, read the industrial control panel standards overview.
Why the conduit runs can stay
The NEC treats fixed raceway wiring between equipment as building wiring, and it sets no stranding rule there. Solid THHN and XHHW are catalog items in the small building-wire sizes, which is exactly how the existing installation went in: conduit, solid conductors, short runs, all under NFPA 70. Re-pulling working circuits to satisfy a machinery standard they were never built to is spent money, not compliance.
Draw the boundary at the machine. NEC 409.2 defines the industrial control panel, and NEC 409.110 puts its enclosure type number on the panel, so the cabinet is a marked object separate from the conduit feeding it. Land the incoming field conductors on a terminal block or power distribution block just inside the enclosure. That block is where building wiring hands off to machine wiring, and each standard is met on its own side of it.
Where stranded earns its keep anyway
Even without the clause, you would choose stranded inside a cabinet. MTW to UL 1063 comes with finer stranding and insulation built for dressing, so it takes a 90-degree bend in duct without fighting back. THHN wears a nylon sheath over a thin PVC layer, built for dragging through conduit, and it handles that way inside a panel too. The panel also moves: every contactor pull-in shakes the backplate, and a solid conductor fatigues where it leaves the terminal, while stranded tolerates the same micro-movement.
Duct fill rewards the flexibility as well. Wiring duct is loaded to 50 percent of its cross-section at most — the figure one duct maker's fill tables set from NFPA 79-2012 section 13.5.2 — and stiff wire reaches that cap with fewer conductors than flexible wire does. The wiring duct and DIN rail layout guide carries the fill numbers. For a deeper comparison of the two wire types, read MTW vs THHN requirements for control panels.
Wire that actually travels is a third category. The listing standard routes flexing service — Class K machine tool wire and welding cable — through its flexing installation rules at 29.3.12, and that means cable that moves: a gantry axis, a crane bridge, a door loop. A conductor bent once and dressed steady buys none of those rules; it only needs a bend kept within its radius.
The worked case: a 40 A starter circuit in a 20 x 16 x 8 in enclosure
Inputs, each with its source. The motor draws 40 A full-load current (example input — read yours off the nameplate or the NEC motor tables). The NEC sizes motor branch-circuit conductors at 125 percent of that current. Copper ampacity comes from the NEC's 75 C copper ampacity table, as this site's guides print it, where 8 AWG carries 50 A. The panel standard sets the 90 C [194 F] insulation floor, and NFPA 79 sets the stranding rule from 22 AWG through 4/0.
The rule in words: multiply the full-load current by 1.25, pick the smallest copper conductor whose 75 C ampacity covers the product, then check the panel-side rules for the portion inside the enclosure.
The arithmetic: 40 A x 1.25 = 50 A. From that table, 8 AWG copper at 75 C carries 50 A, which covers 50 A exactly, so the conductor is 8 AWG. It sits three sizes above the 14 AWG power-circuit floor, and stranded MTW to UL 1063 carries the 90 C rating the dry panel interior asks for.
Decision, with the threshold: from the line-side lug to the starter, and on to the outgoing terminal block, install 8 AWG stranded MTW. Beyond that block, the existing solid run in conduit stays as installed. Any conductor landing inside the enclosure is stranded; everything upstream of the first terminal waits until that circuit is re-pulled for its own reasons.
What to specify
Put this on the wire order and the rebuild drawing:
| Where the conductor lives | What to order | What it satisfies |
|---|---|---|
| Power circuits inside the panel | Stranded MTW (UL 1063), 90 C, copper, no smaller than 14 AWG | NFPA 79 stranding and the listing standard's insulation list |
| Control circuits inside the panel | 16 or 18 AWG stranded MTW or AWM | NFPA 79 stranding; sized to the load switched |
| Fixed conduit between equipment | THHN/THWN (UL 83), solid or stranded | NEC plant wiring; installed solid stays in place |
| Runs that move (gantry, door loop, crane) | Class K machine tool wire or welding cable | The flexing installation rules at 29.3.12 |
| Terminations | Ferrules in screw clamps; compression lugs from 8 AWG up | Every strand held under the clamp |
Order the panel wire in the cabinet's colors at the same time — wire color inside a control cabinet follows NFPA 79 and IEC 60204-1. Terminate stranded properly: one ferrule per conductor in screw clamps, crimp lugs from 8 AWG up, and two wires on one terminal only where the terminal is built for it. For the small stuff, 16 vs 18 AWG control wiring settles the choice by load and run length.
Rebuild the box while the circuit is open
A rebuild is also the moment the enclosure gets resized, because the clearances follow the box. Conductor bending space at terminals is set by NEC 312.6, and working space in front by NEC 110.26(A), which asks for at least 3 ft [900 mm] of depth at 0-150 V to ground. The enclosure sizing guide walks those numbers into a box size before you order.
We build custom 304 and 316L stainless enclosures with every cutout drawn on the approval drawing: gland holes for the new cable runs and drilling for the terminal blocks. They are delivered in 10 business days from your approval of the drawing, rated NEMA 4X / IP66 by the manufacturer and not UL listed. Sketch it in the custom stainless cutouts builder, or start from the enclosure selector if you already know the DIN module count.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 16 × 14 × 8 in | 304 stainless | 16 x 14 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Is THHN banned inside a control panel?
No. Thermoplastic wire to UL 83 is one of the five insulation systems the listing standard accepts, so stranded THHN clears the insulation side. In practice you pay for the nylon jacket in stiffness: MTW to UL 1063 bends into duct more easily and ferrules cleaner, which is why most new cabinets are wired with it.
Can I leave the existing solid wire in the conduit?
Yes. Runs between equipment are plant wiring under the NEC, and solid 14-10 AWG THHN in conduit is standard electrician stock. Put the boundary at the first terminal block inside the enclosure: upstream of it the solid stays, and downstream of it everything is stranded when you rebuild.
Do stranded wires need ferrules in screw terminals?
In screw-clamp terminals, yes. A ferrule keeps every strand under the clamp as you torque the screw, one ferrule per conductor. From 8 AWG upward, stranded wire lands in compression lugs instead. Spring-clamp terminals take bare stranded wire directly.
What temperature rating does panel wire need?
90 C (194 F) minimum for internal power wiring, which the UL 508A panel standard states alongside its list of five accepted insulation systems, starting with machine tool wire to UL 1063. MTW meets it and so does THHN, so the choice between them comes down to handling, not temperature.
How small can power conductors go inside a panel?
14 AWG is the practical floor a UL 508A control-panel design guide sets for power-circuit conductors, all copper. Control circuits run smaller, with 16 or 18 AWG stranded the common choice, sized by the load they switch and the fuse ahead of them.
Does vibration actually break solid wire at terminals?
Repeated movement is what breaks solid wire, and a contactor panel sees plenty of it: every coil pull-in shakes the backplate. Solid conductor fatigues at the point it leaves the terminal, while stranded tolerates the same micro-movement for decades. Cable tracks on gantries are the extreme case, and those get Class K wire or welding cable.
