Parallel conductors from one AC feed must never pass through separate holes in a steel wall. Each lone conductor's magnetic field wraps around the steel between holes, inducing eddy currents that heat the enclosure until paint discolors and steel glows in extreme cases. Group every phase, neutral and grounded conductor of each set through one hole, slot between holes, or fit a non-ferrous gland plate. This article explains the physics and how to lay out the cutouts; for the basics of gland selection and sealing, see installing a cable gland without losing the rating.
What the code and the physics agree on
The NEC addresses this directly: where a single conductor carrying alternating current passes through a metal enclosure and a ferromagnetic field surrounds it, all phase, neutral and grounded conductors of the circuit must run through the same opening, or the opening must be lined with a non-ferrous metal. Grouping conductors per set is not an informal workaround; it is exactly what the rule intends. Check the edition your authority having jurisdiction enforces.
The physics explains why. Current in a conductor creates a magnetic field around it. Bring all phases of a set through one hole and their fields largely cancel, because the instantaneous currents sum to near zero. Put one phase alone through its own hole in steel and the field has a closed magnetic path through the wall around that hole. A changing flux in steel induces circulating eddy currents, and their resistance losses heat the metal. At 4000 A across parallel sets the effect is not subtle.
Where the heat comes from
Three field-driven loss mechanisms act on a steel wall:
- Eddy currents in the steel looped by flux around a single-conductor hole, producing I2R heating in the wall itself.
- Hysteresis loss as the steel magnetizes and re-magnetizes every cycle, adding heat on top of the eddy losses.
- Closed magnetic circuits formed by steel between adjacent holes, which concentrate the flux rather than letting it cancel.
Aluminum behaves differently. Its permeability is nearly that of air, so magnetic flux does not concentrate in it and an aluminum gland does not create a magnetic circuit across a slot. That is why aluminum glands in grouped holes are fine while the same glands in individual steel holes are not. For a deeper treatment of slotting switchgear enclosures, read eliminating eddy current heating by slotting the enclosure.
Three correct layouts
Pick one of these cutout patterns; all three satisfy the grouping requirement.
- One big opening. All conductors of the feed pass through a single hole. Simplest to verify, hardest to seal neatly for many large cables.
- One hole per parallel set, slots between holes. Each set keeps its own conductors, and slots cut between the holes break the steel path so flux cannot circulate. The slots effectively make the grouped openings one electrical and magnetic opening.
- Grouped A-B-C-N through individual holes with slots inside each group. Each hole carries one conductor, but each group contains all phases of the set and slots connect the group's holes so the fields cancel within the group. Slots between groups add little.
A non-ferrous gland plate over the steel is the cleanest version: drill individual holes wherever you like in the aluminum, and remove or open the steel behind it so locknuts touch only the plate or the aluminum. Remember the steel wall underneath still has to meet the same rule; the plate does not magically exempt the enclosure body.
Choosing the gland plate material
- Aluminum plate: unambiguously non-ferrous, drills and taps easily, matches aluminum gland connectors thermally. Paintable to match existing gear.
- Austenitic stainless (304, 316L): practically non-magnetic and accepted by many inspectors, but if the spec says non-ferrous it may not satisfy the wording. Verify with the authority having jurisdiction.
- Non-metallic glands and locknuts: remove metal at the hole entirely, though they do not fix a steel wall with individual holes.
Our custom enclosures can be built with an aluminum gland plate and grouped cutouts from the start, delivered in 10 business days from your approval of the drawing. If you are working from a standard stainless body, review the openings on the custom cutout checklist before anything is cut.
Putting the layout on the drawing
Lay the groups out in the cutout designer: place each set's holes together, mark the slots between them, and check spacing against gland locknut diameters. Number each group so the installer pulls set 1 through group 1, not one conductor per hole. On the approval drawing, confirm the plate material, the slot widths, and that the steel behind the plate is opened as one window per group.
For lower-amperage feeds this is rarely an issue; sizing a cable gland for a connector matters more there. But once you parallel conductors above a few hundred amperes per set, the hole layout is part of the electrical design, not just the mechanical one. If the cables also carry signals, plan the entry the way the RJ45 gland sizing guide does, separately from the power groups.
What to specify
- Gland plate material: aluminum, or austenitic stainless with AHJ approval.
- One opening or slotted group per parallel set, all phases represented equally in each group.
- Slot width and length on the drawing, with the sealing plan for each grouped opening.
- Gland type and size per conductor, torqued per the manufacturer's instructions.
Any enclosure we build for these feeds carries a NEMA or IP rating from the manufacturer for the specified design; it is not UL listed. Grouped, slotted openings change the sealing surface, so review them on the drawing before approving. For the rating fundamentals, see what each NEMA and IP rating means.
Enclosures for this job
Catalog sizes that suit the job this article answers, picked from its subject; open one for its full specifications.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 16 × 12 × 8 in | 304 stainless | 16 x 12 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 30 × 24 × 10 in | 304 stainless | 30 x 24 x 10 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 |
Questions
Does the NEC require grouping parallel conductors through metal enclosures?
Yes. The NEC requires that where a single conductor carrying AC passes through a metal enclosure with a ferromagnetic field around it, the conductor must be grouped with the other phase, neutral and grounded conductors of the same circuit, or the hole must have a non-ferrous lining. The goal is to cancel the magnetic field so it does not induce circulating currents and heat the metal. Check the edition your authority having jurisdiction enforces.
Are aluminum cable glands as good as non-metallic ones here?
For induction, yes. Aluminum is essentially non-magnetic, with permeability close to air, so flux does not concentrate in it and it does not create a closed magnetic circuit through the slot. Non-metallic glands remove any metal at the hole entirely, but the enclosure steel itself is what matters. Fix the hole layout first, then choose the gland material.
Do slots between the holes hurt the enclosure rating?
Slots turn separate holes into one larger opening, so the sealing plan has to treat each grouped opening as one hole. Use one non-metallic gland plate or a grouped sealing arrangement over the slot, and verify the seal design on the approval drawing. Never field-cut slots in a rated enclosure without reviewing the rating basis.
Can I use a stainless steel plate instead of aluminum?
Only if it is an austenitic grade that a magnet will not stick to, such as 304 or 316L. Even then, if the specification uses the word non-ferrous, stainless may not satisfy it literally, and the authority having jurisdiction makes the call. Aluminum is unambiguous.
