Feed a disconnect switch from the top by default. A bottom feed is a legitimate design when two things are true at once: the switch is non-fused, and the manufacturer's instruction allows the supply at either terminal end. The top-feed habit is a convention inherited from knife switches. The rules behind it aim at one thing: the moving contacts must be dead when the switch is open. A fused switch fed from the bottom breaks that condition. On a 480 V supply, its fuse clips sit at 277 V to ground with the handle off. The sections below give the rule, the numbers and the call for each case.
Where the line-in-top convention comes from
Knife switches set the pattern, and two early code rules shaped it. A single-throw knife switch had to be connected so its blades were dead in the open position. It also had to be mounted so gravity would not tend to close it. Supply at the top satisfied both, and in-at-the-top, out-at-the-bottom became the trade default.
A rotary disconnect switch has neither weakness. Its moving contacts turn inside the switch body, gravity plays no part, and the terminals carry no line or load marks on most designs. The convention survived because it puts the live end in the same place on every panel, which keeps the next install predictable. The same habit shows up on contactors, and a separate page works through line-in-top-or-bottom wiring for contactors. Treat the convention as the starting point and the dead-contact rule as the decision.
What the switch-connection rules require
Canada's rule sits in CSA C22.1, the Canadian Electrical Code, Part I. It requires manual single-throw switches, circuit breakers and magnetic switches to be connected so the blades or moving contacts are dead when the device is open. The rule then lists a short set of exceptions. The one that matters here covers switches designed so that all live parts are out of reach when the device is open. A rotary switch with fully enclosed contacts meets that description once finger guards close off its terminals.
The NEC carries the same thinking, written for single-throw knife switches: connected so the blades are dead when the switch is open. Rotary switches were not the hardware those words were drafted for. For them, the deciding document is the manufacturer's instruction.
That instruction is specific. The technical data lists the switch's current and voltage ratings, and the installation instruction states which ends may take the supply. A 32 A frame carries the same 32 A from either direction; the ratings do not move. Some instructions state plainly that the line may land at either end, because the moving contacts open the circuit in two places and stay dead open. Where the instruction marks a line end, that end takes the line.
A fused switch is the case apart. Its fuse holders sit on the load side of the contacts, so the feed direction decides whether the clips go dead with the handle. That is the hardware the dead-blade rule was written to protect.
Working a 480 V, 32 A bottom-fed cabinet disconnect
Here is the worked case, with the numbers a live-panel decision turns on.
- Supply: 480 V, three phase, 60 Hz — a standard North American system voltage under ANSI C84.1, and this example's assumption. Swap in your own system voltage and redo the division.
- Switch: a 32 A frame, non-fused, door-interlocked handle, finger guards on the terminals. The frame size is also an assumption; it does not change the arithmetic.
- Body path: 1 kΩ (1,000 Ω) hand-to-hand, the conservative figure from the IEC 60479-1 shock-current data.
- Let-go current: about 10 mA to 16 mA, from the same IEC 60479-1 data.
The rule in words: on a three-phase system, voltage to ground equals phase-to-phase voltage divided by the square root of three. Current through the body equals the voltage across it divided by body resistance.
The arithmetic: 480 V ÷ 1.732 = 277 V to ground. 277 V ÷ 1,000 Ω = 0.277 A, which is 277 mA. Against the let-go range, 277 ÷ 16 ≈ 17 and 277 ÷ 10 ≈ 28, so the path carries 17 to 28 times the let-go current.
The result: on the non-fused switch, either feed direction leaves one set of live parts with the door open — the three line terminals behind their guards. The moving contacts are dead, so the 277 mA path never exists. The same switch built fused is a different machine. Fed from the bottom, the clips hold 277 V to ground with the handle off, and a fuse change becomes work on live 277 V parts. Fed from the top, the handle kills the clips, and the fuse job happens on a dead switch.
The decision: bottom feed stands while no fuse sits in the switch body and the instruction allows the supply at either end. The first fuse inside the switch sends the supply to the top.
One safety note belongs here: the handle on the cabinet door never de-energizes the supply side. Open and lock out the upstream device before any terminal work.
What a bottom feed changes in the panel
The electrical test is the dead-contact rule; the hardware details move with the feed direction.
Live parts change address. Top feed puts the live terminals at the top of the switch, bottom feed at the bottom. Finger guards go on the live end either way, and on the 480 V example each guarded terminal holds 277 V to ground while the supply is on. A bottom-fed switch earns a one-line door note saying the supply lands at the bottom. That sits on top of the NEC duty to mark every disconnecting means legibly for its purpose.
The door and the interlock stay as they are. NEC 110.26(A) requires working space in front of equipment likely to be serviced while energized, with doors able to open at least 90 degrees. A door-interlocked handle keeps the door shut while the switch is on, and the cabinet main switch that isolates all three phases still leaves its line side hot. That is why the line side of a separate disconnect stays live, and why PPE for the live circuits ahead of the panel main does not change with feed direction.
Bending space gets checked at the bottom. Bottom-fed conductors often enter the wall opposite their terminals. NEC 312.6 sets the minimum wire-bending space at terminals, with the larger Table 312.6(B) space for a conductor entering the wall opposite its terminal and the Table 312.6(A) space when it does not.
The listing paperwork is untouched. NEC 409.2 defines an industrial control panel, and UL 508A is the standard for listing one. NEC 409.110 requires the enclosure type number on the panel. NFPA 79 governs the machine-panel supply disconnect, its interlock and its marking. None of it names a feed direction while the dead-contact condition holds.
The routing win is real. With the supply landing at the bottom, line and load both leave through the bottom gland plate, and the duct runs shorten. Bottom conduit entry into a floor-mounted enclosure works through the entry details, and a conduit-fed box needs its gland plate sized and bonded properly.
The decision for each case
| Switch in the cabinet | Feed the supply | Basis |
|---|---|---|
| Non-fused rotary, instruction allows either end | Bottom, when it saves routing | Moving contacts dead when open; guards on the live terminals |
| Non-fused rotary, instruction marks a line end | At the marked end | The instruction controls the direction |
| Fused rotary, fuses inside the switch | Top, every time | The handle must kill the fuse clips |
| Fused rotary already bottom-fed in service | Top at the next shutdown, or a marked upstream isolator for fuse work | The clips hold 277 V to ground on the 480 V example |
One sentence covers the table: whatever a person would touch for a terminal or fuse job must be dead with the handle off. When bottom feed keeps that true, it is a routing choice. When it does not, top feed is the design.
What to put on the drawing
If the routing win pays for the cabinet, send the layout with the bottom entry detailed. We build custom stainless cabinets in 304 and 316L to your width, height, depth and cutouts, from one piece. See the custom enclosure options, or set the gland-plate drilling yourself in the 3D enclosure builder. The cabinet is delivered in 10 business days anywhere in the world, counted from your approval of the drawing on custom work. The type rating is a manufacturer rating for the specified design, not UL listed. The switch decision then does the rest: guards on the live end, the door marked, and the handle-off position dead wherever hands will go.
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 |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 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 |
Questions
Can you bottom feed a non-fused disconnect switch?
Yes, when the instruction for that model allows the supply at either terminal end and no fuse sits in the switch. A 32 A non-fused rotary switch carries the same 32 A and the same voltage class from either direction, because its moving contacts turn inside the body and stay dead open. Fit finger guards over the live terminals and note the feed direction on the door.
Why is a fused disconnect switch different?
Its fuse holders sit on the load side of the contacts. Fed from the bottom, the clips stay energized with the handle off — 277 V to ground on a 480 V supply, enough to push about 277 mA through a 1 kΩ body path, 17 to 28 times the 10-16 mA let-go range. Top feed lets the handle kill the clips, so a fuse change is a dead-panel job.
Does any code rule literally say line in at the top?
Not in those words. The rules aim at the hazard, not the geometry. CSA C22.1 requires the moving contacts of a manual switch to be dead when it is open, with an exception for switches whose live parts are all out of reach when open. The NEC words the same duty for single-throw knife switches, which also had to be mounted so gravity could not close them.
Do finger guards and door marking still matter with a top feed?
They matter either way. The line terminals are live whenever the supply is on — 277 V to ground behind each guard on the 480 V example, handle on or off. Guards go on the live end whichever way it faces, the NEC marking duty stays, and a bottom-fed switch deserves a one-line door note saying the supply lands at the bottom.
