Fusing a power supply and motor drivers inside an enclosure

Fuse three places in a driver box: one time-delay device at the 120 V inlet, one fuse in each feed from the supply to its driver, and nothing between a driver and its motor. The worked example below is a 480 W 24 V supply on a 120 V inlet feeding three stepper drivers that draw 3 A each. That comes to a 6 A time-delay fuse at the inlet and a 4 A fuse in every driver feed. The sizes come from two labels and a clamp meter, not from the box. The enclosure's part is mechanical: rail space for the holders, short leads from every fuse, and sealed glanded cutouts where the motor wires leave. The steps below size each fuse, place it, and hand you the checks that prove the panel before the first chip flies.

Which circuits in a driver box need a fuse?

Fuse exactly three places: one time-delay fuse or breaker at the 120 V inlet (6 A in the example below), one fuse in each lead from the supply to a driver (4 A each here), and nothing in the leads from a driver to its motor.

Each device guards what sits downstream of it. The inlet device protects the supply's input and the short leads between the inlet terminal and the supply; the 20 A breaker in the wall never treats a fault in them as its problem. Each driver fuse isolates one axis for you, so a shorted driver kills one feed instead of dragging the whole output bus down with it. The fuse map for the example box then fits in three lines.

Check the bus before you pick any of it: three drivers at 3 A plus 0.5 A for the breakout board and fan is 9.5 A of continuous load against a 20 A output. Supply makers set continuous load at or under 80% of rating in their application notes, a 16 A ceiling here, so the example supply has room and is not the device you are sizing. One inlet feeding several devices has its own inlet-side rules; the one power input, several devices article covers them.

How do you size the fuse on the 120 V AC inlet?

Start from the supply's input, not its output: a 480 W supply at 85% efficiency on a 120 V line draws 480 ÷ (0.85 × 120 V) = 4.7 A, the label says 5 A, and the next standard size above it is the 6 A time-delay fuse from the table.

Run the arithmetic the way the label-maker did: 480 W ÷ 0.85 = 565 W of input, and 565 W ÷ 120 V = 4.7 A. On 230 V the same supply draws 480 ÷ (0.85 × 230 V) = 2.5 A, so a 3.15 A time-delay midget fits there.

The delay class matters as much as the amps. A switching supply charges its input capacitors at switch-on and pulls 30-60 A for under a millisecond, a figure the label usually prints. A time-delay element carries several times its rating for seconds and shrugs the spike off; a fast fuse at the same amps can blow on it and cost you a morning of hunting.

Fit a device at the inlet even though the wall has a breaker. The breaker protects the branch circuit at its own rating, 20 A here; the leads inside the box between the inlet terminal and the supply are 18 AWG (0.82 mm²), and the inlet device is what matches them to a real protection level. Put it in the line (L) conductor on a single-phase inlet, land neutral direct, and land earth on the ground stud. Clearances and heat around the supply itself are in the switching power supply mounting article.

How do you size the fuse on each driver feed?

Fuse each driver's feed at about 1.25 times its continuous draw and round up to the next standard size: a driver that pulls 3 A on your hardest continuous move needs 1.25 × 3 A = 3.75 A, so a 4 A fuse goes in its feed.

Get the draw from the driver datasheet at your current setting, or clamp the feed with a DC clamp meter while the machine runs its worst move. The 1.25 multiplier keeps the fuse above working current and its acceleration peaks, and below what the lead can carry. At 3 A continuous the fuse sits at 75% of rating, where a blade fuse lives comfortably for years.

Fit the fuse in the +24 V leg only and leave the 0 V common solid on its distribution block. A blown common fuse leaves the whole bus live at supply potential through the loads, which is the nastier fault to meet in the dark. One fuse per driver: a distribution block feeds three fused terminals, one short lead each to a driver, and a fault in one axis clears one fuse.

Power-up is a non-event at this size. A blade fuse holds twice its rating for several seconds, typical of the automotive blade class, and the drivers' capacitors charge through the supply's controlled output ramp in far less. If your bus hangs off a big linear supply instead, keep the time-delay class here too.

Do the motor leads from driver to motor need a fuse?

Leave the leads from each driver to its motor unfused: they are short, they carry a set phase current of a few amps, and the driver's own electronic current limit clears a lead fault faster than any fuse you could fit.

A stepper or servo at stall draws its set current, the same 3 A it draws at full torque, so a fuse above that never treats a stall as a fault. A phase-to-phase short collapses into the driver's current limit, which reacts in milliseconds. What a motor-lead fuse adds is a nuisance blow at an acceleration peak, mid-job, with the part on the table.

Keep the routed length under 2 m (6.5 ft) and keep the bundle out of the 120 V wiring; the separation rules for 24 V wire beside power wiring are the same ones here. Where the leads leave the box, the gland decides the seal. Our boxes come rated NEMA 4X / IP66 by the manufacturer; not UL listed. The assembly holds that rating only as well as the lowest-rated part in the wall. The glanded arrangement that keeps a sealed box sealed is in the motor-controller power feedthrough article.

How do you install the fuse holders and wire the panel?

Mount the inlet device and the per-driver fuse holders on the DIN rail between the inlet gland and the supply, wire the panel dead and locked out, and land every fuse before the first energise.

What you need:

• the inlet time-delay fuse in a 250 V-rated holder, or a single-pole breaker of the same size
• three per-driver blade or midget fuses, one per driver, plus a spare for the door pocket
• a +24 V distribution block, a 0 V block, and three fused terminals
• 18 AWG (0.82 mm²) leads for the inlet and driver feeds
• rail space: a single-pole module is 17.5 mm wide (DIN 43880), so the inlet breaker plus three fused terminals need about 70 mm (2.8 in) of rail
• glands sized to the inlet and motor-lead bundles

Rail order, grouping and the room each device owes its neighbour are in the backplate and DIN rail layout guide. Then wire it in this order:

1. Isolate the feed, lock it off, and confirm the inlet terminals dead with a meter before the first lead moves.
2. Land the inlet: line to the input of the inlet device, its output to the supply's L terminal, neutral direct to N, earth to the enclosure's ground stud.
3. Fit the inlet fuse and close the holder.
4. Run one short lead from the supply's output to the distribution block, then feed each fused terminal from the block.
5. Land one lead from each fused terminal to its driver's +V input, lengths as short as the rail allows.
6. Land every 0 V return on the common block, unfused.
7. Bond the supply chassis, each driver chassis and the ground stud; the grounding and bonding guide sets the conductor and stud practice.
8. Seat and torque every gland to the gland maker's figure before the door closes.

How do you check the panel before the first cut?

Confirm three readings before the machine moves: about 5 A at the AC inlet with every axis running, about 3 A in each driver feed at the worst continuous move, and under 0.1 V across each closed fuse holder.

• Continuity: check each fuse out of its holder before the first energise.
• Inlet: clamp the AC inlet with all three axes running; about 5 A is the design point, and a reading well above it means an overloaded supply, not a fuse problem.
• Per driver: about 3 A in each feed at the hardest move; a feed at 5 A and up at stall points at that driver's current setting.
• Holders: under 0.1 V drop across each closed holder with your clamp meter; a warm holder means a bad crimp or a fuse working over its rating.
• Isolation: pull each driver fuse with the machine idle; only its own driver loses power and the other two hold.
• Soak: run 30 minutes at the worst load; nothing blows and every holder stays cool to the hand.

One fault no fuse catches: a driver that holds its set current while the motor sits still. That diagnosis runs through the page on a servomotor that draws current but does not turn.

Which mistakes cost the protection or the rating?

The five that cost real money: a fast-acting inlet fuse that blows on every start, one shared fuse for all three drivers, a fuse in a 0 V common, fused motor leads, and an unrated knockout where a gland belongs.

• Inlet blows on every start: change the class, keep the amps. The time-delay element rides the 30-60 A half-millisecond inrush; a fast fuse never will.
• One fuse on the bus: split it into three fused terminals. A shared device turns one shorted driver into a full stoppage of your machine.
• Fuse in the common: move it to the supply-output leg, where a blown device leaves the bus dead instead of live.
• Fused motor leads: pull it and let the driver's current limit do the job it was built for.
• Blank knockout in the wall: fit glands the maker rates for Type 4X / IP66 service, set to their torque. The lowest-rated part in the wall is the rating of the box.

The wider panel around the fuses — e-stop wiring, inlet layout, where the drives sit — is laid out in the CNC enclosure wiring article.

When do UL 508A sizes take over from the label?

If the panel ships as a listed industrial control panel, the label rule stops and the listing takes over: a UL 508A design guide sizes a CC-class branch fuse at up to 300% of the largest motor's full-load current plus the sum of the other motor loads.

The two devices do different jobs, so the sizes differ. A CC-class branch fuse is short-circuit protection for the branch conductors, and because Class CC fuses are current-limiting it lifts the panel's short-circuit current rating. The 1.25 × device in your own box protects the driver's electronics and sits far lower. Both are right; they answer different questions.

For a panel that ships, the sizes come out of the listing document the shop works to, and the UL 508A listing guide explains how that listing wraps a panel. For the box behind your own machine, the rule at the top of this article holds: the inlet fuse and the per-driver fuses as sized above, nothing after the driver. If you are speccing the box itself, set the rail length for the holders and the gland cutouts in the 3D enclosure builder. A custom body drawn that way is delivered in 10 business days from your approval of the drawing.

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.

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