Two servo drives in one enclosure: breaker, filter, fuses

Share the feeder parts, duplicate the branch parts. One 40 A main breaker, a matching contactor and one 30 A EMC filter serve both drives; each drive gets its own 20 A branch breaker on 12 AWG and its own 1 A logic fuse, and only an axis that faults on deceleration gets a regen resistor. Two 1 kW-class drives on 120 V single-phase draw about 28 A together, so a 60 A frame is oversized here and forces a heavier feeder wire. The walkthrough below sizes each part, lays out the backplate, and keeps a sealed stainless box - rated NEMA 4X / IP66 by the manufacturer, not UL listed - inside the 40 C limit in a 25 C shop.

Do two servo drives need two of everything?

No. Split the bill of materials into feeder parts you buy once and branch parts you buy per drive: main breaker, contactor, EMC filter and ground bus are shared, while branch breakers, logic fuses and regen resistors are per drive.

Sharing the filter halves cost and panel space, and it halves the earth-leakage current a downstream RCD has to ignore. Duplicating the branch breakers keeps one drive's fault from dropping the other drive mid-cut. The same split - one feeder, many loads - is laid out for a mixed panel in one power input for multiple devices.

What size main breaker feeds two servo drives?

A 40 A main breaker on 8 AWG copper, worked from the drives' rated input currents: 14 A per drive, times two drives, is 28 A, times 1.25 for continuous duty is 35 A, and the next common frame above that is your pick.

The 14 A figure is the rated input current a 1 kW, 120 V single-phase drive's datasheet prints; substitute your drive's number before you order. Every upstream part follows the same rule: supply components are sized by rated input current, from the breaker through the filter to the contactor.

Standard frames run 15, 20, 25, 30, 35, 40, 45, 50 and 60 A. A 60 A frame forces 6 AWG copper, rated 65 A at 75 C in the NEC copper ampacity table, where 8 AWG at 50 A already covers your main. You buy a wire size heavier and gain nothing.

Mounting carries two code numbers. Keep the main breaker handle center within 2.0 m [6 ft 7 in] of the floor (NEC 240.24(A)). Keep 3 ft [900 mm] of clear working depth in front, with the door opening a full 90 degrees (NEC 110.26(A)).

What size branch breaker and wire does each drive need?

A 20 A time-delay branch breaker per drive, feeding it on 12 AWG copper: that wire carries 20 A at 75 C in the NEC copper ampacity table, and the frame clears the inrush a 14 A drive draws at power-up.

The time-delay curve matters because charging the bus capacitors can pull 10-20 times rated current for a few milliseconds. The magnetic trip of a time-delay breaker sits above that spike, while its thermal curve still clears a genuine overload.

Land each branch on the power terminals your manual prints for single-phase input - L1/L2 on some models, R/S on others - since drives change terminal letters between revisions. Give the wires their turns: NEC 312.6 sets wire-bending space at terminals by conductor size. A 10 in [254 mm] deep body lets 8 AWG feeder and 12 AWG branches land without pinching.

Can two servo drives share one EMC filter?

Yes, one filter covers both drives when its rated current beats the sum: two 14 A drives need more than 28 A, so a 30 A single-phase filter does the job, mounted once to bare backplate metal.

Build the chain in this order: main breaker, contactor, filter, then the two branch breakers, then the drives. One filter then shields both branches and the contactor's switching arcs. Bolt the case flat against paint-free metal and land its PE tab on the ground bus with its own green conductor. Keep the input and output leads short, twisted and separated, so the filtered side does not couple back onto the raw side.

A 30 A single-phase filter typically leaks under 3.5 mA to earth, so one filter instead of two also halves the leakage downstream. Never bolt a filter over paint - its case bond to the plate is half its attenuation. The bond-and-filter order of operations is in VFD enclosure wiring: stop, bond, filter.

How do you fuse the drive's logic supply?

One 1 A fuse per drive, sized to the manual: on a 120 V logic supply you fuse the single line, and on a 220 V supply you fuse both lines, so the count follows the voltage, not the drive count.

These fuses protect the drive's internal control-power input, a small circuit that 20 A of branch power would cook in a fault. Use the manual's type, typically a 250 V time-delay fuse, between your logic supply and the control-power terminal it names. Route the fused logic wiring on the opposite duct side from drive power; the spacing rules in servo drive and computer wiring separation apply to encoder and I/O cables too.

When does a servo drive need a regen resistor?

When the axis dumps more kinetic energy on deceleration than the drive's bus can hold: a DC-bus overvoltage fault during a fast stop, on a heavy vertical axis or a high-speed rapid, is the signature to watch for.

Light horizontal axes at normal ramp rates rarely trip it. A Z axis carrying a heavy spindle, or any axis with an aggressive decel profile, often does. The test is free: run the axis at full speed and command your fastest stop. A clean stop needs no resistor; a bus fault means you fit the one the manual lists, which states a minimum resistance in ohms and a power rating in watts for the model.

Count its heat in your budget. A 100 W regen resistor adds 100 W to the panel, so mount it outside the sealed body where its housing can breathe. What happens when that choice goes wrong is covered in braking resistor overload from a hot panel.

How much space and cooling do two drives need?

Plan 4 in [100 mm] of free air above and below each drive and 2 in [50 mm] at the sides - the clearances most drive manuals call for - and shed about 80 W of heat through the walls of a sealed box.

Here is the arithmetic. At 97% efficiency, the typical datasheet figure at rated load, each 1 kW drive sheds 30 W: two drives are 60 W, filter and contactor coil about 10 W, logic supply 10 W, for 80 W total. A made-to-order 30 x 24 x 10 in [762 x 610 x 254 mm] stainless body has about 1.6 m2 of surface, and stainless walls transfer about 3.7 W per m2 per kelvin. Rise = 80 / (3.7 x 1.6) = 13.5 K, so a 25 C shop gives you 38 C inside - under the 40 C most drive datasheets allow.

A 35 C plant changes the answer. The same box holds 48 C there, and passive walls would need 80 / (3.7 x 5) = 4.3 m2 of surface to stay at 40 C. So at 35 C ambient you cool instead: a closed-loop heat exchanger or air conditioner keeps the 4X rating, while a filter fan drops the assembly to the fan kit's rating, typically IP54.

For backplate arrangement across several drives, read multi servo drive enclosure layout; the single-drive version of this arithmetic is in VFD enclosure rating, size and cooling, and the working-space fundamentals in how to size an electrical enclosure. If choosing the body is your next step, the enclosure selector narrows it by inside height and DIN count.

How do you wire the panel, step by step?

Kill the feeder and prove zero voltage before anything else, then build the chain top to bottom - main breaker, contactor, filter, branch breakers, terminal blocks, drives - so every wire you pull lands on the part below it.

1. Lock out the feeder and check for absence of voltage at the incoming lugs; treat the panel as live until you have.
2. Lay the backplate: the main at top left, contactor beside it, the 30 A filter below, the two 20 A branch breakers in a row, drives in the lower half with the clearances above, terminal blocks down the right edge.
3. Pull the 8 AWG copper feeder into the main breaker, with its handle center within 2.0 m [6 ft 7 in] of the floor.
4. Wire the contactor between main and filter, its 120 V coil in your E-stop chain.
5. Bolt the filter to bare metal: input from the contactor, outputs to the branch breakers, PE tab to the ground bus.
6. Run 12 AWG from each branch breaker to its drive's power terminals, following the terminal letters your manual prints.
7. Fit a 1 A fuse per drive between the logic supply and the control-power terminal.
8. Bond the panel: ground bus to the enclosure stud with paint removed at the contact (NEC 250.12), and every drive PE plus the filter PE on the bus - the full method is in enclosure grounding and bonding.
9. Label both ends of every conductor, torque the lugs to the values marked on the breakers, and dress power and logic into separate duct runs filled no more than 50%, the fill ceiling NFPA 79 duct tables work to.

What checks prove the panel before first power?

Three checks clear it for first power: under 0.1 ohm from every drive PE terminal to the ground stud, a full-speed emergency stop on each axis with no bus fault, and inside air at or under 40 C after an hour of running.

Measure the bond with a low-reading ohmmeter, door closed and hinges bonded. The E-stop test doubles as your regen-resistor decision: any axis that faults on the stop gets the manual's resistor, the others get none. Run the thermal check with both axes cycling and an IR gun on the heatsinks and filter body; a reading over 40 C inside sends you back to the arithmetic above.

Keep the 3 ft working depth clear and let the door swing its 90 degrees. Every cable enters through a rated gland in a factory cutout - the cutout and gland guide sizes them - and custom stainless enclosures cut every opening to your drawing, delivered in 10 business days counted 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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