The Circuit That Tripped Every Time the Robot Started
We built a machine with a 3 kW servo system, a 1 kW heater, and 0.5 kW of control power. Total connected load: 4.5 kW. The customer wired it to a 16 A branch circuit at 220 V 3-phase. On startup, the servo motors accelerated all at once. The inrush current tripped the breaker. The issue: 4.5 kW at 220 V 3-phase is about 12 A running current, but the servo drives draw 2–3× current during acceleration (18–36 A). The 16 A breaker couldn’t hold the inrush. We upsized to a 25 A breaker (with a slow-curve D breaker for motor starting). The machine ran. The mistake was sizing the circuit for the running current, not the startup inrush.
Machine electrical supply sizing is about calculating the total load, accounting for inrush and duty cycle, and picking the right breaker, wire, and disconnect. Get it wrong, and the machine trips the circuit. This article runs the numbers.
Step 1: Sum the Connected Load
List every electrical load on the machine and its power draw.
- Servo motors: rated output (kW) divided by efficiency. A 1 kW servo (output) draws about 1.25 kW input (80% efficient). With peak torque at 2× rated, the peak input is about 2.5 kW.
- Control power (PLC, HMI, sensors): 100–300 W total.
- Pneumatics: air compressor (if on-board) — but the compressor is usually a separate supply.
- Heaters (seal bars, hot melt): resistive load, simple P = V × I.
- Lights, fans, vacuum pumps: small loads.
Example: 3 servos × 1 kW output = 3 kW output. At 80% efficiency, 3.75 kW input. Control: 0.2 kW. Heater: 1 kW. Total running: about 5 kW.
Step 2: Running Current Calculation
For 3-phase power (standard for machines over 1 kW):
I = P / (√3 × V × PF)
Where P is total power (W), V is line voltage (V), and PF is power factor (0.8–0.9 for motors, 1.0 for heaters).
For 5 kW at 220 V 3-phase, PF = 0.85: I = 5,000 / (1.732 × 220 × 0.85) = 5,000 / 324 = 15.4 A.
For single-phase (smaller machines): I = P / (V × PF) = 5,000 / (220 × 0.85) = 26.7 A. Single-phase 220 V can’t handle 5 kW — you need 3-phase.
Step 3: Inrush and Peak Current
The running current is what the machine draws in steady state. But startup (motors accelerating, heaters cold) draws more.
- Servo drives: During acceleration, the drive draws 1.5–2× the rated current. The peak lasts for the acceleration time (0.1–1 second). If all servos accelerate at once, the peak is 2× the running current.
- Heaters (cold): A cold heater draws more than a hot one (resistance changes with temperature). For a resistive heater, the cold inrush is about 1.2× the hot current. Small effect.
- Transformer inrush: A control transformer draws 5–10× rated current for a few milliseconds at startup. The breaker must ride through this.
For our example: running 15.4 A. Peak (all servos accelerating at 2×): 30.8 A. The breaker must handle this without tripping.
Step 4: Breaker Sizing
The branch breaker protects the wiring. It must carry the running current and ride through the inrush without tripping.
Breaker Types
- B curve (standard): Trips at 3–5× rated. For lighting and resistive loads.
- C curve: Trips at 5–10× rated. For general motor loads. Standard.
- D curve: Trips at 10–20× rated. For high inrush (transformers, large motors). Use for machines with big servos.
Breaker Size
Rule: breaker rating ≥ running current × 1.25 (for continuous loads). And the breaker must not trip on inrush.
For our example: running 15.4 A. × 1.25 = 19.3 A. A standard 20 A C-curve breaker would carry the running current. But the peak is 30.8 A. A C-curve (5–10× trip) at 20 A trips at 100–200 A — the 30 A peak won’t trip it. But the problem was that the servo drives’ DC bus charging at startup drew a brief high current that tripped the 16 A breaker. We moved to a 25 A D-curve breaker. It holds the inrush and trips on a real fault.
| Total Load | Running Current (220V 3φ, PF=0.85) | Recommended Breaker |
|---|---|---|
| 1 kW | 3.9 A | 10 A C-curve |
| 3 kW | 11.6 A | 16 A C-curve |
| 5 kW | 15.4 A | 25 A D-curve |
| 10 kW | 30.8 A | 40 A D-curve |
| 20 kW | 61.6 A | 80 A D-curve |
Step 5: Wire Size
The wire from the breaker to the machine must carry the current without overheating. Wire gauge depends on current, length, and voltage drop.
- Up to 16 A: 1.5 mm² (14 AWG) copper.
- 16–25 A: 2.5 mm² (12 AWG).
- 25–40 A: 4 mm² (10 AWG).
- 40–60 A: 6 mm² (8 AWG).
- 60–100 A: 10 mm² (6 AWG).
For longer runs (over 30 m), upsized the wire to limit voltage drop. The voltage drop should be under 3% of the supply voltage. A 5 V drop on 220 V is 2.3% — acceptable. A 10 V drop is 4.5% — too much; the machine sees 210 V instead of 220 V.
Step 6: Disconnect and Safety
The machine needs a disconnect (lockable switch) that cuts power for maintenance.
- Disconnect switch: A rotary switch (or a circuit breaker that locks out) on the machine’s inlet. It must be lockable (LOTO — lockout/tagout).
- Emergency stop circuit: The e-stop doesn’t cut the main power (the controls stay up for diagnostics). It cuts the motor power and drops the safety relays. The main disconnect is for maintenance.
- Grounding: The machine frame is grounded (PE conductor). The ground wire is the same size as the power wire (or one size smaller, per code).
The electrical supply workflow: 1) List all loads (kW). 2) Calculate running current I = P/(√3 V PF). 3) Estimate peak/inrush (1.5–2× running). 4) Breaker rating ≥ 1.25 × running, with D-curve for motor inrush. 5) Wire gauge matches current (with voltage drop check). 6) Add lockable disconnect. 7) Verify grounding.
Single-Phase vs. Three-Phase
Most machines over 1 kW need 3-phase. Single-phase (220 V) can handle up to about 3 kW. Beyond that, the current is too high for a single-phase circuit, and the phase balance on the grid suffers.
- Single-phase (1–3 kW): Small machines, bench-top equipment. Standard wall outlet.
- Three-phase (3–50 kW): Most production machines. Balanced load, lower current per phase.
If the customer’s plant only has single-phase but the machine needs 3-phase, add a phase converter or a VFD that creates 3-phase from single-phase. But this is a band-aid — specify 3-phase for any machine over 3 kW.
Power Factor and kVA
Motors draw apparent power (kVA), not real power (kW). The difference is the power factor (PF = kW/kVA). The breaker and wire size the apparent current (kVA), not the kW.
For our 5 kW machine at PF 0.85: kVA = 5/0.85 = 5.9 kVA. The current (calculated above) already accounts for PF. If you size the circuit for 5 kW instead of 5.9 kVA, you undersize the breaker by 15%.
Servo drives and VFDs often have a low PF (0.6–0.7) at light loads. At full load, the PF improves (0.85–0.9). Use the worst-case PF for sizing.
Transformer Sizing (If Needed)
If the plant supply is 480 V 3-phase but the machine runs at 220 V, a step-down transformer is needed. The transformer kVA must exceed the machine’s kVA draw.
For a 5.9 kVA machine, use a 7.5 kVA transformer (25% margin). The transformer’s primary (480 V) and secondary (220 V) breakers are sized separately. The transformer also has inrush (magnetizing current) — use a D-curve breaker on the primary.
An Electrical Supply Checklist
- List all loads (kW) — servos, controls, heaters, lights.
- Running current I = P/(√3 V PF) for 3-phase.
- Peak current (inrush) = 1.5–2× running.
- Breaker rating ≥ 1.25 × running. D-curve for motor inrush.
- Wire gauge matches current (check voltage drop on long runs).
- Is the supply 3-phase or single-phase? (Match the machine.)
- Is a step-down transformer needed? (kVA size with margin.)
- Lockable disconnect (LOTO) on the machine inlet?
- Grounding (PE) sized correctly?
- Does the customer’s plant supply match the requirement? (Voltage, phase, available current?)
- Is the e-stop circuit separate from the main disconnect?
The Bottom Line
Machine electrical supply sizing isn’t adding up the kW and picking a breaker. Calculate the running current (with power factor), estimate the inrush (servo acceleration, transformer charging), and pick a breaker that holds the peak without tripping on a fault. The circuit that tripped on startup wasn’t undersized for the running current — it was undersized for the servo inrush. Size the breaker 25% above running current, use a D-curve for motor loads, and upsized the wire for long runs. The machine that starts every time without tripping wasn’t lucky — it was sized for the inrush, not just the nameplate.
