"Plugged it in — breaker didn't trip, so it's fine." The 16 A breaker is indeed not tripping — but the cable between the panel and the outlet has been running at 60°C under its insulation for a year.
Quick Glossary
- Load — total power draw of all devices on one circuit, measured in W or kW
- Circuit breaker — protects the cable from overload and short circuit; sized to the cable cross-section, not to the number of devices
- Inrush current — the brief current spike when a motor starts (pump, chiller, fan); can be 3–7× the running current and may trip the breaker even when the steady-state load is normal
- Power factor (cos φ) — indicates what fraction of consumed power is converted to useful work; typically 0.7–0.85 for motors
Why "The Breaker Doesn't Trip" Does Not Mean "It's Fine"
A breaker trips when its rated current is exceeded — but not instantly. A 16 A breaker can tolerate 20 A for several minutes, 25 A for a few seconds. If a circuit runs continuously at 90–95% of its rating, the breaker never trips — but the cable is chronically overheating.
A 1.5 mm² copper cable is rated for 15–16 A in open air, but when routed inside conduit or bundled with other cables, the permissible current drops by 20–30%. A cable carrying 14 A in a bundle where the rated capacity under those conditions is 11 A is chronically overheated — with no breaker trips at all.
Easy to check: clip-on ammeter on the live conductor under full load. If actual current exceeds 80% of the breaker rating — the circuit is overloaded even if the breaker is silent.
How to Calculate Load: Step by Step
Step 1 — List all devices and their power draw. For each device: rated power from the nameplate or datasheet, not "approximately." Fixtures — actual consumption (not watt equivalent), pumps — input power, not flow rate.
Step 2 — Account for cos φ on motors. Pumps, fans, and chillers are motors with inductive loads. Current is calculated as: I (A) = P (W) / (220 V × cos φ). At cos φ = 0.8 and 500 W: I = 500 / (220 × 0.8) = 2.84 A — not 500/220 = 2.27 A as for a resistive load. The difference is small for one device, but adds up significantly across 5–6 motors.
Step 3 — Account for inrush current. If multiple motors on one circuit can start simultaneously (e.g. a pump and fan on the same timer) — calculate the combined inrush current. Inrush = 3–7× running current for 0.1–2 seconds. A Type B breaker trips at 3–5× rated current; Type C at 5–10×. Motors generally require a Type C breaker to avoid nuisance trips on startup.
Step 4 — Distribute across circuits with 20–25% headroom. Total load per circuit should not exceed 75–80% of the breaker rating and cable capacity.
Typical Load in a Grow Room: Real Numbers
Average 3×3 m grow room with intensive lighting:
- Fixtures: 2× 600 W HPS or 2× 480 W LED — 960–1200 W
- Chiller 1 kW for reservoir — 1000 W (with motor cos φ ≈ 0.8, actual current draw is higher)
- Circulation pumps 2× 50 W — 100 W
- Exhaust and intake fans 2× 150 W — 300 W
- pH/EC automation, controllers, sensors — 50–100 W
Total: approximately 2500 W active load. With cos φ 0.8 for the motor portion, actual current at the input is 13–15 A. A single 16 A circuit is already at its limit — and that is before inrush currents. Standard solution: at minimum two separate circuits — fixtures on one, refrigeration and circulation equipment on the other.
Cable Cross-Section Calculation
Cross-section is selected for current, not power directly. Reference values for copper cable in open air:
- 1.5 mm² — up to 15 A (approximately 3.3 kW at 220 V)
- 2.5 mm² — up to 21 A (approximately 4.6 kW)
- 4.0 mm² — up to 27 A (approximately 6 kW)
- 6.0 mm² — up to 34 A (approximately 7.5 kW)
When routed inside conduit, bundled, or in an enclosed cable tray — apply a derating factor of 0.7–0.8. A 2.5 mm² cable in conduit carries not 21 A but 15–17 A.
The breaker should be rated one step below the cable's permissible current — so it trips before the cable overheats, not after.
Three Mistakes That Cost the Most
Adding equipment to an existing circuit without recalculating. Added a second chiller — load increased by 30%, cable and breaker stayed the same. "Doesn't trip" — but the cable is now chronically overheated. Every new device requires recalculating the group load.
Ignoring inrush currents in automated systems. If a timer simultaneously starts a pump, fan, and chiller — combined inrush current may exceed the breaker rating and cause nuisance trips. Solution: stagger startups by 30–60 seconds using time-delay relays or controller settings.
Economising on cable cross-section during installation. The cost difference between 2.5 mm² and 4 mm² over 10 metres is negligible. The safety difference is significant. Cable is installed once and must handle maximum load with headroom for expansion.
Signs That Load Is Distributed Correctly
- Each device group is loaded to no more than 75–80% of the breaker and cable rating
- Inrush currents do not cause nuisance trips
- Actual current under load has been measured — not just calculated theoretically
- A circuit diagram listing all devices and load per group exists and is updated when changes are made