Welding duty cycle calculator
What the rating really means, weld-time vs cool-time at any amperage, and how to plan long jobs without thermal trips.
The definition
Duty cycle is the share of a 10-minute window a machine can weld at a stated amperage without overheating. 20% at 140 A = 2 minutes of arc, 8 minutes of cooling, repeating. Manufacturers test at rated line voltage and 104°F (40°C) ambient — hot garages and long extension cords eat into real performance.
Runtime at other amperages
Heat generated scales with current squared, so runtime scales inversely with the square:
Allowable time % = rated % × (rated amps ÷ actual amps)², capped at 100%.
| Machine rating | Weld time (min per 10) at actual amps | ||||
|---|---|---|---|---|---|
| 70 A | 90 A | 110 A | 130 A | 140 A | |
| 140 A @ 20% | 10+ (cap) | 10 (cap) | 5.2 | 3.7 | 2.0 |
| 140 A @ 40% | 10+ (cap) | 10 (cap) | 10.3 → 10 (cap) | 7.4 | 4.0 |
| 180 A @ 30% | 10 (cap) | 10 (cap) | 8.0 | 5.8 | 5.0 |
| 200 A @ 25% | 10 (cap) | 10 (cap) | 10.3 → 10 (cap) | 7.4 | 6.4 |
| 250 A @ 40% | 10 (cap) | 10 (cap) | 10 (cap) | 10 (cap) | 10.2 → 10 (cap) |
"Cap" rows mean the machine never trips at that amperage within a 10-minute window. Check your machine's plate for its actual rated amps/duty pair — the two numbers always go together, and a rating quoted without amperage is meaningless.
Weld time vs cool time in practice
The 10-minute window sounds restrictive, but arc-on time is naturally low in most work: strike, weld a bead, chip/inspect, reposition. A typical DIY pace is 30–60 seconds of arc per 3–4 minutes — a 10–20% effective duty cycle, right at spec for a hobby-class machine. Continuous runs that actually threaten the limit:
- Long joint welds (a 4-ft seam at 12 ipm is 4 minutes of solid arc — nearly a full 20% budget in one go).
- Aluminum TIG AC work, which runs hot and slow.
- High-amperage stick on thick plate, where rods burn in 2 minutes and you restrike immediately.
Plan around it: mark your machine's arc-on budget at your working amperage, break long seams into passes, and alternate weld/fit-up sections. If a thermal light comes on, wait for the reset — most inverters restore in 5–15 minutes. Forcing a hot machine back on shortens capacitor and IGBT life.
Buying takeaway
Two machines with the same max amps can differ 3–4× at your working amperage. Match the duty-cycle-at-your-amps figure to the job: structural/production work wants ≥40% at your running current; hobby and repair work is fine at 20–25%. Check the settings your material actually needs with the thickness calculator and the MIG wire speed chart, and remember that a machine tripping mid-bead also wrecks your per-foot cost math — restarts and scrap are labor you pay for twice.
Matched welders and PPE for your duty-cycle class: 200 A MIG welders on Amazon · auto-darkening helmets on Amazon · welding gloves on Amazon