Generator Inlet Size Planner: 30A or 50A?
The inlet box on the side of your house — where the portable generator plugs in — comes in two common sizes. Check the loads you want on backup and get the right one, plus the spec to hand your electrician.
No water in an outage without this one checked — the well pump is usually the reason rural homes need backup power at all.
What the two sizes actually mean
- 30A / 240V (L14-30): 7,200 watts max, 5,760 watts continuous under the NEC 80% rule. Typically wired with 10 AWG copper. Covers a well pump, sump, fridge, furnace blower, and lights in most homes.
- 50A / 240V (SS2-50): 12,000 watts max, 9,600 watts continuous. Typically wired with 6 AWG copper. The choice when the electric water heater or big shop loads join the backup list.
The inlet, the wire, and the breaker must all match — a 50A breaker on 30A wire is a fire, not an upgrade. That's electrician territory, and the interlock kit that keeps generator power off the grid is the part that keeps lineworkers alive.
Honest limits
- This sizes the inlet, not the generator. They're related but separate: the inlet caps what can flow into the panel, the generator has to produce it. Size the generator with our generator size calculator — the two answers should agree.
- Starting surge sizes the generator, not the inlet. A 1/2 HP well pump draws 1,000W running but needs ~7,700W for two seconds to start — that surge is the generator's problem (its surge rating must cover it), not the inlet's. A breaker won't trip on a two-second inrush, so the inlet is sized on sustained running load. Corrected October 2026: an earlier version of this tool counted surge against the inlet rating, which pushed recommendations toward 50A unnecessarily.
- Wire gauge is typical, not a design. Distance, conduit fill, and local code move the numbers. Your electrician does the actual sizing; hand them the spec line from your result and let them engineer it.
Where the numbers come from
No invented numbers. Inlet ratings from the National Electrical Code; wattages from manufacturer spec sheets. Well-pump running watts are conservative max-load planning figures — typical running draw is lower (see the table in our well-pump generator guide).
| What | Figure | Source |
|---|---|---|
| Continuous-load limit | 80% of circuit rating (5,760W on 30A, 9,600W on 50A) | NFPA 70 (NEC) 210.19(A)(1) and 210.20(A): conductors and overcurrent protection sized at 125% of continuous load |
| Well pump 1/2, 3/4, 1 HP | 1,000 / 7,700 · 1,500 / 9,800 · 2,000 / 11,700 W | Franklin Electric AIM Manual, Table 13: running watts are conservative planning figures from max-load watts; starting from locked-rotor amps × 240 V |
| Sump, fridge, freezer, furnace blower, microwave | 600/1,800 · 1,200/2,400 · 500/500 · 1,200/2,000 · 1,000 W | Champion Power Equipment wattage reference chart (operator's manual) |
| Electric water heater | 4,500 W, no surge | Generac Generator Sizing Guide, Table 2: hot water 4.5 kW (resistive) |
| Typical wire gauges | 10 AWG copper for 30A; 6 AWG copper for 50A | NEC Table 310.16, 60°C column — typical only; distance, conduit fill, and local amendments change the answer |
Frequently asked questions
Can I just install a 50A inlet “to be safe”?
You can, but the 50A inlet needs 50A wire and a 50A breaker to match — it's a bigger, pricier install, and the generator still has to feed it. Right-size it; oversizing the inlet without the generator to fill it buys nothing.
What's an interlock kit?
A listed mechanical device on your panel that makes it physically impossible for the main breaker and the generator breaker to be on at the same time. It's what keeps your generator from backfeeding the grid. Required by code, installed by your electrician.
Why not just plug the generator into a dryer outlet?
That's backfeeding through a “suicide cord” (male plugs on both ends). It energizes your panel with no interlock — power flows back out to the grid, and it has killed lineworkers restoring outages. It also bypasses every protection device in the panel. Never.
Does the inlet size limit which generator I can buy?
Effectively yes: a generator bigger than the inlet can't deliver its full output to the house. Match them — or size the inlet for the generator you plan to buy, not the one you own today.
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