What an inlet box is for
It is a permanently mounted, weatherproof socket on the outside of the house that the generator plugs into. On the inside, it is wired to the transfer switch.
Without one, the connection between a generator and a transfer switch runs through a window or a door — which means an opening you cannot close, in the weather that caused the outage, with a live cord through it.
Its marginal cost as part of a transfer switch installation is small, because the electrician is already there and the run is already being made. The installation cost page sets out where it falls in the total.
Three specifications, and one of them is the whole decision
| Specification | What it decides |
|---|
| Amperage — 30A or 50A | Must match the transfer switch and the generator's outlet. Getting this wrong makes the whole chain unusable |
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| Connector — L14-30P or CS6375 | Determines which cord fits. These are different standards, not variants of one |
|---|
| Enclosure rating — NEMA 3R minimum | Whether it survives the weather. 3R is rain-rated; anything less does not belong outdoors |
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| Cover design | Whether it is still dry after five idle years. This is the part that actually fails |
|---|
The amperage decision is not really a decision at all — it follows from the transfer switch, which follows from the generator. All three have to be the same number.
The failure mode is water, not electricity
An inlet box does nothing for years. Then, on the worst night of the year, someone needs it to work.
What has usually gone wrong by then is not electrical. It is that the spring cover lost its tension or its gasket, water got in, and the contacts corroded — or worse, the box filled and froze.
So the cover is the part to evaluate. A spring-loaded cap that closes firmly over the inlet when unused is doing the real work, and a stiff one is a good sign rather than an annoyance.
The maintenance is trivial and almost nobody does it: open the cover twice a year, look inside, and confirm it snaps shut. Add it to the routine on the maintenance schedule page.
Where it should go
Three considerations, and they pull against each other.
Close to the panel keeps the internal cable run short, which is where the installation cost lives.
Away from windows, doors and vents, because the generator will sit near the box and its exhaust must not reach an opening. The CO safety page is the constraint that should win any argument here.
Somewhere you can reach in a storm — not behind a gate that ices shut, not down the side passage that floods, and ideally not on the windward wall.
What it does not do
An inlet box is not a transfer switch and provides no safety function on its own. It is a weatherproof connection point, nothing more.
The thing that makes backfeeding impossible — and therefore the thing that keeps a lineworker safe — is the break-before-make switching in the transfer switch or the interlock. The transfer switch page covers that side, and the interlock comparison covers the alternative.
Fitting an inlet box without one of those is not a shortcut. It is illegal everywhere in the United States and it is the specific hazard the rules exist to prevent.
The short version
- Match all three: generator outlet, cord, inlet, switch. One number throughout.
- NEMA 3R minimum. It lives outdoors and does nothing for years.
- Judge the cover, not the metal. Water is the failure mode.
- It is not a safety device. The transfer switch is.
