The switch is not sized on the generator

When to use this: specifying a standby installation, or checking a quote that seems to have the wrong size switch in it.

The answer most people expect is that a 10 kW generator needs a switch rated for 10 kW. For a whole-house switch that is wrong, and the reason is simple once stated:

A whole-house switch sits in the service conductors. In normal operation the entire house load flows through it from the utility. It therefore has to be rated for whatever the service can deliver — 200 A on a 200 A service, regardless of whether the generator behind it makes 50 A or 90 A.

The two kinds of switch

The choice between them is the first decision, not the last, because it sets both the size and most of the cost. A partial-load switch for the same generator is frequently a quarter of the price.

What stops the generator being overloaded

Not the switch. A 200 A switch will happily pass more than a 12 kW generator can make. On a whole-house installation the protection is either a load management system that sheds circuits automatically, or the discipline not to run the range, dryer and air conditioning together. Where a generator is undersized relative to the service, that is worth being explicit about rather than discovering during an outage.

Standard sizes are coarse

UL 1008 transfer switches come in far fewer sizes than circuit breakers: 30, 50, 60, 100, 150, 200, 400, 600, 800, 1000 amps. Note the jump from 200 straight to 400 — a 225 A requirement rounds up to 400 A with nothing in between.

It is worth checking your figure against those steps before assuming a slightly larger service is a small upgrade. Going from a 200 A to a 225 A service can double the switch cost.

Generator output and generator conductors

The generator rating sizes the conductors and breaker between the generator and the switch — a different calculation on the other side of the same box.

NEC 445.13 requires generator conductors rated at least 115% of the nameplate; 125% is normal practice. Check voltage drop on the run too — a generator has no utility stiffness behind it, so drop shows up as poor motor starting rather than as a dim light. Our wire size calculator handles both.

Service-rated or not

A service-rated switch contains the service disconnecting means and the neutral-to-ground bond, and replaces the main breaker position. A standard switch must sit downstream of an existing service disconnect.

They are not interchangeable. Fitting a non-service-rated switch as the service disconnect leaves the installation without a compliant disconnect; fitting a service-rated one downstream creates a second neutral-to-ground bond. Both are inspection failures and neither is visible without reading the label.

The neutral question

Whether the switch should break the neutral depends on whether the generator is a separately derived system, which depends on whether its neutral is bonded to its frame.

• Bonded neutral generator — separately derived. The switch must break the neutral, or the system ends up with two neutral-to-ground bonds and normal load current returning through the grounding conductors.
• Floating neutral generator — not separately derived. The neutral must not be switched, or the generator has no bond at all and a fault has no return path.

Portable generators are commonly bonded; standby units are commonly floating. Check yours before ordering the switch — this is not something that can be corrected afterwards without replacing hardware.

What is never acceptable

Back-feeding through a dryer or range outlet with a double-male cord. It energises the utility conductors outside the house and can kill a lineworker restoring power. It is illegal essentially everywhere, and a manual transfer switch or an interlock kit — both entirely compliant under NEC 702 — costs very little by comparison.

Sizing the generator itself is a separate question, and the surge behaviour matters more than the running load: our generator sizing calculator covers that.

How this calculator is checked

Technical reviewer

Salamot Hok

Electrician · 10+ years of installation work in Bangladesh and the wider South Asian region

He reads the result the way an installer would: are the defaults values people actually meet, does the warning fire where you would stop and think, and is the answer something you could buy and fit? The code figures themselves come from the published standards cited below, not from him — that boundary is set out on his profile.

• The maths lives in a pure function with its own test suite, asserted against worked examples from published references and standards. A calculator does not ship until those tests pass.
• 2 sources cited by name and linked, so any figure on the page can be traced back to the document it came from.
• Last reviewed 17 August 2026. Review dates are advanced only when the page is actually re-read, never to look fresh.
• Unusable input returns no answer. Where the inputs do not describe a real design, the calculator says so and withholds the number rather than printing a plausible-looking wrong one.

The full process is written up in the methodology and editorial policy. Results are engineering guidance, not a code sign-off — see the disclaimer. If a result looks wrong, tell us; corrections are answered before anything else.