I mentioned a while ago about some GFCI problems I'd been having and I'm having to face them now as the kids want outdoor Christmas lights!

So;
If a GFCI is on a circuit with a 15A breaker on it, should the GFCI unit be rated the same 15A, or higher at 20A?

15A I believe but I'm not a spark.

I don't believe it makes any difference. The GFCI should be independent of the breaker rating on the GFCI.

That was my first thought, but then I thought well if the GFCI is higher than the breaker, the breaker will just trip before the GFCI gets overloaded. It has to be better to have a GFCI rated higher than the breaker, rather than the other way round, right? Or am I talking bollox?

The ones that are in say on one side "Class A GFCI 20A 125V" and on the other side say "Receptacle 15A 125V". I appear to have the same ones installed on different circuits, some of which have 15A breakers, and some have 20A breakers...

ETA - that was in reply to Hotscot

Michael - what would happen with a 15A GFCI on a 20A breaker....? That shouldn't happen, should it?

Yes, that's probably the case.
What rating are the lights? Is that not the question?

15A

Instead of replacing the outlet, you might consider replacing the breaker with a GFCI breaker instead. That way all outlets on the circuit are afforded the same protection. I also think this method makes more sense for troubleshooting. You simply check the breaker panel when an outlet isn't working, rather than trying to remember which outlet has the GFCI on it.

If you are not 'handy' and prefer not to pay to have a GFCI outlet or breaker installed, then simply add a GFCI adapter:

Defiant Plug-In GFCI Adapter (3-Wire Grounding)-30339036 at The Home Depot

RCD sensitivity is expressed as the rated residual operating current, noted IΔn. Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their IΔn value.

• High sensitivity (HS): 6 – 10 – 30 mA (for direct-contact / life injury protection)
• Medium sensitivity (MS): 100 – 300 – 500 – 1,000 mA (for fire protection)
• Low sensitivity (LS): 3 – 10 – 30 A (typically for protection of machine)

I believe consumer kitchen and bathroom GFCIs are 6 mA.

Residual-current device - Wikipedia, the free encyclopedia

There are two ratings for a GFCI breaker; the overload current rating and the leakage current rating (The leakage is the "GFCI" bit).

So if the overload current rating is, say 15A, then the breaker will trip if the current supplied through the breaker is greater than 15A for a period of time.

The leakage current rating is usually around 30 milliamps. The breaker essentially monitors the current flowing out on the live and the current flowing back on the neutral and expects them to be the same. If the difference between the two currents is greater than the leakage current rating (e.g. 30 mA) then it assumes that the 30mA is leaking to earth somewhere like out of your hairdryer, through your wet hands and is trying to kill you, so the breaker trips (even if the breaker load is less than the 15A overload current rating).

Simple!

I'm not confident enough to start messing around with the main breaker panel, so I was planning on just replacing like for like on the GFCI outlet. I've got 5 outdoor circuits with separate GFCI on them - each one has 2-3 protected outlets on it, and I've finally got a "map" of which GFCI sockets are on which GFCI circuits, so I'm ok on that one.

A GFCI receptacle protects the whole circuit since it is just looking for an imbalance and the imbalance can occur anywhere on the circuit.

So what you're saying is I could use either the 15 or the 20, and it would be OK as long as it didn't pick up leakage over the rating?

A 20 amp GFCI has a NEMA 5-20R socket, allowing both 15A and 20A rated plugs to be plugged in to it.

A 15 amp GFCI has a NEMA 5-15R socket, allowing only 15A rated plugs to be plugged in to it.

If the circuit is rated at 15A, you don't want to 'advertise' it as being rated for 20A by having a 20A rated outlet.

The GFCI is tripping on my "test" nightlight...

That makes sense because you could potentially be tripping your breaker at a lower current than expected (15A) when plugging into what you think is a 20A receptacle - but would it actually be dangerous to do that, or just inconvenient?

Dangerous, probably not. A GFCI simply compares the voltage leaving the hot wire against the voltage returning on the neutral. If they are not within tolerance then it trips. The GFCI only protects itself, not other outlets on the same circuit, unless you tap off the load terminals on the GFCI to protect downstream outlets. Your breaker is protecting the circuit against overload.

I was trying to find something in the NEC that specifically addresses 20A outlets on 15A circuits. It doesn't seem to be addressed.

'Oversizing' the outlet doesn't offer any gain, and a 20A is typically more expensive. The 20A isn't going to last longer, or offer any notable improvement.