According to :
I'm renovating my basement and have a pretty fundamental question about the
romex cable I'll be running throughout the space for lights and wall jacks.
I'm having my old Federal Pacific breaker panel (150 amps) replaced with a
new Cutler Hammer 200 amp box. Have hired an electrician to do that work for
me. But I wondered why I'd bother with 15 amp circuits (14-2 romex). Is
there a reason not to simply make all three of the circuits I'll be creating
for the new basement 20 amp circuits, using 12-2 romex? Or is there a
fire/shock hazard caused by using the higher amperage circuit breakers on
those standard circuits.

Also, does on GFI protect the entire circuit on which it is located or do I
need to install more? My basement, like so many, sometimes gets wet when the
outside drains are blocked.

How do u tell if u have 15a outlets and/or breakers. Can u use a
multimeter to determine amperage?

The breaker has the amperage stamped on it (usually the handle). The
breaker amps should NEVER (at least in most residential circumstances)
exceed the rating of the wire.

Wire usually has the wire size stamped on it, if it doesn't,
you can tell whether the conductors are 14ga (15A) or 12ga (20A)
simply by comparing it to a known piece of wire.

There's three kinds of outlets you might encounter:

1 The 20A kind you can't plug a 15A plug into (one blade is turned
90 degrees), and only accepts 20A plugs.
2 A different 20A kind has a "t-slot", which will accept both 20A
and 15A plugs.
3 Ordinary 15A outlets (that won't accept a "true" 20A plug). These
outlets are actually rated for 20A - you can draw a total of
20A from the receptacle (if the breaker will allow it), but no
more than 15A from either outlet. Which means you can install
these on 20A circuits, but the devices you plug into it are limited
to 15A plugs.

[In other words, all permissible to connect on a 20A circuit.]

In the US, where 20A general purpose receptacle circuits are legal,
most are wired with outlet (3) only. 20A plug devices are rare.
In those rare cases where it's likely that a true 20A device is
required, you use a T-slot receptacle (2). You won't see (1) on
general purpose circuits - they're primarily for dedicated 20A
appliances.

In Canada, until quite recently, general purpose 20A circuits were
essentially illegal, because (2) simply were never approved for sale.
The only 20A/120V circuits you see were for dedicated equipment,
usually direct-wire. As such, 120V/20A outlets (1) are extremely
rarely used. I've never seen one in residential wiring, only in
workshops and industrial situations for power tools.

I don't think I've ever seen a T-slot outlet in use in Canada, except
for a couple archeological finds that predate plugs with ground pins.
This'll change with the latest amendments to the CEC.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.