Dave Martindale wrote:
"Laurie" writes:
mm wrote:

You have two 20's, but you don't have a 40. First because no one
would call two 20's on the same circuit a 40, and second because it
will trip if you go over 20 amps. The same 20 amps goes through
both breakers.

OK - thanks - I'm learning as I go, which would be why I put the
question mark to indicate my uncertainty. That would be a whole
'nother thread if I asked why use two 20 amp breakers if the same 20
amps goes through both! :-)

That's actually a pretty subtle side effect of the way that North
American single-phase circuits are wired.

On a 120 V circuit, you have "hot" and "neutral" wires. The
difference between these is that the "neutral" one is connected to
ground at some point, and will be at the same voltage as ground when
no current is flowing. The current in the "hot" wire is monitored by
the circuit breaker, and if it trips, then there should be no voltage
present in the circuit at all. So a single breaker is enough.

In Europe, electricity is supplied as 220-240V with two wires (not
counting ground), and one end of the transformer secondary is
connected to ground. So again there are "hot" and "neutral" wires to
each load, and only a single-pole circuit breaker is needed to
protect each circuit in their 240 V system.

But North America doesn't do this. We use 120 V for small appliances
and 240 V for large ones, and in order to supply both voltages the
transformer on the pole is center-tapped. There is 240 V across the
whole winding, and 120 V from either end to the centre. For reasons
that have to do with both safety and reducing the amount of copper
needed, the *centre* of the transformer is grounded, and your house is
supplied with two "hot" wires plus a neutral. Each hot is 120 V to
neutral, but there is 240 V between the two hot wires.

Your dryer may be a pure 240 V type, in which case it uses the 240 V
connection (hot to hot) only, without using neutral at all. In that
case, exactly the same current flows through both sides of your dual
breaker, and you really need only one side to protect against excess
current drawn in the load.

However, that's not the only way to get excess current - you could
have a short circuit. Since both "hot" wires are at 120 V to neutral
and ground, you can get a short circuit from either hot to
ground/neutral. If only one hot wire was protected by a breaker, you
would have no protection against shorts from the other side to
ground. You really need two circuit breakers to protect two hot
wires.

In addition, the two breakers are connected so what when one trips,
the other one shuts off as well. This is for safety - to ensure that
there is no voltage present at all when there is a tripped breaker.
Shutting off just one side would not be safe.

Dave

Thanks Dave - LOTS of good info in there that I'm sure answers my question -
but I'm going to have to get out my "idiots guide to electricity" and refer
to it as I read! ;-) Seriously though, I get the general drift of it and
really appreciate the explanation, which I am going to print out and think
on!