DIYbanter - View Single Post - Load capacity of 200-amp panel

Doug Miller

In article , wrote:
On Oct 24, 2:17=A0pm, (Doug Miller) wrote:
In article =
..com, wrote:

[...]
What voltage do you measure between a and b? Between c and b?

120 Volts
120 Volts

What current do you measure between a and b? Between c and b?

1 amp and it's THE SAME 1 AMP current. It just gets counted twice.
Which once again is my point.

Umm, no, actually, that's *my* point: it's counted twice. *Two* amps at 120V.

There is only 1 amp flowing in the
actual complete circuit, just like there is only a maximum of 200
physical amps flowing in a 200 amp service.

OK, so there's 120V @ 1A flowing between a and b = 120W. And there's 120V @ 1A
flowing between b and c = 120W. Total = 240W.

240W / 120V = 2A

In a house, here's how the same thing happens. I hook a 120Volt
light bulb that draws 1 amp on one hot leg and a 120volt fan that
draws one amp on the other hot leg. The 1 amp current comes in one
leg, goes through the bulb, through the fan and out the other hot
leg. That's still an actual current of only 1 amp, though it runs
through two 1 amp loads.

Wrong. Two 1 amp loads = *two* amps, not one.

If you want to get techical, since it's AC,
the current direction switches each cycle.

Kapisch?

I "kapisch" that you don't understand this.

That's what I meant when I said a 200 amp service cannot supply 400
amps of current.

But it can. 200A at 120V on each leg is a total of 400A at 120V. The two legs
of a residential electrical service are, in effect, two parallel circuits..
200A flowing in each of two parallel circuits is 400A total, not 200A.

They are NOT parallel circuits. That would imply that each has it's
own seperate return path. They do not. The return path is through
the other hot conductor for the balanced part of the load and through
the shared neutral for the unbalanced portion. Again, at any point
in time there is only 200 amps moving through that service going into
the home, which is why it's called a 200 amp service.

Suppose that each one had its own separate return. Does that change your
answer?

Consider a house with only 120V loads, no 240V circuits anywhere, and 200A
service. Suppose that one leg of the service is fully loaded, and the other
leg is unloaded. I think we'd both agree that the power being drawn is 200A at
120V, right?

Yes

Now fully load the other leg too.

And now you have 200 amps flowing from one hot and back on the other
hot. Zero flows through the neutral. Hence, again, it's only a
200 amp total current moving through the service.

So, according to your reasoning, since it's "only a 200 amp total current",
then 200A at 120V on only one leg of the service is the same as 200A at 120V
on *each* leg of the service.