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Posted by on October 24, 2009, 1:47 pm
On Oct 23, 3:54=A0pm, spamb...@milmac.com (Doug Miller) wrote:
> In article <9d65d12b-f49b-4fd4-abcd-5f30827cc...@p15g2000vbl.googlegroups=
.com>, trad...@optonline.net wrote:
> >On Oct 22, 4:10=3DA0pm, spamb...@milmac.com (Doug Miller) wrote:
> >> In article <eac71d20-b0a7-49ff-970c-7068c6290...@l35g2000vba.googlegro=
ups=3D
> >..com>, trad...@optonline.net wrote:
> >> >Aside from the 80% rule,
> >> Which doesn't apply...
> >> > you can't get 400 amps out of a 200amp service.
> >> Of course you can; it just depends on which circuits are in use. If yo=
u're
> >> using only 120V circuits, you can get 200A on *each* leg. 200A @ 240V =
is the
> >> same power as 400A @120V.
> >Yes but physically, a current of 200 amps is all that is actually
> >flowing. =A0 Put a meter on it and you will measure 200 amps, not 400.
> >It's a simple matter of Kirchoffs law.
> 200 amps on _each leg_. It's a total of (up to) 400 amps at 120V.
> >What you are arguing is like saying a resistor that has 1 amp flowing
> >in it is actually carrying 2 amps, because 1 amp goes in and one amp
> >goes out.
> It's not at all the same. You are failing to consider that the two legs o=
f a
> residential service can be treated as two *separate* parallel 120V circui=
ts.
> >> > The service consists of two hots, and a neutral. That
> >> >means with a 200 amp service you can get 200 amps at 240V flowing
> >> >between the two hots. Or you could get say 150 amps at 240V plus 50
> >> >amps at 120V. In the latter case you have 200 amps flowing on one
> >> >hot, 150 on the other hot, and 50 on the neutral.
> >> Ummm....no. You could have up to 150A at 240V plus *100A* at 120V -- a=
nd if
> >> the loads are distributed evenly across the two legs, the current in t=
he
> >> neutral is zero.
> >That is not true. =A0 If you have 150A running at 240V then you have
> >150A coming in on one hot, 150A going out on the other hot during each
> >half cycle and zero flowing through the neutral. =A0 =A0 =A0The next hal=
f
> >cycle, it reverses. =A0 =A0 If you now add another 50 amps between one h=
ot
> >and neutral, you now have 200A running through one hot, `150 amps
> >through the other hot, and 50 amps through the neutral. =A0 =A0
> Right so far...
> >There is
> >nowhere for your extra 50 amps to come from
> Sure there is: the other hot leg still has an extra 50A capacity.
Yes, you are right on that point and I was wrong.
> > to give you 250amps =A0
> Ahh, _there_ is the source of your misconception.
> The neutral carries only the unbalanced current. When the other hot leg
> carries 200A as well, the current in the neutral _drops to zero_.
> > The
> >max current flowing is still limited to 200 amps.
> Suppose that the residence has no 240V loads of any sort -- gas furnace, =
gas
> WH, gas dryer, gas stove, no large power tools, no double-pole breakers, =
every
> circuit in the panel is a 120V circuit.
> Do you maintain that the maximum power that could be drawn from this serv=
ice
> is 200A @ 120V =3D 24kVA?- Hide quoted text -
> - Show quoted text -- Hide quoted text -
> - Show quoted text -
The issue here is what defines the current at the service. In a 200
amp service there is only 200 amps of actual physical current running
through the service conductors. The conductors are sized for 200
amps, not 400 amps.
Consider this simple circuit analogy which is exactly what you would
have with a balanced load on a 240V service. It's a 240V voltage
source powering two 120ohm resistors.
____________ 240V___________
I I
I I
I I
---------120ohm---------120ohm---------
a b c
There is only 1 amp of actual current flowing in the circuit. Across
each resistor there is 120Volts and 1 amp of current flowing. So,
yes you have 1 amp flowing in EACH load, it is supporting two 1 amp
loads, but it's the same physical current flowing through each load.
The "service" is only supplying 1 amp of actual current, not 2.
That's what I meant when I said a 200 amp service cannot supply 400
amps of current.
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> wrote: