My house has the normal 3 wires coming in from the street pole.
Two being the 220 V phase to phase, and the neutral/ground.
When they say a residence has, e.g., 150 amp service, does that mean
150 amps at 115 V, or 150 amps at 220 V ?
150 amps connected load. Draw more than 150 amps either 120 or 220 and the
main breaker will open.
I had a home with 2-3 ton a/c's, 3 swamp coolers, a spa and all of the other
electrical equipment you would expect and never had a problem with the 150
| My house has the normal 3 wires coming in from the street pole.
| Two being the 220 V phase to phase, and the neutral/ground.
| When they say a residence has, e.g., 150 amp service, does that mean
| 150 amps at 115 V, or 150 amps at 220 V ?
In the US the nominal voltages are 120 and 240. I'll use those in the
The answer is either. Both phases are 150 amps capable, which means you
can get 150 amps at 240 volts, or you can get 150 amps on both 120 volt
circuits at the same time. Or you can do any combination such that the
total current does not exceed 150 amps on either phase wire (for example
90 amps at 240 volts plus 60 amps on one 120 volt phase and 60 amps on
the other 120 volt phase).
The power company typically assumes residential users won't use the full
power level on a long term continuous basis. Thus they will often put in
a transformer actually rated less. They can handle overloads for a while,
sometimes even hours, possibly days. But if for some reason you expect to
pull the 150 amps all the time, 24x7, you need to make sure the equipment
is rated for continuous service at that level.
Thanks for reply; appreciate it.
Funny, you think you understand something, and then when you think about it
a bit more, you realize you really don't.
I am still, a bit, confused over yyour explanation, even though it
is very clear. Please bear with me, as I'm a bit out of my field here:
The 120 V circuits are, of course, Phase A to neutral, and/or Phase B to
The 220 is Phase A to Phase B.
Forgetting for a moment that the wires wouldn't take it, but as a
hypothetical only, suppose
you had 150 amps at 220 V between Phase A and Phase B.
Are you suggesting that you could "still" pull more off of either phase (to
neutral) although at 115 V ?
the 150 amp rating means that wire, connectors, busses, main breaker are all
sized to provide a peak load of 150 amps. (and probably 80% of peak load on
a continuous basis)
the object of the main breaker or fuse is to prevent excess current in the
wires up to that point. in other words from the pole to the house.
brief instantaneous peaks of current at greater then the rated value may
occur due to a time delay function depending on the type and specs of the
safety device used.
it may help to think of the 2 120V circuits as "legs" or "branches" instead
of phases because most residential service is referred to as single phase
service to distinguish it from 3 phase industrial.
the act of distributing loads between the 2 branches is called "load
if i understand your question correctly: "how much power could i get from a
150 amp circuit if the voltage 220 instead of 120?" the answer would be
voltage times current or 220 * 150 = 33,000 watts (peak) however one would
be foolish indeed to actually attempt to run continuously at maximum. around
where i am at least the electricians and power co. use the formula 220 *
150 * .8 = 26400 watts to determine safe maximum loading. even then the
panels get warm to the touch (or hot), the breakers run hot, the
connections tend to burn up.
| Thanks for reply; appreciate it.
| Funny, you think you understand something, and then when you think about it
| a bit more, you realize you really don't.
| I am still, a bit, confused over yyour explanation, even though it
| is very clear. Please bear with me, as I'm a bit out of my field here:
| The 120 V circuits are, of course, Phase A to neutral, and/or Phase B to
| The 220 is Phase A to Phase B.
| Forgetting for a moment that the wires wouldn't take it, but as a
| hypothetical only, suppose
| you had 150 amps at 220 V between Phase A and Phase B.
| Are you suggesting that you could "still" pull more off of either phase (to
| neutral) although at 115 V ?
Once you have 150 amps current flowing, that's it. If you add on a 30 amp
load on just phase A, then it will be at 180 amps and that will heat up the
wire even more, beyond what it's supposed to be (though it count take a
little while for 180 amps to trip a 150 amp breaker). You'll have 150 amps
on phase B and 30 amps on the neutral at that point.
This is assuming a normal 120/240 volt circuit. A 120/208 volt circuit
derived from 2 phases of 3 phase service will behave differently.
No, he's not.
Do you now how many amps of load you have between 'Phase A' and Neutral?
Call that X. Do you know how many amps of load you have between 'Phase B'
and Neutral? Call that Y. Do you know how many amps of load you have
between 'Phase A' and 'Phase B'? Call that Z. The amps you'll see on your
incoming 'Phase A' wire will be X+Z. Keep this less than 150A. Well, keep
it less than 120A, because of the 80% factor others have mentioned. In
fact, keep it well under 120A. The amps on your incoming 'Phase B" wire
will be Y+Z. Keep this under 150A (120A) as well.
If this doesn't seem to make sense, it may be because of ac electricity and
phase angles (+ and - signs in the case of a 120/240V system where the
phases are 180 degrees apart).
The above is simplified, but for typical residential loads, it should be
adequate in terms of the math. If you're actually going to make decisions
with safety and/or monetary impact, you will want to get more info. There
are probably Code rules for determining demand. One thing that has a huge
impact is diversity. Say you had 400A worth of equipment and receptacles
hooked up on 'Phase A' in your home, you might find that the most current
you ever draw for any sustained period (say, 15 minutes) is 80A, because not
everything is on full tilt all of the time.
Hope that helps, or at least, doesn't make it worse.
It may be easier/better for you to think of this
in watts, rather than amps. Your 150 amp service
can provide 36,000 watts (240 volts times 150 amps).
Each 120 volt half of it can provide 18,000 watts.
Exceed 18,000 watts on either half, or exceed 36,000
watts total, and the breaker trips.
In the scenario you asked about, there were already 36,000 watts
consumed (I changed your 220 to 240). You can't pull more
without exceeding 36,000.