Standard houshold 20 ampere AC receptacle to feed 12Vdc 200 ampere battery jumper /charger but how?

Jpflex

Jpflex

Electrician big leagues
Location
Victorville
Occupation
Electrician commercial and residential
Many years ago when I was a mobile mechanic working in automotive electrical systems, I wandered how I could make lighting on a house dim when jumping a 2000 Ford f450 7.4l turbo diesel with my 12 volt dc 200 ampere jump box for the trucks dc starter motor but yet not trip the household breaker for the receptacle circuit rated for only 20 amperes AC?

Hypothetically, what in the conversion process from AC to DC allows this to work?

If comparing power in both circuits (ignoring power factor, impedance, reactive qualities) you have

AC

120 volts x 20 amperes = 2400 VA on AC side of circuit

And

DC

12 volts x 200 amperes = 2400 VA on dc side


Although power whether in VA apparent or watts with power factor, I understand power is consistent in both but are not the AC household breakers rated to trip from excessive amperes current rather than power VA and WATTS?


I also understand that breakers have an inverse trip setting which would make me think the dc jumper would cause the breaker to trip thermally

I guess the equal power is the reason but I thought this was an interesting subject
 
It is kind of the same thing since the voltage applied to the circuit is known and mostly fixed, e.g. 120, 208, 240, ...

So a 120v 20A breaker is good for 2400 watts. A 20A breaker doesn't trip at 20.1 amps, it will hold much longer but diminishing as the current goes up. It would probably hold 25 amps for 10 minutes. Jumping a car is pretty quick, maybe 5 to 10 seconds. You could probably pull 30A or more out of a 20A breaker for that short period.
 
In typical ambient temperatures, a breaker and fuse is UL tested to carry 133% for up to 15 minutes.
 
I was thinking that this concept may prove that ac home breakers are not so much about amperes but about watts power or VA

With voltage varrying current limitation can be any number
 
No, strictly line current, but with the voltage drop across the breaker, it becomes power.
 
Jpflex said:
Many years ago when I was a mobile mechanic working in automotive electrical systems, I wandered how I could make lighting on a house dim when jumping a 2000 Ford f450 7.4l turbo diesel with my 12 volt dc 200 ampere jump box for the trucks dc starter motor but yet not trip the household breaker for the receptacle circuit rated for only 20 amperes AC?

Hypothetically, what in the conversion process from AC to DC allows this to work?

If comparing power in both circuits (ignoring power factor, impedance, reactive qualities) you have

AC

120 volts x 20 amperes = 2400 VA on AC side of circuit

And

DC

12 volts x 200 amperes = 2400 VA on dc side


Although power whether in VA apparent or watts with power factor, I understand power is consistent in both but are not the AC household breakers rated to trip from excessive amperes current rather than power VA and WATTS?


I also understand that breakers have an inverse trip setting which would make me think the dc jumper would cause the breaker to trip thermally

I guess the equal power is the reason but I thought this was an interesting subject
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If the primary side is 240 volts, what's the issue?
 
gadfly56 said:
If the primary side is 240 volts, what's the issue?
Click to expand...
The subject or issue is pulling 200 amperes for a few seconds from a 20 ampere breaker or even 100 or 50 amperes for an extended period of time without tripping a normal 20 ampere AC household receptacle circuit breaker
 
it’s only pulling that amperage (200) in the secondary side of the power supply. If I read everything correctly.
 
LarryFine said:
Yes. In theory, 20a at 120v is the same power as 200a at 12v.
Click to expand...


LarryFine said:
Thus, the 200a output draw "only" requires 20a at the input.
Click to expand...
And that's what I posted originally. 12v x 200 I or 120v x 20i which made me think that breakers trip from over power rather than excessive amperes exclusively.


Why not rate breakers also in watts or VA for this reason?
 
jim dungar said:
In typical ambient temperatures, a breaker and fuse is UL tested to carry 133% for up to 15 minutes.
Click to expand...
I have actually monitored a breaker draw more current than it's rating for an extended period before tripping
 
Jpflex said:
And that's what I posted originally. 12v x 200 I or 120v x 20i which made me think that breakers trip from over power rather than excessive amperes exclusively.


Why not rate breakers also in watts or VA for this reason?
Click to expand...

Because a 20A breaker will trip in one case, but not the other even though the power is the same. Think about it….considering the current that is flowing through the breaker in each case.
 
Jpflex said:
And that's what I posted originally. 12v x 200 I or 120v x 20i which made me think that breakers trip from over power rather than excessive amperes exclusively.


Why not rate breakers also in watts or VA for this reason?
Click to expand...
Because breakers are 100% current rated devices.
Their long time thermal element is based on I2R heating and their short circuit instantaneous is based on the magnetic field caused by the current flow. The voltage rating is only a function of arcing path length and insulation to ground so it is a maximum, not operating value.
 
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Jpflex said:
And that's what I posted originally. 12v x 200 I or 120v x 20i which made me think that breakers trip from over power rather than excessive amperes exclusively.

Why not rate breakers also in watts or VA for this reason?
Click to expand...
As I said, they are purely current-sensing. Voltage is only involved in arc-withstanding.
 
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