Re: [Phys-l] Power

[brian whatcott <betwys1@sbcglobal.net>]

On 4/6/2012 3:53 PM, Paul Lulai wrote:
> Sorry, resending with a more appropriate subject heading.
>
> Does anyone know actual power or current levels for residential and transmission lines? I've been looking, but can't find anything reliable. Thanks for any help you can offer.
>
> I would think that the stepdown transformers would step down voltage and increase current. That is my understanding if how they work. As such, I would think that power in equals power out at least roughly).
> In my mind, that implies that the power at each voltage would be fairly constant. Each split would drop current and therefore power. But I assumed each standard house has  the same current and voltage limits (at the single phase level of use at least).
> Any thoughts on current or power ratings at these levels?
> How wrong am i?.
>
> Paul Lulai
> St Anthony Village Senior High
I notice that  nobody picked up on your query.  It's not about how much 
power the average house can consume from the electrical supply, but 
rather your desire to get some comfort level about what's going on, I 
imagine. I can suppose St Anthony Village is a US location, where most 
contributors to this list are located. That means there's a good chance 
that if you live in a house - you can see power lines stretching back to 
a post with a transformer on it. That "pole pig" might well supply seven 
or eight houses, each of which might be able to take as much as 150 
amperes from the 120-0-120 supply.
  Why so many transformers, anyway?    It would be much much more 
efficient to carry power into the houses at 1000's of volts.     Any 
desired power consumption needs so much less current, so much less 
copper, if the supply voltage is high.
And the air, and glass thimbles etc., make rather effective insulators, 
unless conditions are very wet or dirty.   Unfortunately, so high a 
voltage would be much more frequently lethal.    The business of 
domestic electrical power supply is a balancing act, to some extent.   
If a consumer is exposed to 240 volts instead of 120 volts referred to 
ground, he is more likely to get a nasty, even lethal jolt.
On the other hand, if a consumer has to deal only with 120 volts, it 
needs more current to give the desired power - so that a bad connection 
subject to higher current causes more localized heating, and perhaps 
encourages  the consequential fire.   So nations choose between 
electrocutions, and house fires, to some extent.

Power is transmitted long distances at the highest possible voltage, and 
is supplied domestically at 120-0-120 volts in the US. The intermediate 
distributions are at decreased voltage, usually several steps down, in fact.

But that idea of "transmission lines" can be misleading: if you imagine 
a transmission line of the radio/radar type, you expect the source and 
load to present a similar impedance as that of the line itself. This is 
quite different from the mains supply, which is provided at quite low 
impedance: so that if you drew 150 amps, you would not expect the 
voltage to drop by much more than 10 volts: that represents a source 
impedance of  10/150 or  about 0.07 ohms.
Imagine a crow bar dropped across the house supply: its resistance might 
be as high or higher than 0.07 ohms, so it would want to take a very 
large current. But there are fuses and breakers to make this overload 
fleeting.   If by ridiculous contrast - the supply were arranged to 
balance the impedances at and into the house, so that 150 amps might 
drop the supply voltage to half its unloaded voltage  value, a short 
circuit would take LESS power than the balanced impedance. But in this 
ridiculous case, the voltage of the supply would be bouncing high and 
low as the consumption varied.

But I have now rambled quite long enough on this topic - hope it helped.

Brian Whatcott
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l