1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Electric current in household wires

  1. Aug 3, 2014 #1
    See, we know that different electrical appliances in our house require different amount current to work .now first if some appliance is working which requires less current then less current will be passing through wires but when we use electrical appliance which uses high current then high current will be passing through wires.So, how is this current change regulated
  2. jcsd
  3. Aug 3, 2014 #2


    User Avatar
    Science Advisor

    Didn't you just answer your own question? The appliances (their resistance) determine how much current flows.
  4. Aug 3, 2014 #3
    But doesntmit depend on the sourcs, my thinking is that more the potential difference more will be current but then how it depends upon resistance
  5. Aug 3, 2014 #4


    User Avatar

    Staff: Mentor

    We regulate the voltage, not the current; the voltage is held constant throughout the power distribution system. As long as the voltage is held constant, the current takes of itself - adding a load increases the current flow required to maintain the voltage, so the generator back at the power plant has to work a bit harder to push a bit more current through the wires. So at the power plant, they just monitor the voltage; if it starts to drop below the desired line voltage they increase the power (from steam, falling water, or whatever) into the generator and if starts to rise above the desired line voltage they cut back on the power input.
    (In practice, these adjustments are completely automated and implemented by keeping the shaft of the generator turning at a constant speed even though the load on it is constantly changing).
  6. Aug 3, 2014 #5
    Current draw follows the formula I = E/R


    I = Current in Amps
    E = Volts
    R = Resistance in Ohms

    Essentially, turn on your toaster and the resistance in the heater elements determine the current draw for the circuit for a given household voltage.

    Adding additional appliances or lights on the same circuit just adds parallel resistance to the circuit.

    The formula for parallel resistance is 1/RTotal = 1/R1 + 1/R2 + ... Rn

    Simply said, if a toaster's resistance is 100 Ohms and you turn on a light that is 300 Ohms, the total circuit's resistance is now 75 Ohms.

    Plugging in the total resistance to the original formula yields I = 117 / 75 = 1.56 Amps.

    Now, as for the source, the limit on the current supplied is actually the power company and the wires to your building. Typically, that can be 100s of Amps, but you don't want that much current running through house wires, so you install breakers in the breaker box that trip when you exceed the limit for a given circuit.

    So, it is not the power company that limits the current flowing through a given circuit, but the devices themselves and their impedance (similar to resistance) and the household voltage (i.e., 117 VAC or 220 VAC).

    On last thing. For devices that use transformers like TVs, stereos, computers, etc., the correct term is impedance, not resistance. The reason is that impedance is the resistance at a specific frequency, whereas resistance is a term that is not frequency dependent. Transformers generally run at 50 Hz or 60 Hz (depending on where you live) and some do both, but their efficiency may suffer if you use the wrong frequency. I am probably getting off-track here, so I hope this helps answer your question.
  7. Aug 3, 2014 #6


    User Avatar
    Science Advisor
    Gold Member

    Ok to here, where you get it a bit wrong.
    Impedance has two components, called resistance and reactance. Resistance causes a current to flow that is in phase with the applied voltage, using the case here of 60 or 50 Hz AC power mains. Reactance causes a current that has a phase shift compared to the applied voltage.
  8. Aug 3, 2014 #7

    Absolutely correct. I was trying to avoid going into this any deeper than I needed and bringing impedance into the subject in the first place was probably just muddying the waters.
  9. Aug 3, 2014 #8


    User Avatar
    Staff Emeritus
    Science Advisor

    There's two sets of wires that you need to think about here. The first is the wiring of the household itself that runs through the walls and connects to different outlets. The other is the wiring from the appliance to the outlet. Most houses are set up so that there are multiple circuits with each circuit running through its own fuse. In turn, each circuit typically has several outlets set up in parallel.

    Each appliance only draws the current required for its own operation through the wiring leading from the outlet to the appliance. But turning on more than one appliance will draw more current through the wiring of the household circuit those outlets are connected to. This is why you can blow a fuse if you try to run too many things off of one circuit. The current flow through the circuit exceeds the fuse rating and the fuse activates, cutting off power to the circuit before it overheats and causes a fire.
  10. Aug 3, 2014 #9


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    You don't need to consider Impedance with mains electricity except when you are concerned about wasting power or over stressing the supply - in which case the term used is Power Factor (1 is perfect).
    When you use a (good) transformer to alter the mains volts to suit a low voltage load (say a 12V lamp) the effect is to present the mains supply with a much higher resistance than the lamp, so that the current into the primary is reduced and the VI is the same (higher volts and lower current). Resistance is 'transformed', too.
    Of course, many appliances (large electric motors with coils, for instance) will present a significant Reactance as well as a Resistance and affect the Power Factor - which it is possible to correct for.
  11. Aug 4, 2014 #10


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    There are two ways to look at power supplies and loads...

    1) The power supply pushes current through the load
    2) The load draws what current it needs from the supply

    Its sometimes more constructive to think one way rather than the other.

    The first approach is usually better when thinking about constant loads and variable supplies.
    The second approach is usually better when thinking about constant supplies and variable loads.

    Appliances such as washing machines have motors and heaters inside that are switched on and off by a microprocessor - so best think of them as a variable load. The mains supplies them with a more or less constant voltage and they draw what current they need.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook