# The contradictions in relation between current and voltage with regards to power and impedence

What happens in a house if there is a change in voltage due to voltage fluctuations? assume voltage decreases from its normal value

case 1) V = I*Z

Z does not change unless there is a change in frequency (z=R+jwL)
so V proportional to I ....

so result is if Voltage decreases current also decreases

case 2) P = V*I

So if there is a drop in Voltage (v) ,current has to increase to compensate the loss in voltage to deliver the power the load requires

This implies Voltage decreases current increases

Is this not a contradiction? If not how is it resolved??

• Why do you assume power must remain constant? – JustJeff Jan 4 '14 at 4:39
• In nature there are no contradictions - just lack of knowledge. In generalised electronics and "electrics", where ideal systems 'do not make sense' the sense will be found in gaining a better understanding of the theory. Once this is understood much becomes clearer easier. ie when you KNOW the 'problem' must be one of your understanding and not that the text books are all wrong, then it is easier to work out the correct solution. Occasionally the text books ARE all wrong - but that's rare enough that it can be ignored in everyday life :-). – Russell McMahon Jan 4 '14 at 6:00

Depends on the type of load. If all your loads are resistive, then the current will decrease with the voltage. However, not all loads are resistive. Your computer power supply, for example, is most likely a complex switching power supply that is set up to efficiently convert a high voltage, low current supply into a low voltage, high current output. Assuming your computer requires a constant current and voltage, decreasing the input voltage to the power supply will certainly result in an increase in current draw. The impedance of the power supply is most certainly not as simple as Z=R+jwL.

It's hard to say exactly what might happen to the current draw of a whole house if the voltage droops. It depends on what loads are turned on - it could either increase or decrease depending on how much load is constant impedance and how much is constant power.

• so yeah ..Finally it all depends on the type of load .right? – Sri Jan 4 '14 at 12:58

so if there is a drop in Voltage (v) ,current has to increase to compensate the loss in voltage to deliver the power the load requires

Except that doesn't happen. Power decreases. There is no conflict.

• Doesn't this happen though in constant power loads? Imagine a PC power supply that demands 600 watts. For a 10% decrease in voltage, it will draw increased current to deliver constant power to the load. I know that this is not the case for resistive loads like light bulbs or maybe electric dryers. – HL-SDK Jan 4 '14 at 4:05
• @HL-SDK: Constant power loads are not constant resistance loads, which is what the OP brings up. – Ignacio Vazquez-Abrams Jan 4 '14 at 4:07
• I thought devices like motors consume higher current in case of low voltage conditions – Sri Jan 4 '14 at 4:19
• Motors consume more current when they stall out. – Ignacio Vazquez-Abrams Jan 4 '14 at 4:20
• Considering the number of devices or appliances that now use switch mode power supplies, TV, computer, phone, sound system, LED lights, chargers, etc. that are always on, it is a tossup for the house in total. If electric heat and fans or washing machines are on, expect a drop since their rated power is based on assuming a certain voltage. Is there a particular configuration you have in mind? – C. Towne Springer Jan 4 '14 at 5:39

There is no contradiction. There are two ideal cases, neither of which shows up in real households, although tendencies in both directions do show up at times.

Household loads are almost never linear resistors. Their impedance varies, both with line voltage and with other factors.

Although most load impedances vary, only a few vary in such a way so as to draw something approaching constant power.

There are sources of information for each individual kind of load you might encounter in a house - all separately, of course - if you need that level of detail.

In general: Heating elements, including incandescent lights, increase in resistance as the voltage increases, but not enough to make them constant-current. They just don't appear to follow Ohm's law because their resistance changes with current.

Most small-scale electronics approximate constant-current loads, but the actual power consumed is trivial.

Larger items with switching power supplies (computers, game systems, TVs, monitors, video gear, etc.) may approximate constant-power operation over as much as a 90-250v input range.

Most of the current drawn by many electronic devices is delivered to their load or wasted as heat in the attempt, and it varies widely for reasons other than line voltage. A rock band in the basement with ugly-but-efficient digital equipment can draw kilowatts on program peaks, dropping to dozens of watts during silence. A microwave oven on half-power just turns full power on and off every few seconds.

Electric motors, though, do often draw more current as their voltage is decreased. Not perfectly consuming constant power, but trending in that direction. It doesn't often affect households, but construction workers using long extension cords are familiar with the effect, where the voltage decrease comes from line losses.

Typical household loads are often dominated by a few large electric heaters that turn on and off at random, so to speak. When they're all off, the remaining load can be quite different, and very small. If the result of a 5% increase in line voltage is that twenty kilowatts of electric heat in the floors get warm enough to all turn off at once, the current will certainly go down, but it's not exactly Ohm's law.

The rare situations where a large load may increase current with decreased voltage include an electric car charger pushing a constant charging current into a large battery, a server farm (racks containing dozens of constant-power computers), or a busy wood shop with a large number of big electric motors running at once.

• So there is no contradiction overall.. just that there are different loads that behave differently to changes in voltages. – Sri Jan 4 '14 at 13:08