Watts = Amps * Volts (or, more scientifically, P = I * V)
If something draws 48 Watts of power, you can either supply it with 4 Amps @ 12 Volt, or 1 Amp @ 48 Volt.
It's the amps that determine how thick the wiring needs to be, so by lowering the amperage, you lower the amount of wiring needed.
I believe the only reason cars are 12 V is because that's a practical voltage to build lead-acid batteries at (which is actually closer to 14 V in practice). Early car electric systems were only 6 V back when the only accessories were the front lights and the horn, but as more stuff was added on cars moved to 12 V.
This makes such little sense to me. I'm probably just stupid, but if the same amount of power is flowing through the wire, don't you need the same size of wire? A higher voltage system will push more energy through the same component, so you need thicker wires. If you up the resistance of the component to consume the same amount of energy as before, you arrive at the same size of wire you were using before, don't you?
What exactly is magical about higher voltages that makes them suddenly able to carry more power across the exact same wire? I know you just said it's "lower amperage" but I don't understand how amperage can be the only thing dictating the required size of a wire.
What matters is energy but what's being carried by the wire is charge. A given amount of charge can have more or less energy, which is what voltage is. So if you give each unit of charge more energy then you can carry more power by only moving the same amount of charge. You get a similar thing in mechanical systems: a shaft is sized based on the maximum amount of torque it can withstand, but the amount of power going through it also depends on the RPM. More RPM = more power, for the same amount of torque, so if you are for some reason limited in the size of the shaft, you can use a gearing mechanism to rotate it faster and then gear down the other side and get more power through a smaller shaft. It's just that in mechanical systems gears are a lot less convenient than changing voltages is in electrical systems.
(RPM and voltage are in fact related in EVs as well: generally speaking a higher voltage motor will be able to produce more power for the same amount of space and materials, but mostly by spinning faster, not producting more torque. You then need to gear it down to be practical in a car. But the trend is that things that make higher voltages and higher RPMs possible (the technology in transistors, insulation, bearings) are already cheap or getting cheaper, while things that make higher currents and torques possible (generally more raw material like copper and steel) are staying the same or getting more expensive. So EVs in general are pushing to higher voltages for lots of things)
A higher voltage system enables the same energy at a lower amperage, so constrained to wanting to do a particular thing (loud stereo, bright headlights, ...), higher voltages result in lower amperages. I'll opt to not try to explain that intuitively, hoping that somebody else chimes in.
For the other half, "why do amps determine wire size?" Amps are coulombs per second -- how much electrical stuff moves through a cross section in a second. The amount of heat produced is directly proportional to amperage (every bit of movement has a chance to whack into something and produce heat, and more movement per second or more seconds generates more heat). Wire size is ultimately constrained by being able to release that heat while still acting like a wire (not melting, not sublimating, still conducting, not setting anything else on fire, ...). The amount of heat released is proportional to surface area (proportional to the square root of diameter, but that difference doesn't matter a ton right now), and the amount of heat produced is proportional to amperage and approximately nothing else. The point where those two terms are equal is the limit for the system (engineers add in huge safety factors to account for conduit and insulation and other imperfections), above which your wire melts and below which it behaves reasonably. Higher amperages require higher surface area to release the extra heat (so much higher thicknesses), so a given amount of power (stuff you want your electrical system to do) requires lower amps or thicker wires to work appropriately, because you can achieve the same power by increasing the voltage on the same amperage, thereby solving your power problems without extra heat.
Edit: Heat released is also proportional to the temperature differential between the wire and the outside world, as well as a constant associated with how well it's thermally insulated. The wire temperature rises till either some sort of breakdown occurs or that rate of heat release balances the rate of heat production. The spirit of that original comment is correct, but this feels like a nuance that might matter to somebody new to the topic.
Imagine you've got an object you want to move by shooting water at it from a hose.
If the pressure coming from the hose is high, then you don't actually need a lot of water, and so you can use a narrow hose. On the other hand, if the pressure is low, you'll need to use a lot more water at once, so a wider hose.
It's the same thing for electrical components. If you'll excuse the mixed analogy, if you need to push an object with 480 watts of water to get it to move the speed you want it to, you can either do it with 48 volts of pressure with a hose 10 amps wide, or do it with 12 volts of pressure 40 amps wide.
Just as further confirmation, remember that long distance power delivery uses high voltage, often measured in kilovolts. Because of reduced losses that way.
The power loss by the resistance scales in the square of the current and linearly to the resistance. So doubling voltage reduces resistance power loss to one quarter so you could lower wire size and increase its resistance to double and still resistance power loss would be halved
Car electrical systems typically run at 13.8-14.4V while the alternator is charging and drops slightly after the battery is topped off. When the engine is off the battery regulates a 12v output
Nope. Consumed power is voltage*amperage, and it’s constant, so 4x more voltage means 1/4 the amps. Wire heating is I squared R, so with both I reduced and R increased the losses are just 1/4, not 1/16 of the 12V system. Because you wouldn’t keep the same wire gauges you’ve had in the 12V system, you’d reduce them to actually have some benefits from conversion.
Isn't the resistance of a wire usually modeled as a constant value?
Power dissipated in the wire as heat is proportional to the square of the current. It's equal to the voltage drop times current (P = I * V). Voltage drop is current times resistance (V = I * R). So P = I^2 * R.
48V is good because a lot of safety standards have the cut off for 'extra-low voltage' at 60V DC, gives you a bit of margin (when battery charging it pushes up to be in the 50s for example). Some battery systems I've seen at 56 volts nominal too. so sometimes people do have a bit higher-voltage ELV systems.
48V still lets you touch both conductors with dry hands and it's still very unlikely for you to be able to get any current to flow through you, you can't even feel it (although with safe working practices you'd only touch either positive or negative at one time, not both). Obviously it's very much not the case to be able to touch the 800V conductors safely in the traction systems in EVs, that side is extremely dangerous and requires extreme caution and safety procedures!
As long as the battery pack is still sealed and the interlock systems are undamaged and working correctly, it's no big deal. You follow the manufacturer's instructions to make it safe, then work on it like there's no voltage present.
If the battery pack is broken or a prototype or whatever, yeah, then you need to think things through carefully before doing them. And have a plan for what you're going to do when things go wrong.
Source: I work in electric aviation and work on battery packs.
As an analogy, if you have a hose squirting out water at a certain rate, and you increase the pressure, then all other things being equal you'll get a higher rate of water. But then that means you could now reduce the size of the hose, keeping the new higher pressure, and achieve the original rate of water delivery with a smaller, cheaper, lighter hose.
It's very similar with electricy: water pressure is voltage and the rate of water delivery is power.
The reason to not keep going higher is purely practicality because stepping down bigger voltage gaps for lower voltage applications begins to become less efficient/more waste heat.
If you design 100v computers, monitor, headlights, seat adjustment motors, window/windshield motors, pumps, etc. you could cut these 48v wires in half again but if you end up stepping everything down anyway it loses its utility.
The safety rules about voltage and the human body are often poorly overstated, as is the addage that 'its the amps that kill you, not the volts.' in reality it takes a whole lot of both [0].
As further research, here[1] is styropyro touching 2 contacts which have more current than the largest bolts of lightning (enough to almost instantly vaporize a crowbar) with his bare hands, but because it is 12V it is not able to pass through him.
You've already gotten a bunch of answers but to be honest I find all of them a little incomplete if you don't have any electrical background, so here is my attempt to be quite thorough from (almost) first principles.
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The equation for power delivered by an ideal system is P = IV, or power (P) = current (I) * voltage (V). Power is measured in watts, and that is generally the overall number that matters, in terms of what you can run off of your system at the same time.
So to increase the wattage (power) of your system you can either increase the voltage or the current.
- Increasing the voltage of a system increases the amount of resistance it can "break through". In "danger to human" terms, our skin is generally not a great conductor, so voltages lower than 50V usually won't enter the body (read: vital organs) at all. Voltages above 50V will start to enter the body depending on conditions, which is when electricity becomes much more dangerous.
- However even if the voltage is high enough to enter the body, if the current (I) isn't very high it still won't be dangerous. Current is measured in Amperes (A), and the usual number at which a current inside the body becomes dangerous is above 30mA. 30mA can cause respiratory failure if it passes through the lungs, current as low as 100mA can cause cardiac arrest if it passes through the heart. In a car's electrical system, the currents we're operating with are definitely going to be over the 30mA threshold we just established, so we want to keep the voltage under 50V instead.
Anyway back to cars and ignoring danger to humans for a second. Resistance is the main thing you want to overcome when it comes to the efficiency of such a system. The equation for power loss is P = I^2 * R, or "power dissipated (as heat) is proportional to the square of the current times the resistance". So if you increase the current of your system (in order to deliver more watts) you will also increase the amount of power you lose as heat. You can decrease the loss by decreasing the Resistance.
The equation for resistance through a material is: R = ρ * (L/A), or Resistance = resistivity (an innate property of the material) * Length / Area. In other words, the longer your wire is the more current you will lose to resistance. But if you increase the cross-sectional area of your wire (A) by making the wires thicker, you decrease the resistance.
So in short: if you have a high current (amperage) system, you use thicker wires in order to ameliorate resistive heat loss. But you can alternatively just decrease the amperage to reduce your resistive heat loss, which means thinner (and therefore much lighter) wires. But then you need to increase the voltage of your system in order to offset the power you're losing by decreasing the current. If you increase the voltage by 4x, from 12V to to 48V, this keeps it under the human danger zone (of 50-60V) and means your wires can be up to 16x thinner, taking up less space and less weight. Having it be a nice multiple of the previous system (4x) should make upgrading the relevant circuits a little more straightforward as well.
Bad analogy: for the electrons, Volts are (inverse) latency, and wire thickness is maximum bandwidth (or max amp). That is, make electrons go faster, instead of pushing more electrons at the same time.
Of course none of this really makes sense, for example you can't increase wire thickness to increase amps etc.
For your first question, power = voltage x current. To transmit the same power, if you double the voltage you only need half the current. This is important because the resistance losses in conductors are related to the current not voltage, so you can use thinner wires without overheating.
Yes but state of your body/skin (R) and current (I) is what matters with regard to 60 volts being really dangerous (also the current path as the article below states).
60 volts is nothing UNLESS you are completely wet or sweaty.
"It is estimated at 150 ohms for completely wet skin (in water), 1000 ohms for sweaty skin, and 100,000 ohms to 500,000 ohms for dry skin."
Assuming a worst-case scenario with dry skin providing a resistance of 100,000 ohms, fatality becomes a possibility if the current exceeds 50 mAmp.
Therefore, the lethal voltage would be above 0.05 (50 mAmp)×100,000=5000 Volts. [1]
So if you are wet or sweaty, it could be 7.5V to 50V that gets dangerous.
So it makes sense why 60 volts is a safety threshold, especially for those that live in Florida or Arizona.
> 60 volts is nothing UNLESS you are completely wet or sweaty.
From experience I'm not touching 60v, sweaty or dry as sand - that voltage hurts! And 48v is seriously uncomfortable. People have died from less DC voltage in industrial settings.
Following EN61010, the max safe DC voltage in laboratory equipment is 35v for wet locations. For a car, we ought to assume that being wet is a possibility.
