A car battery can produce several hundred amps for starting a cold engine. At work, the battery charges for our forklifts put out over a hundred amps EACH (and we’ve got nearly 2 dozen running off the same circuit*).
However, the circuit breakers in my house top out at 60 amps (on HVAC, water heater, other high-draw circuits). Furthermore, I’ve heard that breaker for the WHOLE HOUSE is typically just 200 amps (in modern homes).
**If that’s the maximum current you can safely put through house wiring, then the wires at work, and especially in a car should be massive. But they aren’t. **
Yes, I know the voltages are MUCH lower (12v in a car, ~30v in the battery chargers), but that makes even less sense because AFAIK, circuits with low voltage and high current have worse power losses (or require a larger cable). Also, the industrial scale stuff at work does have much beefier cables and safety features, but still less than I’d expect given the current levels.
Can somebody please help me resolve this logical disparity?
actually, each charger rack (max of 2 chargers) connects (via circular outlets) to it’s own junction box with a switch. But, 1. I’m pretty sure that box is just a switch (for safety purposes), and 2. all the boxes are supplied by a single conduit running back to a larger subpanel
For one thing those chargers which can put out 100’s of amps can not do it continuously, but just for a start attempt and usually will auto switch off/lower setting after a time if not manually switched.
The current carrying capacity of a wire is determined by the cross section of the metal wire (thickness). The voltage decides the thickness of the insulation.
Twice a thick wire conductor will carry 4 times the current. While 12v to 120v wire will have 10 times the thickness of insulation (if using the same insulating material and not taking mechanical strength into consideration)
Another thing to consider is that 200 Amps is usually only CCA (that is cold cranking amps) and by definition transient. It is incapable of heating the wire up too much.
The majority of the wires in your car aren’t massive. They’re also protected by little 10-30 amp fuses.
However, go look at your car battery. From the positive side, you’ll see one massive red wire that goes to the starter. From the negative side you’ll see a massive black or bare copper wire running to the frame (or engine or body etc) and possibly one running down to the starter.
The current rating on house wiring is quite conservative. It assumes that, for example, multiple conductors carrying maximum rated current are passing through the same hole in a combustible wood stud. The goal is not to prevent the wire from fusing, but to keep the temperature well below the ignition point on materials it may be in contact with.
If the environment the wire is in is relatively well controlled, the acceptable current for a given wire gauge can be much higher. Look inside a appliance that plugs into a 20A kitchen outlet - you are going to see wire considerable smaller than the 12 AWG in the house wiring leading up to the outlet.
This. Typically from the hot terminal you’ll see one humungous wire going down to the starter solenoid, which handles The Big Current that the starter motor needs in order to spin the engine - and you’ll also see a much smaller wire that goes to the fuse panel, feeding everything else in the car. The starter needs hundreds of amps, but the “everything else” that’s fed by the fuse panel - radio, headlights, engine computer, etc. - uses relatively little current; the biggest single draws are likely to be headlights (about 3 amps each for HID headlights) and seat heaters (another 3 amps each). Add it all up, and it’s maybe a few dozen amps for the fuse panel.
Welding cable is pretty big for the huge continuous current, too.
I haven’t heard anything about it in a while but there were some stories about automotive electrical systems going to 48 volts, in part to reduce copper weight.
To expand on this a bit, ampacity charts such as this one have separate columns for “chassis wiring” vs. “power transmission”. The chassis wiring current limits are several times that of power transmission. The power transmission ratings assume conductors which may be bundled together, have fairly strict temperature limits, and long lengths. Chassis wiring will be better ventilated and have shorter lengths, and can get away with higher currents.
This article says a car starter wire is typically 4-gauge or 6-gauge. 4-gauge is 0.204 inch diameter, and that’s the diameter of the metal wire itself, not including insulation. Whereas 20-amp wiring in a home is 12-gauge, which is 0.0808 inch diameter…
But a 200A service entrance requires 2/0 AWG copper (0.364 inch diameter) or 4/0 aluminum (0.46 inch diameter). Way larger than 4 AWG. So if you were going by ampacities for residential wiring you would be expecting a much more massive wire to the starter. But because of the aforementioned conditions (low duty cycle, isolated conductor in a controlled environment, short wire length) the 4 AWG is enough.
Starter wires can be smaller then you might expect because they are very short and the duration is also short. Ampacities for home or industrial wiring are based on the current being supplied continuously and also enclosed in conduits. Conduits have a fill ratio that should not be exceeded. Free air ampacities for conductors can be higher.
As for battery chargers I am familiar with ones for electric golf carts. A 36 volt cart battery charges at about 40 volts so the current draw on the home wiring is one third of the charging current. Most charge at 20 amps so the draw on the house wiring is about 8 amps. The golf carts use 4 gauge wiring for the most part and for short periods of time can handle 600 amps.
I think the towmotors at work were 48 volts so they would charge at 55 volts or so. They are plugged into a 240 volt line so the draw on the building wires for a 100 amp charge rate is about 22 amps.
Batteries can produce tremendous currents until the pathways in the plates saturate. The research into ball lightning used a bank of submarine batteries and produced tens of thousands of amps for a few milliseconds. They used a copper rod swinging like a pendulum that barely contacted the other electrode as it passed the bottom of the swing. BLAM - fireworks.
An old golf cart I bought had two of the battery terminals melted and sunk into the tops of the batteries from bad connections.
Thank you, all. A lot of great replies with multiple complementary factors covered. I hadn’t even noticed thinner insulation on the charger cables, just that they’re a LOT heavier than I’d expect for a residential cable of that thickness, which makes sense if they’ve got more conductor and less insulation.
Nobody covered the battery chargers at work, though. These are for large batteries (weighing over 2,000 lbs each) that are the sole power source for our forklifts (and similar vehicles), there’s over 2 dozen of them running from the same panel (presumably through the same cables), and most of them are going to be in use at the same time (between shifts and during breaks).
I’m guessing the fact that the conduit they’re connected to is buried beneath the concrete floor means they can get away with thinner cables, as a couple people have explained (the concrete should make a rather effective heatsink).
Much of what you are confused about is due to the magic of transformers.
A battery charger supplying 100 amps into a load is probably doing it to a mostly discharged battery bank. This discharged bank may have a terminal voltage at around 5 to 10 volts. As the battery’s voltage climbs to x volts during charging the charger’s amp delivery will drop in proportion.
The chargers transformer is constructed to accept a high voltage with low current IN and produce low voltage with high current out.
In (very) simplified terms the power (watts) formula is Power = Volts x Amps
So simplified 100 watts going in is around 100 volts and 1 amp in current but 100 watts heading out is 5 volts and 20 amps out.
This is VERY simplified but you can see the principle of how a step down transformer can be used to produce high amperage.
Did you ever stop to wonder how a city is powered through those skinny transmission lines? After all, cities take millions of amps - wouldn’t that take wires the size of a storm sewer? No, because the voltage on those lines is extremely high - sometimes over a million volts. That reduces the current by a corresponding amount.
As an extreme example of how the environment can change the required wire thickness:
“Wire bonding” is the process used to connect the external pins to the semiconductor in devices like power transistors or LEDs. These are very thin wires: one rule of thumb is that a 25 micron diameter wire (thinner than a typical human hair) can carry about 1 amp of current safely.
By way of comparison, 14 AWG wire is considered sufficient to carry 20 amps in a household environment. This has a cross-section of 2.08 mm^2 in copper.
The 25 micron wire has a cross-section of 0.0005 mm^2. Scaled up to the same as the 14 AWG wire, it would be carrying over 4 thousand amps.
4000 amps vs. 20, for the same amount of metal. In fact it’s more than this because bond wires are made from gold, which has lower conductivity than copper. So the ratio is really more like 6000 amps vs. 20 if we stick to copper.
The reason why semiconductors can get away with this is that the bond wires are so short (~2 mm long), the voltage drop from the resistance is minimal, and that they are connected to a relatively large pin that acts as a heatsink. They can also get much hotter overall since they are not in close proximity to flammable material.
In other conditions, a wire that thin would get red-hot and break almost immediately, but in this particular situation it works fine.
Trucks (in Europe) almost all run on 24 volt systems. They typically use diesel engines with high compression, so powerful starters are required. Reliability is increased due to fewer problems with voltage drops over the longer length of the vehicle.
The downside is that 24-volt bulbs are more fragile than 12-volt, so some trucks had a voltage dropper so that they could use the more robust bulbs. This no longer matters since they all have LEDs these days - much more robust, but a lot more expensive to replace after accidental damage.
Multiple circuits can run in one conduit, so just because all of the boxed are supplied by a single conduit does not mean they are on the same circuit.
Sidetrack: when I was a kid we had a VW beetle with a 6V, positive ground electrical system. Battery died on the PA turnpike. We had to wait for another VW to happen by before we could get a jump. Fortunately VW drivers in those days where a pretty communal bunch…
No VW was ever positive ground. I used to be a VW mechanic at a dealership. The Beetles, (Type 1s), were all 6 volt up to 1966, in 1967 the Beetles came out with 12 volt electrical systems. The buses, (Type 2s), and the Square-backs, (Type 3s), all switched over to 12 volts at about the same time. Type 4s were always 12 volt systems.
IME, VW drivers are still a pretty communal bunch, especially air cooled VW drivers / owners.
I am not trying to argue here, I just do not want a reader to be misinformed.
BTW, I found that replacing the 2 gauge factory battery cables with 00 gauge ones fixed many of the air cooled 6 volt electrical issues. So much so, that now when I repair any 6 volt systems, they end up with 00 gauge battery cables right away.