A wire size chart that I found was incorrect or I was reading it wrong. I can see on a different chart that #6 works at 50A for quite a long distance. My inverter/batt/SCC will be very close to each other. I also get that the OCPD should be 50A.
I've watched a lot of videos and I never heard anyone explicitly say that wire size from battery to inverter is based on the AMPs that the inverter can pull from the battery so at least that is clear now. This is probably common sense to most people, but I'm one of those types that wants to read or hear it so I'm without a doubt.
So I can either meet the minimum or go over the minimum - If I had a 200 AMP BMS laying around (I don't), is there any harm in using it?
I ask about 200 as just a way to find out if there is any upper limits.
So I don't know if you are up for learning a little about the math behind the selection of components used in these power systems. But I have Covid 19 and am not going anywhere for at least 2 weeks.
There are 2 major considerations when designing these systems. The needed Power Capability, the actual Voltages, Currents, and Wattage, that can be present during operation, and the Safety margins used to design the physical characteristics of the components used for safety factors so that you don't catch fire or blow up and meet N.E.C. Standards. In most cases, N.E.C requires a 25% safety factor. These calculations can in some cases be very complex especially when it comes to AC circuits. XC/XL R True Power and power factor. But for the most part, DC equivalents can be used for designing these systems.
There are 4 basic formulas that are needed I=E/R, E=I*R, R=E/I, and P=E*I, where E is Voltage, I = Current (amps), R resistance, Resistance to electricity–that is, electrical resistance–is
a force that counteracts the flow of current. Also, Power which is the combination of Voltage, Current, and time is defined as the unit of measure that performs work.
Power is the rate at which work is done. In this case, rate means per unit of time. Power is calculated by dividing the work done by the time it took to do the work. The unit of measure for the watt is really watt/second or joule. So when calculating these values it is important to keep the Units Straight, especially time, watt/sec, watt/hours, etc. Most multi-meters only consider the values relevant to the "Second" for instance
One ampere is equal to 1 coulomb or 6.241509074×1018 electrons worth of charge moving past a point in a second.
The standard unit of electric potential is the volt (V), in honor of Count Alessandro Volta (1745 - 1827), known for his development of the electric battery. A volt is equivalent to one joule of energy per coulomb of charge (V = J / C) or Joules/coulomb electrical Pressure or potential. You have to realize that no current flows until the resistance of the circuit is reduced to a value where electrons move in response to the circuit's closure. A switch is basically defined as low resistance. I= E/R so you also have to have a load that uses or dissipates the power to do some job, I.E. does work. So the Design of the Circuits has to be designed to handle 125% of the maximum circuit amps. The Loads have to be capable of dissipating the 125% of the energy delivered so they don't overheat of course depending on what type of load it is. THE N.E.C. code deals with these minimum requirements. A simple series Circuit could be as simple as a Battery, a Switch, a Load, and connecting wires. The values in the circuit determine the selection of the components. If I want to design a 100-watt heater circuit, this is simple, I need a power source capable of delivering at least 100 watts, and a load "Resister" that can dissipate that 100 watts. I need a switch that will switch the current flow. The components that determine the Voltage, current, and watts are what is important. If my system is a higher Voltage then the current will be lower with the same watts. This is because of what is called the current squared law. P=I*I*R, also P=I*E, and E=I*R Substituting E for I*R you get the P=I*I*R.
If I have a circuit where I use a 1 Volt battery and a .01-ohm resister, the current would be 100 amperes of current and the power is 100 watts. But I could re-design it to be a 100 Volt circuit and use a 1.0-ohm resistor This would be 1.0 Amps of Current but the power dissipated is still the same 100 watts. The real advantage here is the size and cost of the wire as well as the other components such as the switch. The Cost of the battery source is also an issue because 100 Volt batteries cost more than a 1 Volt battery with a capacity of the current needed to source the load in ampere-hours or watt-hours. You can see that with higher currents the higher the cost in general. This all has to do with the physics of Mass. The real Crux of the matter is the cost of the material. It is generally all about saving money in manufacturing costs. This is why most systems are moving to higher voltages. This may or may not be possible because there are limitations at both ends and a balance. Silicon devices cost more to manufacture at higher voltages and lower voltages are typically safer.
The other thing that is not obvious to most people is that Resistance in a circuit with current flowing always exhibits an associated voltage drop. So even though the resistance on a very large conductor is very low it is not zero and as such has a voltage drop if there is electrical current flowing there are voltage drops across the wire and connections that also exhibit wattage dissipation. So Poor or bad connections can heat due to these unintended resistances. Only Super Conductors exhibit little or no resistance, and of course, the power loss follows the P=I*I*R formula called I squared R loss. This is why Wire Ampacity Tables and Wire Resistance Tables are so important to understand.
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