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I'm working on building a Tesla battery powered off-grid system in our Airstream 22FB Sport. The idea is to be able to run the 120V AC system from the 5.2KV Tesla battery which is with the existing 12V Battery system not possible.

Here the current schematic: Schematic

I'm now very confused about the correct wire sizing for the 24V side of the system. With this calculator and the following settings:

24V, 125A, 6 Feet, 1% allowed Voltage Drop, 90°C Wire Temp

It calculated me that a 4 AWG Wire is good enough. So I went with a 4 AWG Wire from Home Depot and their Datasheet

After I installed everything I went through the spec sheet again and realized that the spec sheet says max 100A at 90°C on the 4 AWG Wire.

Therefore my questions:

  1. I assumed that we have a max amperage draw of 125A (3000W / 24V = 125A), is that assumption correct? Or should I work with higher numbers?
  2. Is the 4 AWG too thin for the application? Should I use 2 AWG?
  3. If too thin, can I just double each of the lines (as I already bought and soldered the connectors it would be much cheaper to just double them)
  4. Is maybe the Circuit Breaker Rating too high?
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  • \$\begingroup\$ If you used a 1% drop as the allowable drop in your calculations, then I would not worry so much about the fact that you uncovered a datasheet detail that disagreed with the calculated value you wanted to achieve. Reasons: (1) The NEC "ampacities" for a gauge were NOT written with your use in mind. (2) Everything is theoretical, anyway. Your actual situation will have so many uncontrolled variables in it that it's best you realize you are close to your goals and work out some ideas for validating what you have and worry about changing things if you find a problem. \$\endgroup\$
    – jonk
    Jan 28, 2019 at 1:02
  • \$\begingroup\$ Assuming that you really are targeting a 1% drop, this is \$240\:\text{mV}\cdot 125 \:\text{A}=30 \:\text{W}\$ dissipation in the wire. Arrange a worst case test (get the load up to \$125\:\text{A}\$ somehow) and just measure the actual voltage drop and plan on measuring the temperature rise in the wiring and connector systems you have. There are probably other things to test. Just make up a test plan and develop decision criteria for the results you get. Let that guide you. Your \$5.2\:\text{kW}\$ battery is capable of \$\frac{5.2\:\text{kW}}{24\:\text{V}}\approx 220\:\text{A}\$. Just a note. \$\endgroup\$
    – jonk
    Jan 28, 2019 at 1:09
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    \$\begingroup\$ I recently found out, much to my surprise, that there's a section of the US National Electrical Code (NEC) that covers RVs. It would be a really good idea to follow it, for various reasons involving liability and resale value. I have no clue, unfortunately, of what site or forum would be the place to go for specific advise on it. \$\endgroup\$
    – TimWescott
    Jan 28, 2019 at 1:31
  • \$\begingroup\$ @TimWescott -- DIY.SE normally handles building-wiring questions of this type. I've flagged it for migration there. \$\endgroup\$
    – ThreePhaseEel
    Jan 28, 2019 at 1:35

1 Answer 1

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If you appreciate power efficiency is like rising interest rates on borrowed money, choose your tolerance level vs cost of cable to get a payback period and factor the average percent peak power demand.

For example 1% of 5.2kW = 52W in 6' of wire pair is 12' or wire. That's about the same as waste heat in a 60W light bulb.

But that rarely happens, however when the ACU starts it draws up to 5x the current and if the voltage drops, it takes longer to get up to speed which is hard on the motor. Generally, a 10% voltage drop is the spec but the source impedance is much lower.

Due to motor/compressor start surge currents on DC you want a <1% voltage drop to minimize loss of motor torque during start.

The Tesla 5.2kWh battery is 24V, 233Ah, 5.2kWh with 23.1V @ 70% SoC and 24.9V @ 100% SoC. ( it is not 12V nor should label say +24,-24 rather 24+,0V )

Thus assuming a 3kW load draws 5x or 15kW surge momentarily or 15kW/24V = 625A

With 6' of wire pair or 12' total with a 1% drop in voltage 0.24V/625A=0.4mΩ

This would be slightly bigger than the size cable used for a car starter or 5/0 welding cable.

But you can get away with 4/0 Welding Cable which depending on your supply of surplus auto cable or new 15' will be much less than $10/'

Connection torque is important just like on a batttery.

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