If I have a 4Awg copper conductor being used as a bus bar for a lithium ion battery pack, the chart at https://www.powerstream.com/Wire_Size.htm seems to imply that I can run 135 amps through that bus-bar. If I take two 4Awg wires and twist them together, is it correct to assume that bus bar can now conduct 270 amps?
Do these values continue to scale in a linear fashion?
From the chart you linked to, I'll reproduce the definition of ampacity, with my own highlights
Definition: ampacity is the current carrying capability of a wire. In other words, how many amps can it transmit? The following chart is a guideline of ampacity or copper wire current carrying capacity following the Handbook of Electronic Tables and Formulas for American Wire Gauge. As you might guess, the rated ampacities are just a rule of thumb. In careful engineering the voltage drop, insulation temperature limit, thickness, thermal conductivity, and air convection and temperature should all be taken into account. The Maximum Amps for Power Transmission uses the 700 circular mils per amp rule, which is very very conservative. The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle. For short lengths of wire, such as is used in battery packs you should trade off the resistance and load with size, weight, and flexibility. NOTE: For installations that need to conform to the National Electrical Code, you must use their guidelines. Contact your local electrician to find out what is legal!
Two parallel strands of wire will have a higher rating than a single strand. Will it be 2x the rating? It depends on what the 'rating' is based on.
If it's based on the long term temperature rise, then no. Both heat-producing wires are sharing the same thermal environment, heating the same air-space, heating the same battery faces. As the thermal heatsinking capacity has not doubled, the ampacity rating will not double.
If it's based on a very short term adiabatic (no time for heat from the wires to diffuse into the environment) pulse, then yes. Double the mass of the wires, double the heat that can be absorbed.
But as the description says, those are guidelines. It's not even clear what the single strand rating really means in your context. You are told the ratings are conservative, so it sounds like you could get away with some stretching.
'Short wires for battery packs' are specifically mentioned, in that you should do your own tradeoffs for that application. Note that short wires are effectively heat-sunk by the battery tags/faces. If you are happy for the batteries to absorb some heat from the connecting wires, then this will increase your rating of the ampacity of the wires. That will depends on your thermal budget for the cells, which will of course be heating themselves at 135/270 A.
$$ R = \frac{\rho\ l}{A} $$
Using a second cable with identical physical properties (length, cross-sectional area, conductor type & quality, etc.) theoretically doubles the ampacity. Because you double the cross-sectional area (CSA).
However, it's not always true -- as Neil has highlighted in his answer.
From the table that you linked to:
A 4-AWG cable has a CSA of ~21mm2, and its ampacity is 136A.
A 1-AWG cable has a CSA of ~42mm2, and its ampacity is 211A. The CSA is doubled but the ampacity is not.
You should also note that the cables will heat each other if twisted. And the resistance increases with temperature.
There is also the matter of different resistance, even if paired from the same spool of wire. A slightly greater resistance in one wire of the pair, not easily measured but real, will cause more than half the current to flow in the other wire, increasing heat and loss. Reducing the total amps from 2X the amps of one wire, say 90 amps drawn through paired 50 amp wires, will avoid trouble.
Pair 2 wires of any size will not double the amp capacity but at best increase 50 percent or one size stronger like 4 to 3 or 10 to 8. Example: 8 awg and bigger is high a rating and cannot Double to use for 6 awg. Normally 14 or 12 is the limit.
