300A 12VDC Battery Cable Sizing

[ee.van]

Hi Mike,

My 3000W inverter has recommended a 4/0 cable paired with a 300A class T Fuse. I would likely use welding cable for flexibility.


I always double check UL Table 28.1, which states that 4/0 copper cable is only rated for 230A.


This Chart from Blue Sea Power (doesn't go up to 300A) indicates that 2/0 Battery Cable <20ft is suitable for 200A even though the chart above indicates that it is undersized when referencing table 28.1.



I've always been taught that its amperage that affects the wire size, so my gut feeling is off on this one.

Thanks for the help and expertise

 

[ron]

Use the table from the code (which is similar to the UL table you posted). For 300A, you need 300kcmil which is good for 285A and because it is less than 800A, you can round up to the next breaker size.

 

[petersonra]

I think it is safe to follow the manufacturer's instructions. You likely have wires out in free air where they can dissipate heat better than when stuffed in wire duct or conduit.

The ampacity of conductors is largely governed by how hot the insulation and/or terminations can get without failure. The hotter the conductor insulation and terminations can go, the more ampacity you can get.

The NEC and UP508a are very conservative in this respect because they try to accommodate the absolute worse case it might see.

 

[JimInPB]

The colorful table from Blue Seas is actually the ABYC (American Boat & Yacht Council) ampacity table. It rates wire size by voltage drop. NEC rates wire size for fire safety. ABYC will most often tell you to use larger cable compared to NEC, especially for longer runs. In a 12v system, loosing 2 volts means a lot. In a 120v system, 2 volts difference hardly even gets noticed in most cases.

The ABYC table has two ratings, one for critical systems, with a very small drop (3%) & one for general use, with a 10% allowable drop. When using that table, be aware that the wire lengths listed are "full loop" lengths. If you run 10 feet of 2 conductor wire from a source to a device, that is interpreted as 20 feet of wire in the ABYC table.

There is also a formula that can be used to calculate voltage drop across a conductor for situations where you want to use lengths that are between entries in the table or outside the limits of the table. A little google work should bring up that formula if you need it.

Welding cable is leaded for lubricity. It's a little heavier than standard copper. Tinned conductor cables are recommended for corrosive environments & required by ABYC standards for use on boats.