Which wire gauge for a 250 ampere source?

Question

I have two 6V 220Ah DC golf cart batteries that I want to put in series to make a 12V 220Ah output that I will connect to a 3000W power inverter. My problem is finding the proper connecting wires for the series hookup between the batteries. This website says I would need a 250 gauge wire:

http://www.cerrowire.com/ampacity-charts

but I haven't got a clue where to buy it since, when I went to Home Depot and another local electronics store, their wires were mostly from 10 to 50 amperes which makes sense for the normal 120V AC home wiring systems, but not for my situation.

A thought I had was to use two 2 gauge wires in parallel between the two batteries, but I literally have no experience so I need help.

Answer 1

Current will be determined by the load, not the battery. If you're planning on operating something which requires 250 A continuously, you could run two 4 AWG wires to share the current.

However, notice that your inverter probably does not have massive lugs to handle "0000" (quad-aught) or thicker gauge wire. In fact, it probably uses "0" (aught) gauge, if it's similar to this Energizer EN3000 inverter.

The manual for this inverter provides a handy gauge for determining what wire to use for your battery bank:

Basically, it comes down to continuous vs intermittent operation. You can use thinner wires if you're not loading them fully or using them at high temperatures. The manual also discusses duty cycle so you can determine what inverter to use for various loads.

If your inverter supports it, and you plan on running it at 100% of capacity (3kW), then you might want to use two 4 AWG wires (per terminal) to share the current. (I used this current capacity (ampacity) chart (chassis wiring).)

If you really want to find the thickest gauge wire for your application, you'll need to visit an electrical contractor supply store.

Edit:

The wires connecting the batteries can be the same gauge as those connected to the inverter, as the current will be the same:

^{simulate this circuit – Schematic created using CircuitLab}

Edit 2:

Selecting the correct wire for current carrying capacity is based on a variety of factors: Ambient temperature, wire size, airflow (cooling), duty cycle, conductor type, insulation type, etc.

Here is an excerpt from the site I linked to for current capacity:

As you might guess, the rated ampacities [current capacities] 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.

(Emphasis mine.)

The value I selected to recommend 4 AWG is based on the Chassis Wiring (135 A), which is for wires in free air (not in a bundle). Power transmission wiring (the other values provided) assumes wiring in a bundle.

Note also that my recommendation is using two 4 AWG (2 * 135 = 270) wires if you can't obtain 0 AWG.

The temperature given in the chart is the rated temperature of the wire. Wires with higher temperature ratings may safely carry more current. The 75° you are referring to corresponds to a temperature rating of 75°C (167°F). According to your chart, which I assume to be for wiring in bundles (more conservative), a 4 AWG wire can carry 85 A up to this temperature. Wiring in home attics, for example, can reach these kinds of temperatures, which is why you would want higher temperature-rated wire.

If you were to open up the inverter and look at the wiring that the DC input connects to, you will probably find that it is not 250 MCM. Using anything heavier than what the inverter uses means the inverter itself would contain the "weak link in the chain," so to speak.

You only would need the very large gauge wire if you were operating at full power for long durations. Your inverter would probably burn out, unless you have an industrial unit designed for such things.

I hope this helps clarify a bit more.

Answer 2

As a quick test to see how hard it'd be to buy, I just did a search on amazon and ebay for "4 AWG wire", "2 AWG wire" and "0 AWG wire".

4 AWG and 0 AWG had both more than 20 choices, 2 AWG less. It is not cheap, but it seems to be straightforward to buy.

Edit:
AWG means American Wire Gauge. I am a European, so we use metric gauge or millimetres. More precisely I'm British, so I might also use BWG (originally Birmingham Wire Gauge, they made a lot of wire in Birmingham, England :-).

Essentially, as JYelton explains, you need a low-resistance cable, that won't get too hot.

If you really want 3kW, from those two 6V batteries connected in series to make a 12V battery, then calculating the power through the system:
Power = Voltage x Current
Hence Current = Power / Voltage
Current = 3000W/12V = 250A

So the cable needs to have a low resistance to get a low voltage drop, or else it will heat up a lot.

For example, if the cable had a resistance high enough to drop 1V across its length, then it will convert electricity, 250A x 1V = 250W, to heat. That will get hot in an enclosed space, so it is well worth ensuring that the cables won't get hot by over engineering it.

I assume the batteries will be close together. So the cable should be short. JYelton links a way to estimate cable size, and this is one using cable cross-sectional area vs current capacity

You could convert the area to any local wire gauge (using a web-based converter), or use callipers to check.

If the batteries are mounted side by side, you might use a solid bar of copper (even aluminium/aluminum), and drill it. You'd need to worry about corrosion at these currents.

Answer 3

Try an electrical supply house - one which caters to professional electricians. If they don't have it, they can probably get it for you.

Or, if you want an internet source, https://www.platt.com/platt-electric-supply/Electrical-Wire/search.aspx?SectionID=15&xGauge=250 was the first one that popped up on a google of "250 gauge wire".

Answer 4

The manual for my Xantrex 2000 watt inverter recommends 250 MCM cable if less than 6 feet of cable, and 350 MCM between 6 and 12 feet. You won't find this size cable at Home Depot - you will have to go to an industrial electrical supplier. You may also be able to find suitable cable at a marine supplier, like West Marine, as boaters often use large inverters.

I used #2/0 marine cable with my inverter - that is a little smaller than recommended, but I don't intend on using the full 2000 watts. #4/0 would be better, but that is still a little smaller than 250 MCM.

Answer 5

Are you sure that your batteries can handle a continuous load current of 250 amps. I don't believe golf cart batteries are rated for that level of current. Be very careful because the batteries could overheat and even explode.

Answer 6

250 A is lots of current. This table says you need AWG 0000 to support 250 A if it is a bundled power line and AWG 00 if the wires are short and far apart ("chassis wiring").

It already does not look like just a wire, and you may easier find it where plain copper is for sale. You need a copper pipe where the copper would make at least 107 mm^2 in the cross section. If it is a rod rather than a pipe, it should be about 11 mm in diameter.