Battery Cable Size for 2000W Inverter

[sgharford]

I am laying out the installation of a remanufactured Xantrex Prosine 2000W inverter I just purchased. This little box needs some big cables going to it from the battery bank. I am going to power inverter with two Rolls “Big Red” 8D’s rated at 275AH each. Alls I am using inverter for is to a run fridge that is already cold, 110 volt lighting at night, and probably about 3 hours of TV over a 12 hour period before generator or alternators go back to work. I figured about 390AH of usage max.

So my question has 3 parts:

1) Planning on wiring inverter directly to battery disconnect switch. I will have to have larger cables made from 2 batteries to battery disconnect to handle additional 300 amp breaker needed for the inverter. So, if I got this right, the starter for this bank draws 100 amps + 1st DC panel sized for 100 amps + 2nd DC Panel sized for 100 amps + 300 amp inverter breaker = 600 amps for battery wire size.

Does anyone know where I can find a cable sizing chart for wire lengths at this amperage? Does my calculation of 600 amps look about right? If I recall correctly, believe you can discount about 20% or so for “diversity” – that is, not everything will be on at once, so probably could get away with 500 amps with no problems.

2) Does any one know where I can get cable this large? Thinking I am going to need something like 500 MCM cable from batteries to disconnect and 250 MCM from disconnect to 300 amp breaker to inverter. Would be great to know a place that not only sold wire but would crimp/solder the battery terminals and lugs as well.

3) Anyone know of a marine rated surface mount 300 amp breaker? Really like the Blue Sea breakers, but largest I could find was 150 amp.

As always – Thanks for your input, Scott.

 

[SKYCHENEY]

How about welding cable? It comes is large diameters and has finely stranded wire and this easy to bend. Just a thought.

 

[Walter P]

Welding cable is nice to work with and is a great conductor, but I don't think it's approved for use in our boats. Especially for a positive cable.

I may be wrong and I hope I am since I have a good source of it at good prices, but I am terrified of fires at sea and I don't know enough about electric. Remember the old story: "Get away from that wheelbarrow, you know you don't know anything about machinery". That's me and electric so if anyone can enlighten us, please do.....

Walt

 

[sgharford]

I was hoping for OK on welding cable too as you can actually find it online, looks reasonably priced, and heard easier to work with too. I hear you about fires - thought I had one this summer when generator overheated and spewed steam. Scared the crap out of me - just me, my wife, 2 year old, and 2 month old on anchor at time. Never want to see the real McCoy - can just imagine how quickly 500 MCM wires hooked up to two high output 8D's can start a fire..

 

[MikeP]

I was shocked at the amps/cable size you are suggesting for a 2000W inverter until I suddenly realized... your system is 12V, not 32, right?

Here's a table and calculator that might help: http://www.powerstream.com/Wire_Size.htm

 

[sgharford]

The system is 12V - I now realize why the big boats use 32V systems.

Thanks for link Mike. I noticed they had voltage drop calculator that handled the big sizes (like 500 MCM) at bottom of chart. I assume I can size wire based on length and 3% voltage drop (max 0.36 volts). So if one way length of cable from battery to disconnect switch is 10 feet long with 600 amp load, I would get 0.32 V drop with 400 MCM wire, or 0.256 V drop with 500 MCM wire. The manual for the inverter says to use 350 MCM wire for the inverter alone if the run is over 6 feet, so I just may play it safe and use 500 MCM for battery to disconnect in this case.

So where can I get 500 MCM cable made up with battery terminals and lugs? I tried googling “Custom Battery Cables” just now and everything came back max cable size of 0000.

 

[Pascal]

i have no idea what MCM is, never seen that mentioned in any of the manuals for inverters I've installed.

The answer is in you inverter manual, in AWG based on length.

Ideally, you want to keep the DC run to no more than 6 to 8'. More than that and you're going to have voltage issues.

Check your manual, i'm sure it's going to say the same thing as the Xhantrex Freedom I have or others: 6 to 8', 2000W inverter, you'll need 2/0 cable, possbily 4/0.

You need to use the cables needed for your inverter, not for what you think you will pull out of it. Just wait till you wife, girlfriend or daughter plugs in a blow dryer without telling you....

You can get away with increasing the cable run a little bit, maybe to 10, 12' if you go one size higher to 4/0.

4/0 marine cable isn't cheap but easy to find. Look it up on Ebay, there is a vendor that specializes in marine wiring supplies and i think they have far better price than WM. The fitting can be crimped with a crimping tool if you can borrow one, although i have done some excellent crimps with a big hammer and a BIG screwdriver. (making the "crimp" length wise, not across the wire/terminal)

dont' cut corners... welding cable will be a big no no if you ever get the boat surveyed, or if anything happens and your insurer looks at it.

 

[MikeP]

Scott -

One thing you might consider if it's practical, is to NOT use a single cable of the size you need. Instead, make up two smaller cables that total the needed ampacity and connect them in parallel between the appropriate connectors.

As an example, lets say you need 5 feet of 0000 to meet your needs. You could make up two 5 ft cables of 0 and connect them both between the same terminals and have essentially the same amp capability (2 amps less) as the single 0000.

It's just an option to consider, especially if the wire gauge needs to be so large that it is difficult to find/fabricate.

 

[REBrueckner]

What you are looking for is specified in the owner's manual for your inverter. You'll also need a special FUSE to be properly hooked up. If you don't have the owner's manual, download one from online. If you can't find the manual for your brand, use a manual for another brand same size....

They are very clear and very cautious about heavy cabling arrangements!

 

[sgharford]

I had never heard of MCM wire size either until I opened the user’s manual and looked at their recommended DC wire and breaker size chart. It specifies 250 MCM wire for DC length 0 to 6 feet with 300 Amp breaker. For 6 to 12 feet recommends 350 MCM wire with 300 amp breaker. I just called Xantrex to confirm. They say they oversize because of the inductance created by the initial inrush of current when you start the inverted up could give under voltage fault as a result. However, he said most people use 0000 and it works just fine.

I am sizing cable for ‘worse case’ – the Keurig coffee maker I have on board is 1500 watts alone. Would not use welders cable unless ABYC acceptable – know how these things bite you in the rear when you sell, kind of like not getting a building permit and getting away with it until you sell and have deal pending that goes south because it comes to light when you need it least.

I would use two cables as Mike suggested, but would be really ugly installation as this is from battery terminals to battery disconnect. I would have to put 2nd set of cables off screw holding existing terminal to battery post. That paints an ugly picture.

Here’s a thought, wonder if anyone makes a battery terminal post that turns one post into two post? This way I could leave existing wire and run new set for just the inverter. That would be a clean installation. Any ideas on how to get two sets of 0000 wire (believe existing DC wire is 0000) off one post cleanly?

 

[Pascal]

connecting two 4/0 terminal to a single post isn't a problem. When you look at the shape of the terminal (ancor) you cna see that the flat part is not centered with center of the cable but lines up with the side of the terminal. Not sure if that's a clear description. So you can put two of them against one another and fit them on a single post.

when you wire a large bank, you end up with multiple terminal per posts, not a problem.

One thing you may want to consider is not feeding the inverter from your main battery switch but directly from the battery bank terminal, obvioulsy with a dedicated switch for the inverter. This way, when you have a problem with the inverter (it is a Xantrex...), you can just turn off the inverter feed and not kill power to your engine, alternator and the rest of the electrical system.

It will also reduce the cable run and may allow you to mount the inverter closer to the bank where 4/0 cable will be fine.

maybe i'm a little over cautious, but i woudln't hook up an inverter to the main battery switch.

 

[sgharford]

Pascal - think I know what you are saying – talking about terminal like picture below. I like idea more of wiring separate to battery - mostly because I don't think I am going to find 500 MCM marine rated wire. Either way, I figure I could use 300 amp breaker for disconnect on inverter.

If I wire inverter directly to existing bank I really have 4 x 0000 cables coming off the second battery in parallel with the first. On the original set up, I have one wire coming from first battery post to second battery post where it then extends to disconnect switch. I would have to add same thing for inverter, this time going to 300 amp circuit breaker (which could be used as disconnect). So now I have 4 x 0000 cables coming off one battery post. So I would need a terminal like one below, but with 4 crimp ends instead of 2. Just tried googling and couldn’t find. This is tougher then I thought.

 

[sgharford]

What do you guys think about below? - think a rather elegant solution - just need to add flag teminals like one pictured above to ends of exiting wire and splice in new wire.

 

[sgharford]

Did find one potential problem with my diagram - the negative ground would not be big enough where it's tied to what I believe is engine block. I could add 2nd ground cable from inverter negative back to appropriate ground as per below.

 

[NAN-PO 41]

Using 2 smaller cables that when added together will give you the capacity of the required larger cable is a no no. If at some point in the future you get a poor connection on one of the 2 cables the remaining good cable will try to carry the load and can overheat.
Fred

 

[NAN-PO 41]

OOPS ! I forgot to add that the wire dealer on e-bay is named Genuine Dealz. I bought some wire from them last year and was very satisfied. As always no connection etc. just a customer.
Fred

 

[sgharford]

Fred - can't argue with that logic about using two wires. However (note this is to get everone off the hook), assuming the negative grounds have bullet proof connections, it looks like it should work, right? Thank for suggestion about Genuine Dealz - have used them too. That is where I got pictire of flag termminal above.

 

[Sonny]

A couple things to add here guys. Welding cable isn't 100% tinned like marine grade wire is and can therefore corrode inside its insulation as humid air moves in and out of the cable due to temperature fluctuations of well over 100F in your engine room.
The cable size is determined by amperage carried and distance from the battery x2, so measure your cable route then double it for correct sizing and don't forget the neg has to be the same size as the pos conductor. If you connect via a switch then the cable from the switch to the battery should be of sufficient size to carry the inverter load and whatever else is to be connected to the switch. Use the calculator to size the wires you use to parallel the batteries as well.
It's straightforward enough to make up your own cables. Put the terminal in a vice, upright like a cup. Heat with a torch and feed in solder til the terminal is about 1/4 full, then insert the cable slowly so you don't send molten solder flying all over the place. Don't forget to put a sleeve of heat shrink on the cable to seal the connection.

 

[sgharford]

Agreed about using welding cable - although I don't think you much choice once you go over 0000. Since current plan uses 0000 wire I will have no problem getting marine grade tinned (from Genuine Dealz like Fred suggested). Learned about doubling length when sizing wire in prior post - but thanks for the suggestion. I had to make a 0000 wire with lugs last year and splurged $50 on the Ancor crimper that you whack with a hammer in anticipation of using again one day. Definitely more comfortable with mechanically crimped connection then with solder on this one and will follow thru with marine grade shrink wrap.

Out of curiosity - If your inverter is wired to 12 volt house bank, how was it done on your boat? Kinda feel like I’m reinventing the wheel.

 

[Keith]

I have been in the wire and cable business for twenty five years and can probably offer a little insight into your problem. First of all, don't go cheap on the cable. Use only tinned copper stranding. The higher the strand count the more flexible the cable is and when you're talking about 4/0 and larger, you want it to be flexible. Use only quality crimp contacts - not the kind you can put on with a hammer! The worst thing you can do is buy expensive cable then put a poor quality crimp contact on the end. Remember, poor contact leads to high resistance which lead to high heat which ultimately could lead to FIRE. Buy or borrow a large crimping tool to install the contacts. Use adhesive shrink tubing over the barrel of the crimp which will extend about 1 - 2 inches back over the insulation. Use a quality heat gun and shrink the tubing until the adhesive oozes out so you know you'll have a water tight connection.

Look on line at cablecotech.com to find a on-line source for heavy very flexible cable.