Cable size for 1000W inverter (And battery q)

[Doodlebug]

Hi all,

 

I'm about to get a 1000w inverter for the boat which can peak at 2000w

 

That means it could be drawing up to 166 amps from the batteries, and I went looking for battery cable to use, because I want to add more batteries to the bank and I cant find anything near 166 amp cable.

 

So what should I use?

 

Also, when using our current inverter the batteries struggle to give the full amperage we require, so how many should I have to run the 1000w inverter?

 

Thanks

 

Doodlebug

[Grace and Favour]

OP - - Have you yet done a power audit and carefully summarised your total daily power consumption/requirements?

 

I'd suggest that's the very best place to start.......

 

 

After you've that figure - we can calculate your optimum battery bank size(s), and what replenishment systems you could have.

 

(remember, the bigger the battery bank - the greater the power generation facility you may need)

[Doodlebug]

Yes, each day we use

 

24ah lights

 

36ah laptops

 

96ah fridge

 

And then 700ah once week (for washing)

[smileypete]

16mm² for up to 2 meters between inverter and batts

 

8mm^₂ per metre for distances beyond that.

 

This'll give 0.425V drop at 1kW operation and 0.85V drop at 2kW surge, which allows a bit of wriggle room for an ammeter shunt.

 

Maybe go bigger if the inverter will get used a LOT and you're concerned about efficiency, but for occasional use it's fine.

 

Don't forget a fuse in the positive near the batt end, use the inverter makers recommended size, failing that 100A should do.

 

cheers, Pete.

~smpt~

Edited May 4, 2013 by smileypete

[Doodlebug]

We have 360w of solar panels, and a 1000w generator, but rather than run the washing from the genny I would rather run it for a few hours a week to recharge the batteries and then be able to run everything on the boat without having to make noise.

[Arthur Brown]

-As you say that your batteries struggle with the current inverter what are you planning to do to the batteries? No cable can carry current that the batteries cannot supply.

[Doodlebug]

Well I want to increase the size of the battery bank.

 

So if I bought 16mm2 cable to join the battery bank together, and to join it to the inverter, as close to the batteries as possible, how many batteries can I get away with to give the current I need?

 

I'm confusing myself, maybe I should just run it all from the generator. I'd just rather run the genny when its not going to disturb people but then be able to run things at night without problems.

 

But if I need 700ah for the washing, ill need at least 1400ah of battery power just to run the washing.

 

Thanks,

 

Doodlebug

[Justme]

If the genny can cope why not do the washing via the genny whilst charging the bats?

 

 

How many washing loads is 700ah?

 

Thats 8.4kWh.

 

So an average washer would use that in about 7 full loads on a low temp cycle.

 

Fill it with hot water & it would use less.

 

Do you really do 7 loads per wash day?

Edited May 4, 2013 by Justme

[Doodlebug]

If the genny can cope why not do the washing via the genny whilst charging the bats?

 

Well, it was because I figured in the summer, when we have more solar power than we need, we can just 'save up' the excess power to then do the washing at the weekends.

 

At the moment our batteries are 100% charged by 11am. So from 11am onwards we are wasting all that power.

[Doodlebug]

I might have messed up my maths a bit here, its 1kwh, for 2 hours, thats the maximum it could be, including heating water so I wanted to be prepared for that.

 

And no, it would be done once a week.

[smileypete]

Looks like there needs to be at least 25mm² between batts to meet the BSS:

 

"3.2.2 Are battery cables of a sufficient current-carrying capacity?

Check the size of the following cables by comparing them against a typical sample cable.
· battery to master switch;
· battery or master switch to starter solenoid;
· battery to battery;
· engine return to battery or master switch;
· battery to bow thruster motor;
· battery to anchor winch motor;
· battery to inverter system (over 1000w size).

The battery cables prescribed in the check must be approximately 25mm2."

 

So maybe go up to 25mm (or more?) in case you get a bit bigger inverter later on.

 

BTW Sounds like the existing batts may need a bit of a health check, eg with a hydrometer, or even a timed discharge test.

 

cheers, Pete.

~smpt~

Edited May 4, 2013 by smileypete

[system 4-50]

16mm² for up to 2 meters between inverter and batts

 

8mm^₂ per metre for distances beyond that.

plus?

[smileypete]

plus?

Well, by 8mm per m, I meant for 3m between batts and inverter at least 24mm² should do, for 4m use at least 32mm.

 

That said the 16mm² minimum would need to be derated to about 25mm² for high ambient temps or running in cable bundles/conduit (but still more derating needed for both at the same), so 25mm² is probably a good sensible minimum and for the OP will meet BSS when used for batt interconnects too.

 

Sooo.... our revised rough rule of thumb for 12V inverters could be 25mm² per kW for up to 3m between inverter and batts, then 8mm² per kW per metre for 4m and above, rounding up to the next standard cable size. Wonder what Victron, Mastervolt and the like say?

 

For batt interconnects, gotta remember that they aren't fuse protected, so if a spanner slips and short things 16mm² will catch fire about 2.5x more quickly than 25mm²...

 

cheers, Pete.

~smpt~

Edited May 4, 2013 by smileypete

[bizzard]

Well, for 3m between batts and inverter 25mm² should do, for 4m use at least 32mm which gives 35mm as the next standard size.

 

That said the 16mm² minimum would need to be derated to about 25mm² for high ambient temps or running in cable bundles/conduit (but still more derating needed for both at the same), so 25mm² is probably a good sensible minimum and will meet BSS when used for batt interconnects too.

 

Sooo.... our revised rough rule of thumb for 12V inverters could be 25mm² per kW for up to 3m between inverter and batts, then 8mm² per kW per metre beyond that, rounding up to the next standard cable size. Wonder what Victron, Mastervolt and the like say?

 

For batt interconnects, gotta remember that they aren't fuse protected, so if a spanner slips and short things 16mm² will catch fire about 2.5x more quickly than 25mm²...

 

cheers, Pete.

~smpt~

If a spanner falls upon pos-neg battery lugs and causes a dead short, the spanner and the battery-batteries it falls upon try to become the fuse 'direct', heating the spanner, boiling battery cells and if not removed pronto going off pop or at the least destroying battery cells, but the interconnecting cables should be left in peace and unaffected. Me being an expurt in the subtle art of mega heavy battery drop testing like.

[Doodlebug]

Woah, made a mistake, its 70ah not 700 once a week.

[bizzard]

Woah, made a mistake, its 70ah not 700 once a week.

Doodlebug you are gradually reducing your usage down and will soon be able to use nice cheap doorbell or telephone wire from the likes of Wilkinsons.

[Doodlebug]

Doodlebug you are gradually reducing your usage down and will soon be able to use nice cheap doorbell or telephone wire from the likes of Wilkinsons.

 

Yes, although the machine is rated at 1250W so in theory, if the heating element kicks in it could go beyond what i've calculated.

 

Ive found the correct wire now and bought it, its 175 amp cable.

 

Thanks for the help.

 

Am I right in saying a 4 battery bank should be suitable?

[Richard10002]

 

Ive found the correct wire now and bought it, its 175 amp cable.

 

How many mm2 is it? I have a 2500W inverter and use 95mm2 cable.

[Justme]

You dont want to size the cable by the max amps it can carry.

(in this case, some times you do)

 

You need it sized for the voltage drop.

 

To much drop & stuff stops working.

[MtB]

I'm a bit puzzled. A single leisure battery should be able to deliver 166 Amps easily.

 

Also, when using our current inverter the batteries struggle to give the full amperage we require, so how many should I have to run the 1000w inverter?

 

I'm a bit puzzled. A single leisure battery will deliver 166 Amps easily, if not for very long.

 

I think you should find out why yours doesn't before upgrading the inverter. Fixing one problem at a time is always a good strategy.

 

MtB

[Paul C]

I'm a bit puzzled. A single leisure battery will deliver 166 Amps easily, if not for very long.

 

Can you quantify "not for very long"? Less long than a washing machine cycle? Less long than 2½ minutes? etc

[smileypete]

Yes, although the machine is rated at 1250W so in theory, if the heating element kicks in it could go beyond what i've calculated.

 

Ive found the correct wire now and bought it, its 175 amp cable.

 

Thanks for the help.

 

Am I right in saying a 4 battery bank should be suitable?

 

The mm² of the cable is important, as the current rating for welding cables is often given at 60% duty cycle so looks higher than it should be.

 

For normal PVC insulated cable, 25mm² per kW is a good start. This allows some derating, eg for bundled cabling OR hot ambients, and will allow for up to 3m between inverter and batts.

 

But where double derating is needed, eg for bundled cables AND hot ambients, 35mm² per kW is a good minimum. This bigger size will also allow for up to 5m between inverter and batts.

 

The latter figure gets you close to what Mastervolt and Victron call for, Victron also suggest doubling up 2 identical cables to get higher mm², using say 2x50mm² where 100mm² is needed.

 

Would have though 4 healthy leisure batts would power a 1kW inverter no probs. But if they're 'past it' they may not last long or the inverter may start beeping very soon.

 

cheers, Pete.

~smpt~

Edited May 5, 2013 by smileypete

[Doodlebug]

Its 16mm2 and will be very short, because the inverter will be right next to the batteries.

 

Its not that the batteries cant give out the amperage, i'm sure they can, but the issue is the inverter cuts out when it gets below 10.5v.

 

The problem is that when drawing smallish loads, the voltage drops below and cuts the inverter. Then, as soon as it stops working it jumps back up to being pretty much fully charged.

Might not have pointed out that at the moment we only have two batteries.

[Paul C]

Have you measured the voltage drop across the cables eg voltage at battery terminals, voltage at 12V connections on the inverter? Don't forget there's other sources of voltage drop in the circuit too, eg isolation switch (you do have one, for the inverter, don't you...?), fuse/circuit breaker, negative bus, etc.

[Doodlebug]

Well when measured at the inverter it says 12.7v when fully charged.

 

Its about 2-3m away from the batteries, its right where the solar regulator is.

 

Could the cable be causing it?

 

What cable should I use for, say 3 metres between a 30a regulator and a 300w inverter?

 

Thanks

 

Doodlebug