doubling up on battery bank cables

[solarstuff]

Recently the forum helped me understand something-you can double up on cables to get better AWG (for example two 2/0 AWG'S act as 4/0 cables between a fuse an an inverter).

Is there any problem doing the same on the cables connecting my battery bank?

I have 4x100a AGM's connected with 1 AWG cables. I'd like to double the cables up to simulate the load carrying capacity of 1/0.

The reason is to give me help me feel better about routinely drawing 1500w loads out of the battery (via 2200w inverter connected via 4/0 cables through a 250a fuse).

Am I missing anything or is this a good idea?

 

[Ampster]

Am I missing anything

No but unless we can read your mind or assume you are talking about 12 volts? 1500 Watts at 12v is 125 Amps. How many Amps can 1/0 wire carry? If less than 250 your fuse should be changed

 

[Hedges]

Sure, doubling up can help. Problem might occur if one is open and current draw exceeds ampacity of what's left.
Your 1 awg isn't large; you can get 2/0 or 4/0, pre-assembled if you wish. Using find strand cable they're limber enough.

I bought ten ready-made 10" 4/0 cables with ring terminals and glued heat shrink. That went between 8 (6V) batteries, from battery to negative bus bar, and from battery to two fuses.
I made eight 2/0 cables to go from fuses to my inverters.

So you've already got 4/0 for the inverters?
Looks like 1 awg 90C is good for 220A in free air. 105 degree is 245, round up to 250A fuse is OK.
If short enough that IR drop isn't a problem, and temperature rating is 105 degree, shouldn't be a problem.

 

[solarstuff]

Sorry, right, 12v.

Specs say 250 is right but I always like to double check here

No but unless we can read your mind or assume you are talking about 12 volts? 1500 Watts at 12v is 125 Amps. How many Amps can 1/0 wire carry? If less than 250 your fuse should be changed

 

[Hedges]

If your batteries are in parallel, the wires between them carry less so no need for larger wires.
But you should design the interconnect scheme so all get exactly matched IR drop, charge and discharge identically.

Something like this:

 

[solarstuff]

Much thanks, Hedges!

Sure, doubling up can help. Problem might occur if one is open and current draw exceeds ampacity of what's left.
Your 1 awg isn't large; you can get 2/0 or 4/0, pre-assembled if you wish. Using find strand cable they're limber enough.

I bought ten ready-made 10" 4/0 cables with ring terminals and glued heat shrink. That went between 8 (6V) batteries, from battery to negative bus bar, and from battery to two fuses.
I made eight 2/0 cables to go from fuses to my inverters.

So you've already got 4/0 for the inverters?
Looks like 1 awg 90C is good for 220A in free air. 105 degree is 245, round up to 250A fuse is OK.
If short enough that IR drop isn't a problem, and temperature rating is 105 degree, shouldn't be a problem.

 

[Hedges]

Sorry, right, 12v.

Specs say 250 is right but I always like to double check here

View attachment 23762

What's the source of that table?
Current looks higher than what I see for 105C, not engine room:

http://assets.bluesea.com/files/resources/reference/21731.pdf

 

[John Frum]

Sorry, right, 12v.

Specs say 250 is right but I always like to double check here

View attachment 23762

Sssuming pure copper with insulation rated to 105 celcius 1 awg has an absolute max ampacity of 245 amps.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

But you need to include fuse headroom so derate by .75.
Which means the maximum non fault current you should be passing is 183.75 amps.
BTW your wires will get very hot carrying that amount of current.

 

[JoeHam]

Recently the forum helped me understand something-you can double up on cables to get better AWG (for example two 2/0 AWG'S act as 4/0 cables between a fuse an an inverter).

I don’t believe two 2/0 equals a 4/0.

I will have to look up a reference.

 

[John Frum]

I don’t believe two 2/0 equals a 4/0.

I will have to look up a reference.

2x2/0 is 140 mm square.
300 mcm is 150 mm square.
4/0 is 120 mm square.

 

[solarstuff]

I want that headroom....so 1/0 for the battery interconnect?

Sssuming pure copper with insulation rated to 105 celcius 1 awg has an absolute max ampacity of 245 amps.

Interactive Wire Size Calculator

Battery Chargers, Inverters, Solar Components, and Wiring Supplies for Boats, RVs, and Off-Grid Applications.

baymarinesupply.com

But you need to include fuse headroom so derate by .75.
Which means the maximum non fault current you should be passing is 183.75 amps.
BTW your wires will get very hot carrying that amount of current.

 

[John Frum]

I want that headroom....so 1/0 for the battery interconnect?

What is the maximum current you plan for and how hot do you want your wires to get?

 

[solarstuff]

The max current is 1500w at any one time, but the inverter can handle 2200w...and a wise man told me to spec for what could happen, not what I wanted to happen.

Is 1/0 sufficient for the batteries or should I go 2/0?

What is the maximum current you plan for and how hot do you want your wires to get?

 

[John Frum]

The max current is 1500w at any one time, but the inverter can handle 2200w...and a wise man told me to spec for what could happen, not what I wanted to happen.

Is 1/0 sufficient for the batteries or should I go 2/0?

Go with 2/0 awg and a 275 amp fuse.

 

[Hedges]

I want that headroom....so 1/0 for the battery interconnect?

Don't think you have any need. Sketch out a wiring diagram that matches physical implementation (something equivalent to that battery wiring picture I plagiarized) and we can check it out. Label which awg where.

 

[solarstuff]

Circling around to the start of this thread, can I use 2 1/0 cables in between each battery terminal to:simulate 2/0?

Go with 2/0 awg and a 275 amp fuse.

 

[Dzl]

What's the source of that table?
Current looks higher than what I see for 105C, not engine room:

http://assets.bluesea.com/files/resources/reference/21731.pdf

Yeah that table looks way off to my eye.

Some of those ratings (2/0 400A) even exceed 200*C rated wire

 

[Hedges]

2x2/0 is 140 mm square.
300 mcm is 150 mm square.
4/0 is 120 mm square.

There's this simple phrase "skin depth" or "skin effect", which leads to extremely complex electromagnetic physics and calculus. It's not just for RF, where even surface roughness is a significant factor.

Skin effect - Wikipedia

en.wikipedia.org

At 60 Hz, cables this fat don't use their full depth; current is pushed toward the surface, where it encounters more resistance than cross-sectional area would suggest, and more heating.
None of that matters for DC battery cables (except if there is a lot of ripple.) But I haven't found any good tables for DC.

At least for 60 Hz, summing ampacity of 2/0, 300 + 300 = 600A is what we would compare to 4/0, 405A
If we use 120mm^2/140mm^2 x 600 we get 514A, well over the 405A NEC table.
Part of the issue could also be less surface area of insulation for heat to escape.
(careful, the table I used looks different from what Dzl just posted.)

 

[Hedges]

Yeah that table looks way off to my eye.

Some of those ratings (2/0 400A) even exceed 200*C rated wire

I've seen some bigger numbers before. May be not continuous, like starter motor. Lots of applications are cranking, rather than feeding inverters.

 

[Hedges]

Go with 2/0 awg and a 275 amp fuse.

In the beginning he said, "inverter connected via 4/0 cables"
If accurate, that's plenty for wires carrying the full current.

" I have 4x100a AGM's connected with 1 AWG cables."
Since those are in parallel, ideally each 1 AWG carries half or 1/4 what the 4/0 does. 1 awg has ampacity > 50% of 4/0
Worst case, carries 3/4 as much. 1 awg has ampacity 70% of 2/0

So looks to me like fuses and wire gauge are fine. Just want to make sure battery wires provide symmetric draw.