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Author Topic: Cable confusion, running multiple smaller cables to equal a larger ones capacity  (Read 4915 times)
Stevieboy118
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« on: February 01, 2017, 12:12:21 PM »

I'm looking around to get some cables, the inverter i've got says it runs 170A nominal but peaks at 600A.

After checking a few size/current capacity charts i need 120mm2.

But cable to carry 300A is only 40mm2, if i was to double up on the 40mm2 cable would it carry the 600A peak, even though it only has 80mm2 cross section or would it turn into cheap(?) fireworks?

Thanks

Thought i'd put a fresh thread up for this, be easy for the next person to find with the search function.
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Stig
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« Reply #1 on: February 01, 2017, 12:23:34 PM »

I guess the 2x40mm2 vs. 1x120mm2 is that the smaller cables don't get so hot as they've got proportionally more surface area.

What voltage are you running at to get 600A?!
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camillitech
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« Reply #2 on: February 01, 2017, 12:30:40 PM »

I'm looking around to get some cables, the inverter i've got says it runs 170A nominal but peaks at 600A.

After checking a few size/current capacity charts i need 120mm2.

But cable to carry 300A is only 40mm2, if i was to double up on the 40mm2 cable would it carry the 600A peak, even though it only has 80mm2 cross section or would it turn into cheap(?) fireworks?

Thanks

Thought i'd put a fresh thread up for this, be easy for the next person to find with the search function.

Dunno where you picked up that info the CSA is the important factor so, if 120mm square can carry 600A then 40mm can carry 200A. I use this for all my calcs http://www.paul-pelletier.com/LDCalculator/LJ_Calculator_Download.htm Happy to be proved wrong, I could save a fortune  Grin

Cheers, Paul
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Stig
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« Reply #3 on: February 01, 2017, 12:42:39 PM »

Yeah but no but...

CSA will directly affect resistance but there are two problems with resistance: energy loss and heating effect.  A coiled up extension lead can't safely take as much current as when it's unwound.  I'm guessing Stevieboy's charts are setting max current for safe working temperatures.
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camillitech
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« Reply #4 on: February 01, 2017, 12:53:20 PM »

Yeah but no but...

CSA will directly affect resistance but there are two problems with resistance: energy loss and heating effect.  A coiled up extension lead can't safely take as much current as when it's unwound.  I'm guessing Stevieboy's charts are setting max current for safe working temperatures.

I appreciate that Stig but generally speaking it's the CSA that's important or car manufaturers would use double starter leads instead of one thick one. Sure there will be some cooling advantage but not as to make a 33% reduction in copper surely, or everyone would be doing it.

Cheers, Paul
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'Off grid' since 1985,  Proven 2.5kW, Proven 6kW direct heating, SMA SI6.OH, 800ah Rolls, 8kW PV ,4xTS45, Lister HR2 12kW, , Powerspout pelton, Stream Engine turgo, 60 x Navitron toobs and a 1500lt store. Outback VFX3048 and 950ah forklifts for backup,
Stevieboy118
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« Reply #5 on: February 01, 2017, 01:12:39 PM »

I guess the 2x40mm2 vs. 1x120mm2 is that the smaller cables don't get so hot as they've got proportionally more surface area.

What voltage are you running at to get 600A?!

24v

Dunno where you picked up that info the CSA is the important factor so, if 120mm square can carry 600A then 40mm can carry 200A. I use this for all my calcs http://www.paul-pelletier.com/LDCalculator/LJ_Calculator_Download.htm Happy to be proved wrong, I could save a fortune  Grin

Cheers, Paul

I was assuming it was to do with the volume of copper allowing for a greater amount of current to flow without overheating, now i've engaged my brain it does make sense that two cables (even with lower cross section) will have a greater surface area for cooling.

Thanks for the calculator link Smiley

(This is part of the reason i bore my wife to tears with this stuff, i'm not telling her about it, i'm telling myself about and trying to workout where i've gone wrong!)



I'm guessing it won't be running at the full 600 amps for more than a couple of seconds when the inverter is surging to handle motors starting up.
« Last Edit: February 01, 2017, 01:14:29 PM by Stevieboy118 » Logged

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Nickel2
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« Reply #6 on: February 01, 2017, 01:25:06 PM »

I have used this calculator for all my recent stuff, it saves me having to sit down with calculator, pen and paper:

http://photovoltaic-software.com/DC_AC_drop_voltage_energy_losses_calculator.php

It is worth remembering that AC and DC conductors behave differently at different voltages and currents, so be wary of applying AC calculations to DC circuits. (Don't forget that this is for single conductor, so work it out for the out-and-back-path.)

N2
« Last Edit: February 01, 2017, 01:26:55 PM by Nickel2 » Logged

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Of course it'll work. (It hasn't caught fire yet).
Stig
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« Reply #7 on: February 01, 2017, 01:27:34 PM »

Yeah but no but...

CSA will directly affect resistance but there are two problems with resistance: energy loss and heating effect.  A coiled up extension lead can't safely take as much current as when it's unwound.  I'm guessing Stevieboy's charts are setting max current for safe working temperatures.

I appreciate that Stig but generally speaking it's the CSA that's important or car manufaturers would use double starter leads instead of one thick one. Sure there will be some cooling advantage but not as to make a 33% reduction in copper surely, or everyone would be doing it.

Cheers, Paul

I agree, 33% does seem a bit much.  A rough calculation says the increase in surface area is more like 15%.   Huh
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Tombo
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« Reply #8 on: February 01, 2017, 02:39:00 PM »

Unless you plan on flogging it to death, then I suggest you wire it for it's normal loading.  The inverter will cut out if you are pushing it too hard. This will happen long before the cable starts glowing cherry red.  sh*tfan
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camillitech
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« Reply #9 on: February 01, 2017, 03:06:10 PM »

Unless you plan on flogging it to death, then I suggest you wire it for it's normal loading.  The inverter will cut out if you are pushing it too hard. This will happen long before the cable starts glowing cherry red.  sh*tfan


+1 Stevie, this is well worth a read http://www.xantrex.com/documents/Discontinued-Products/SW2512MC-SW4024MC2UserGuide.pdf it has a very comprhensive section on cabling, pages 106/110 and is written in plain English.

Cheers, Paul
« Last Edit: February 01, 2017, 03:08:05 PM by camillitech » Logged

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Tinbum
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« Reply #10 on: February 01, 2017, 03:16:46 PM »

Not a good idea to parellel up cables. It's a bit like the ring main discussions previously. If you had a fault on one of the cables you wouldn't know.
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al barge
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« Reply #11 on: February 01, 2017, 04:42:39 PM »

Not a good idea to parellel up cables. It's a bit like the ring main discussions previously. If you had a fault on one of the cables you wouldn't know.

I was also under the impression electricity will always take the route of least resistance too, so (hypothetically) if one cable was 1cm longer and not terminated with as good a connection, the power would flow in the other cable, which could then be overloaded causing fire etc..
« Last Edit: February 01, 2017, 04:44:25 PM by al barge » Logged

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HalcyonRichard
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« Reply #12 on: February 01, 2017, 04:49:31 PM »

Problem is the continuous current and peak are not the same. Do they recommend what fuse to fit ? On a consumer unit they are sized to protect the cable not the appliances plugged in. So you could get an idea of likely cable required from the fuse fitted. And is the maximum peak only at start up ? i.e. do the recommend a slow blow fuse ?

Richard
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knighty
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« Reply #13 on: February 01, 2017, 08:24:20 PM »

how many kw is the inverter ?

work that back to amps in, over rate it a bit and then work with that ?

170amps @ 24v is 4080watts


4080watts input looks like 4kw output +2% loss


EDIT: using this calculator here: http://photovoltaic-software.com/DC_AC_drop_voltage_energy_losses_calculator.php

with 50mm csa cable, at 170amps you'd lose 55watts into 5m of cable

at 300amps 414 watts

at 1.6kw


I feel like there must be an error there but can't see where... putting 55 watts of heat out of the cable is too much for my liking  414watts is way too much and 1.6kw is crazy

(at work, the heaters inside the drains for freezers when they defrost are 40watts/meter... those heat up enough to melt ice... inside a freezer... in 20min)
« Last Edit: February 01, 2017, 08:28:36 PM by knighty » Logged
rogeriko
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« Reply #14 on: February 01, 2017, 08:42:50 PM »

The most important thing that no-one has mentioned is to keep the cables short. Arrange the batteries so that the +/- terminals are back against the wall and you will only need 50cm of cable to reach the inverter mounted immeadiately above. 50mm csa is plenty you dont need any thicker the peak loads are only for a fraction of a second just 1 or 2 cycles at most.
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