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Author Topic: Cable confusion, running multiple smaller cables to equal a larger ones capacity  (Read 4328 times)
Stevieboy118
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« Reply #15 on: February 01, 2017, 09:50:35 PM »

Thanks for all the advice folks, i'm going for some 50mm2, supposedly good for 340A.

The inverter is a 3kva unit that i acquired from offthegridandy a while back, the only time it will be supporting the whole house will be overnight when it's just background loads. The rest of the time will (hopefully) handling the peaks when the vanishes for a couple hours and the panels drop off.

If i get any warming, i'll go for a fatter cable and put the 50mm into the "useful stuff" pile(s)

The batteries will be approx 1.5-2m cable length away, it's the nearest i can get the inverter to an outside wall, the batteries are going into a small insulated enclosure against the exterior wall.
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Nickel2
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« Reply #16 on: February 01, 2017, 10:08:38 PM »

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

It often puzzles me when folk ask questions then don't read the answers

I posted this link, knighty posted this link.

Mods: Are the posted links not showing on the forum?

I love helping people with tech problems, but if a link does not appear in a way that they can read is there any way of resolving the problem
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1.140kW mono south-facing at 49*
EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
EpEver STI1000-24-230 pure sine inverter
Of course it'll work. (It hasn't caught fire yet).
rogeriko
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« Reply #17 on: February 01, 2017, 10:26:39 PM »

I used your calculator to work out the losses on 1 meter of cable at 170a useful tool.
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Stevieboy118
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« Reply #18 on: February 01, 2017, 11:45:42 PM »

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

It often puzzles me when folk ask questions then don't read the answers

I posted this link, knighty posted this link.

Mods: Are the posted links not showing on the forum?

I love helping people with tech problems, but if a link does not appear in a way that they can read is there any way of resolving the problem

I was unsure if it was working right as the voltage drop didn't change regardless of the cable size I selected.
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4kW pv (16x250+good-we inverter)
Wood burner
Pile-o-tatty-batteries
107L water tank solar dump
Anything else the wife will let me play with.
Iain
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« Reply #19 on: February 01, 2017, 11:58:31 PM »

Hi

Quote
I was unsure if it was working right as the voltage drop didn't change regardless of the cable size I selected.

Same for me as well, tried various csa's and doesn't change the volt drop, when you press calculate.

Iain
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Scruff
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« Reply #20 on: February 02, 2017, 02:34:40 AM »

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.

Well you find out when the remaining cable tries to carry twice it's rated load and spontaneously sets fire.


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..

Up to a point increased heat in the leader makes it lag. It's a rubbish circuit but it works until it starts a fire. Looks pants two, compression glands don't seat properly, takes more space in conduit.

50mm˛ I'd say 200A. Battery cables should not even get warm, stop wasting eletrons.  fight
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camillitech
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« Reply #21 on: February 02, 2017, 07:19:39 AM »

Hi

Quote
I was unsure if it was working right as the voltage drop didn't change regardless of the cable size I selected.

Same for me as well, tried various csa's and doesn't change the volt drop, when you press calculate.

Iain

That's why I always use this one http://www.paul-pelletier.com/LDCalculator/LJ_Calculator_Download.htm
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http://lifeattheendoftheroad.wordpress.com/

'Off grid' since 1985,  Proven 2.5kW, Proven 6kW direct heating, SI6.OH, 800ah Rolls, 4.75kW 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,
billi
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« Reply #22 on: February 02, 2017, 07:43:32 AM »

Hi , make sure that your cable connections are done right and that the battery poles are clean  my cables got hot 120mm2 and I think it was caused by corrosion on my battery connection
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camillitech
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« Reply #23 on: February 02, 2017, 07:50:10 AM »

And buy one of these http://www.ebay.co.uk/itm/New-50-550-Temperature-Gun-Non-Contact-LCD-IR-Laser-Infrared-Thermometer-UK-/291780291532?hash=item43ef75ebcc:g:NkUAAOSwwpdW87KQ to point at your battery connections (and anything else for that matter) monthly it will save you a whole lot of grief for not much more than a tenner.
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http://lifeattheendoftheroad.wordpress.com/

'Off grid' since 1985,  Proven 2.5kW, Proven 6kW direct heating, SI6.OH, 800ah Rolls, 4.75kW 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,
jonesy
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« Reply #24 on: February 02, 2017, 10:00:18 AM »

Quote
I was unsure if it was working right as the voltage drop didn't change regardless of the cable size I selected.
Same for me as well, tried various csa's and doesn't change the volt drop, when you press calculate.
Iain
That site is not the best.  It's not clear from the boxes what to put where. For VD, you have to fill in the current field in the box to the left and press the calc button on the right.  It does work, but you have to make sure you fill in all the boxes. However, what it doesn't say is the type of cable or installation method, both of which have an affect on the volt drop and current carrying capacity. The calculator uses 0.1663 V/A/m which is a tad on the low side for domestic T+E, for example (0.017V/A/m) -well it gives an answer 2.3% low for 19A on a 22m run on 2.5mm2
I've done a couple of calcs vs the regs and the quoted volt drop is for the installation, not per core.
It's not difficult to do a VD calc. Get the data sheet and find the volt drop per amp per metre. For T+E single phase or DC, 2.5mm2 it's 0.017V/A/m. Take that figure and multiply by length and current. EG for 19A on a 22m run on 2.5mm2, it will be 0.017 x 19 x 22 = 7.11V in total for the job so about 3.55V in one core and 3.55V in the other. I find it counter intuitive that the volt drop is given on the data sheet for a pair, but as was pointed out to me once, a tad sarcastically, you need at least 2 cores to get a circuit. The VD figure is slightly lower for 3 phase.
Don't forget that the calculations assume a cable temperature of typically 70C, so a hot cable is not a bad cable; it's a working cable, or a not-very-good electric heater. It's rare to feel insulation above say 40C; just goes to show how conservative the calcs are.
I've seen paralleling of cables on projects carrying 6kV/600A.  IIRC the cable has to be the same production run and pulled in without exceeding the maximum pull force.  Sometimes you have to pull in several smaller cables as the ducts simply cant take the required size.
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Stevieboy118
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« Reply #25 on: February 02, 2017, 04:10:31 PM »

Right, once again thankyou all for your guidance and advice, apologies if i'm sometimes too dense to understand things (that calc program is a bit intimidating when it first opens!), after much hunting i've ordered a couple lengths of 4/0 AWG (107mm) cable, (hopefully) i can't go wrong using the manufacturers specification.
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4kW pv (16x250+good-we inverter)
Wood burner
Pile-o-tatty-batteries
107L water tank solar dump
Anything else the wife will let me play with.
Nickel2
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« Reply #26 on: February 02, 2017, 05:21:17 PM »

Sorry Stevieboy!  surrender I've got so used to electrickery, I forget that other folk need a hand in some areas. I'll post a screen-shot of a sample exercise, to help steer you in the right direction:



When you open the site, the first thing you see is 'AC voltage drop and energy losses calculator'.
Scroll down a bit and you get to 'DC voltage drop and energy losses calculator' which is what you need for your application.

The first column is called 'DC POWER'.
In the first yellow box, 'DC voltage (U)', I've entered your battery voltage, say about 25V.
In the yellow box just below 'DC current (Ib)', I've entered what I think the current will be, say about 120A.
Then I press the grey 'calculate' button just below and the resultant power is shown in the pale green box, 3000W.
You can change the values in the yellow boxes as you wish, then press the 'calculate' button to see the power in the pale green box.

The next column is called 'DC Voltage drop'.
The first yellow box says 'copper' which we'll leave as it is. (unless you wish to try aluminium cable.)
The second yellow box says 'wire size (mm^2)' it needs to be quite a beefy cable for 120A, so I'll click on the box and start at say, 95.
The third yellow says 'simple length (one run)'. I reckon the distance between my battery and inverter is 1 metre, so there and back = 2 metres.
Then I press the grey 'calculate' button just below and the resultant voltage drops are shown in the pale green boxes below, (0.12V or 0.48%).

The third column is called 'DC Energy losses'. I press the grey 'calculate' button, the energy losses show in the two pale green boxes as 13.95W or 0.46%.

If I change any of the parameters in any of the yellow boxes, I need to click the 'calculate' button at the bottom of each column; left, middle and right.

You may decide that you are not using the thing full-time, so don't need cable this chunky. Change some values until you find a happy medium between power-loss and copper-cost. Too skinny = voltage drop, inverter struggling, lights dip when the hair-drier goes on, over-heating and burnt shed. Too fat = perfect system, empty bank balance. In the example, the heating effect of 7w per metre will be noticeable, but harmless.

I hope this helps, please don't think me patronising, it may help someone else!  Grin


« Last Edit: February 02, 2017, 10:57:05 PM by Nickel2 » Logged

1.140kW mono south-facing at 49*
EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
EpEver STI1000-24-230 pure sine inverter
Of course it'll work. (It hasn't caught fire yet).
jonesy
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« Reply #27 on: February 03, 2017, 12:06:30 PM »


If you are 1m from your battery, you enter 1 metre, not 2.  The calcs assume a return trip.
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Nickel2
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« Reply #28 on: February 03, 2017, 05:48:37 PM »

Even better still Cheesy
I haven't done the maths to prove it either way, but I'll certainly have a close look at it when I have the time. If I was doing it professionally, I'd cross-check everything by full calculation and make no assumptions.


Edit:
Sat down and ran the calculator over it, so you're absolutely right. I had assumed that the 'simple length one run' was a single run and did not involve the return trip. As Vinnie Jones says in lock stock and two smoking layer cakes, 'Assumption is the mother of all Fake-ups'.
So for the purpose of the exercise, 50mm^2 is probably quite adequate. My tinkering system uses the stuff lying around in the workshop cupboards. One  reason I go large on conductors is to avoid any chance of unwanted oscillation or effects caused by a high impedance current source.
« Last Edit: February 03, 2017, 06:24:46 PM by Nickel2 » Logged

1.140kW mono south-facing at 49*
EpEver 4210A at 24v
24V 400 Ah battery. (4x200Ah FLA)
EpEver STI1000-24-230 pure sine inverter
Of course it'll work. (It hasn't caught fire yet).
jonesy
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« Reply #29 on: February 04, 2017, 10:53:27 AM »

Phew.  I copied my example from the C&G 2381 course I took, so I assumed that was right  Grin
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