% loss of solar cable acceptable?

Tim Good

Tim Good

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I'm uprating my 120w panel to 240w and the existing cable twin 2.5mm. I'm using these solar wire calculators online and you select the % loss.

If I select 20ft distance, 15 amps power with 5% max lost then is says I need 10 gauge (2.5mm cable) which I already have. If I select 3% then it says 8 gauge.

Is 5% generally acceptable or should I really be looking at upping my wiring?
 
northcave said:
I'm uprating my 120w panel to 240w and the existing cable twin 2.5mm. I'm using these solar wire calculators online and you select the % loss.

If I select 20ft distance, 15 amps power with 5% max lost then is says I need 10 gauge (2.5mm cable) which I already have. If I select 3% then it says 8 gauge.

Is 5% generally acceptable or should I really be looking at upping my wiring?
Click to expand...

That does not look right for a 20ft distance.......... are you sure you should not be putting the total "there and back" wire length ie 40 ft into the calculator or is the cable run distance really really only 10 ft.

3% is the usual allowed volts drop

For a 20ft total" there and back" wire length that comes out as slightly less than 5mm²

For 40 ft total; between 9 and 10mm²
 
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VicS said:
That does not look right for a 20ft distance.......... are you sure you should not be putting the total "there and back" wire length ie 40 ft into the calculator or is the cable run distance really really only 10 ft.

3% is the usual allowed volts drop

For a 20ft total" there and back" wire length that comes out as 5mm²

For 40 ft total; 10mm²
Click to expand...

Most of the calculators seem to consider only the distance from panel to battery. So this one:

http://www.freesunpower.com/wire_calc.php

Says: This is the distance in feet from your solar panels to the charge controller and battery bank location. Even though you will actually be running 2 wires, one negative & one positive, do NOT double the distance. The Wire Size Calculator assumes this and does it for you in the calculation.
 
northcave said:
Most of the calculators seem to consider only the distance from panel to battery. So this one:

http://www.freesunpower.com/wire_calc.php

Says: This is the distance in feet from your solar panels to the charge controller and battery bank location. Even though you will actually be running 2 wires, one negative & one positive, do NOT double the distance. The Wire Size Calculator assumes this and does it for you in the calculation.
Click to expand...

Thats OK

Some calculators use cable run distance and some use double. They don't all say which!

By calculation from a figure of 2.2ohms/1000m for 8AWG I make that a 3.3% volts drop at 15amps. for 6m cable run distance
 
Don't forget that calculated % is at full power.
In lower light (or non-optimum panel angle etc), the loss will be a lower %.
Also will the few % of full power you save really be of use, or will it most often be dumped by the regulator?
will the batteries use it?
Will the fridge use it?
etc!

You have to look at the value of that couple of % against the cost/difficulty of uprating the cable.
Alternatively go for a higher voltage panel and convert it down to 12V at the regulator/battery, but look at regulator efficiency first of course.
 
More of a question than an answer, but am I right in thinking that undersize 12V wiring is merely inefficient, meaning the % loss is a matter of what is personally acceptable, whereas undersize 230V wiring is a fire hazard?
 
Quiddle said:
More of a question than an answer, but am I right in thinking that undersize 12V wiring is merely inefficient, meaning the % loss is a matter of what is personally acceptable, whereas undersize 230V wiring is a fire hazard?
Click to expand...

Nope. Undersize 12V can most certainly be hazardous, although in most routine practical applications it will simply cause excessive voltage drop.
As well as tables for volt drop, you can find them for 'ampacity', both on-line and in reference works. Allowable ampacity will vary with use/circumstances: for instances it will be lower for a given cable in an engine room than elsewhere on a boat. The requirements for temperature tolerance of the insulation will also be higher in an engine room.
 
lw395 said:
...Alternatively go for a higher voltage panel and convert it down to 12V at the regulator/battery, but look at regulator efficiency first of course.
Click to expand...

One of the reasons I went for 4 smaller panels configured as series/parallel was to get a higher voltage down the 'long' run to the regulator. Panels on stern arch & centre cockpit with regulator under the companion way
 
VicS said:
By calculation from a figure of 2.2ohms/1000m for 8AWG I make that a 3.3% volts drop at 15amps. for 6m cable run distance
Click to expand...

Ok thanks. So basically I use my current 2.5mm (10 AWG) cable and get 5% loss or upgrade the wiring to 8AWG and achieve 3%.

Given that mounting the new panels and re routing a new cable is arguably a different job I will just get the new ones installed and then do the cable down the line.
 
northcave said:
Ok thanks. So basically I use my current 2.5mm (10 AWG) cable and get 5% loss or upgrade the wiring to 8AWG and achieve 3%.

Given that mounting the new panels and re routing a new cable is arguably a different job I will just get the new ones installed and then do the cable down the line.
Click to expand...

...or you could just wire them in series if the regulator would be happy with the extra volts.
 
northcave said:
Ok thanks. So basically I use my current 2.5mm (10 AWG) cable and get 5% loss or upgrade the wiring to 8AWG and achieve 3%.

Given that mounting the new panels and re routing a new cable is arguably a different job I will just get the new ones installed and then do the cable down the line.
Click to expand...

Thats not an unreasonable approach

northcave said:
I think it can handle 24v but not sure I see the advantage of putting them in series?
Click to expand...

It will be a MPPT controller if it can accept 24 volts ( at "max power point" that will probably be around 35 to36 volts) and convert that efficiently to the 14 or so volts your battery needs for charging. A PWM controller wont do it.

The advantage of series connection will be that you will have only half the current ( in amps) in the wiring to the controller that you would have if connected in parallel. This means that very much lighter wiring is needed
between the solar panels and the controller. Because the current will be halved and the voltage being doubled wiring only 1/4 of the cross sectional area will be suitable. This means your existing wiring will be more than adequate.
This leaves only the wiring between controller and battery carrying the full 15 amps and which if short will not need upgrading.
 
RobbieW said:
Nominal voltage of most panels found on boats is 18v, in series will give you 36v. That reduces the volt drop enough to at least think about it
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True but thats the voltage at the "max power point" to achieve which a MPPT controller is needed. ( sorry I am repeating myself . I already said that)
 
The Voltage loss % calculations are limits

based on what you may wish to "run"from the solar panel, while most of us are running a battery charge: as opposed to an actual lamp or fridge say. For this reason, I suggest the % loss is more academic and your cable size calculations are based on how much you can afford to lose between panel and battery. For us it has made sense to use the larger sized cable we can afford and or route, based on 3% loss and moving to next size up if appropriate.

Thank you
 
Quiddle said:
More of a question than an answer, but am I right in thinking that undersize 12V wiring is merely inefficient, meaning the % loss is a matter of what is personally acceptable, whereas undersize 230V wiring is a fire hazard?
Click to expand...

All wiring must be able to safely handle the current it is carrying without overheating......... thats the basis of cable sizing for 230 volt systems.

In 12 volt systems once the cable run exceeds very roughly a metre the volts drop becomes the more significant factor. This then means that the size of the wiring needed will be above the safe minimum size for the current involved, well above in the case of long runs.
 
northcave said:
I'm uprating my 120w panel to 240w and the existing cable twin 2.5mm. I'm using these solar wire calculators online and you select the % loss.

If I select 20ft distance, 15 amps power with 5% max lost then is says I need 10 gauge (2.5mm cable) which I already have. If I select 3% then it says 8 gauge.

Is 5% generally acceptable or should I really be looking at upping my wiring?
Click to expand...

The difference is 2%. How much is it worth to you to have 232.8W rather than 228W at the controller? I wouldn't rewire for that, myself. It's worth having good chunky wires between controller and batteries, so the controller sees close to the actual voltage, but a bit of extra loss between panel and controller doesn't matter much.
 
VicS said:
The advantage of series connection will be that you will have only half the current ( in amps) in the wiring to the controller that you would have if connected in parallel. This means that very much lighter wiring is needed
between the solar panels and the controller. Because the current will be halved and the voltage being doubled wiring only 1/4 of the cross sectional area will be suitable.
Click to expand...

So double the voltage, half the amps and you get the same power with thinner wire.

It's basically magic is what your saying.
 
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