Questions about wire size and solar panels

[tymonk]

Hi. I replaced a solar panel of unknown wattage (but similar physical size) to a 30W panel. It would be infinitely easier to use the existing wiring. With calipers the inner conductor measures 1.2mm2 in diameter (although I find it quite tricky to get an accurate reading), so seems like this could be 1mm2. It's 2mm in diameter including the insulation. The solar panel’s short circuit current rating is 1.82A and the optimum operating current is 1.67A. The panel connects to a 15A MPPT controller on the other end. Is this fine to use if I use connectors of appropriate current rating, etc.?

A few extra questions:
- What's the reason all panels come with seemingly excessively high-gauge wires? Is this just industry standard and/or to prevent voltage drop?
- Being unsure about the above, I left the panel unplugged. How long (looking for rough estimate, I understand this depends on many factors) can a reasonably new, fully charged engine battery be left and still start an engine?
- What the most useful wire amp rating chart? Do any include overall diameter?

Thanks.

 

[Trident]

1mm is good for around 14a in the open so fine though depends how long the wire run is as to the voltage drop you may see

Most panels use 4mm or 6mm wire in very thick double cases so look bigger but they expect quite long wire runs and the higher the voltage when it reaches the controller the better

The self discharge on a battery depends one many factors like the temperature and the type of battery but if yours won't start the engine if its disconnected in the meantime, with no parasitic loads and in good condition , after many months then you should get a new battery. Assuming you'll be back at the boat sometime this summer and that the above is correct (battery switched off with no loads) you'll not need to worry.

Google will show lots of charts - the casing diameter is not a useful guide so you do need to know the actual wire size inside . The type of wire and how it is fixed (in conduit or just clipped effects heat dissipation and thus carrying capacity) also make a difference but check the charts online and they're all reasonably close - and then go a good chunk lower for your max amps. So 1.5mm is good for about 21 amps but I normally limit it to at most 14 am loads. 2.5mm is rated at around 29A so when doing wiring I use it up to about 20 amp unless a very short run

Never be tempted to try and use a wire to its maximum rating (or as one customer of mine had done add more and more devices to one 19amp rated wire to avoid having to run more or bigger wires until he had 4 devices that together came to exactly 19 amps on a 5m run of wire) - always leave yourself a margin and check the tables online for voltage drop over distance (and its there and back distance remember)

 

[tymonk]

Great, thanks. That is useful. So even if my measurement was off and the diameter was 1mm rather than 1.2mm (diameter rather than squared), that should still be completely fine in terms of safety for this <2A load? Voltage drop relates to efficiency of the panel to charge rather than safety, right? The run is about 2m (so 4m). Of that I would say 1.5m is through a pvc tube that goes through the bulkhead and carries multiple single core cables.

 

[andsarkit]

Ultimate current capacity is rarely a problem on 12V systems as the power to overheat the cable comes from volt drop x amps and it is only with an unacceptably high volt drop that this would be significant.
Volt drop calculator here: 12volt planet
A volt drop of a very few percent or fraction of a volt is usually considered acceptable.
Putting your figures into the calculator gives : The voltage drop is 1.23% (0.148 volts).

 

[tymonk]

Ultimate current capacity is rarely a problem on 12V systems as the power to overheat the cable comes from volt drop x amps and it is only with an unacceptably high volt drop that this would be significant.
Volt drop calculator here: 12volt planet
A volt drop of a very few percent or fraction of a volt is usually considered acceptable.
Putting your figures into the calculator gives : The voltage drop is 1.23% (0.148 volts).

Thanks. So the numbers above would not concern you?

 

[William_H]

Ok to use lighter existing wiring. Consider that the 30w panel will have an internal resistance of 13 ohms any additional resistance in wiring will be insignificant. Of more concern is the robustness of the wiring. ie resistance to damage and corrosion. Wiring insulation on domestic (large) PV panels is seemingly over rated but considering they are usually wired in series to give a voltage over 350 volts you need good insulation. ol'will

 

[noelex]

If the wire conductor has a diameter of around 1mm the cross sectional area of the wire would be about 0.8 mm squared.

Such fine wire is not ideal on a yacht particularly when leading outside to solar panels as the physical strength of the wire is not high. However, electrically it is fine for 30w panel. The voltage drop is acceptable as is the current capacity.

You can use it safely and it should perform well, but make sure their is a suitable fuse between the solar controller and the battery. The fuse that would normally be used with a 15A controller is a little high (probably 20A) to protect this fine wire so swap it for lower value otherwise the wire will not have adequate protection (around 5A or 7.5A would be OK).

Try and protect the wire from physical damage (using conduit, cable ties etc) as much as possible.

 

[sarabande]

May I raise a concern about the common run of multiple cables or wires through the PVC tube.

Active DC generates a magnetic field around a wire, and this can induce a current in adjacent wires. Solar panels, especially small ones, can have quite quickly changing output (clouds and other shadows) and the resulting change in magnetic field can send a spurious signal into adjacent wires.

It would be worth knowing if the OP's other cables and wires in the PVC tube are carrying sensitive data or power.

 

[tymonk]

Ok to use lighter existing wiring. Consider that the 30w panel will have an internal resistance of 13 ohms any additional resistance in wiring will be insignificant. Of more concern is the robustness of the wiring. ie resistance to damage and corrosion. Wiring insulation on domestic (large) PV panels is seemingly over rated but considering they are usually wired in series to give a voltage over 350 volts you need good insulation. ol'will

Thanks. The series wiring needing larger cables makes sense. I will check the cable for damage.

If the wire conductor has a diameter of around 1mm the cross sectional area of the wire would be about 0.8 mm squared.

Such fine wire is not ideal on a yacht particularly when leading outside to solar panels as the physical strength of the wire is not high. However, electrically it is fine for 30w panel. The voltage drop is acceptable as is the current capacity.

You can use it safely and it should perform well, but make sure their is a suitable fuse between the solar controller and the battery. The fuse that would normally be used with a 15A controller is a little high (probably 20A) to protect this fine wire so swap it for lower value otherwise the wire will not have adequate protection (around 5A or 7.5A would be OK).

Try and protect the wire from physical damage (using conduit, cable ties etc) as much as possible.

Indeed there is a 20A fuse between the controller and the battery. The wiring used there is larger though (2mm2 rated at 25A) but I may as well put in a lower rated fuse. It's just the wires between the the controller and the panel wires that are finer. The wires are protected on either end.

May I raise a concern about the common run of multiple cables or wires through the PVC tube.

Active DC generates a magnetic field around a wire, and this can induce a current in adjacent wires. Solar panels, especially small ones, can have quite quickly changing output (clouds and other shadows) and the resulting change in magnetic field can send a spurious signal into adjacent wires.

It would be worth knowing if the OP's other cables and wires in the PVC tube are carrying sensitive data or power.

The PVC tube carries wires for the engine monitor (revs and hours), an electrical socket that's never used, and the tillerpilot (ST2000+) socket. A separate tube carries wires for nav lights and antennas on the stern.

 

[andsarkit]

May I raise a concern about the common run of multiple cables or wires through the PVC tube.

Active DC generates a magnetic field around a wire, and this can induce a current in adjacent wires. Solar panels, especially small ones, can have quite quickly changing output (clouds and other shadows) and the resulting change in magnetic field can send a spurious signal into adjacent wires.

It would be worth knowing if the OP's other cables and wires in the PVC tube are carrying sensitive data or power.

This is very unlikely to be a problem unless you are mixing large very rapidly changing currents and millivolt signals from something like a strain gauge and over a long distance of many metres. Most data signals (i.e. NMEA 0183 based on RS422 or NMEA2000 based on CANbus) are differential balanced two wire connections which are immune to common mode interference. This means that any interference picked up by both wires of the twisted pair does not affect the data signal.
The auto pilot with it's rapid motor switching is more likely to cause a problem but again this is very unlikely.

 

[noelex]

Indeed there is a 20A fuse between the controller and the battery. The wiring used there is larger though (2mm2 rated at 25A) but I may as well put in a lower rated fuse. It's just the wires between the the controller and the panel wires that are finer. The wires are protected on either end.

That is sensible. In a fault condition the battery can deliver a high current. The 2mm squared wires between the battery and the controller are adequately protected with a 20A fuse, but this fuse size is too large for a 0.8mm squared wire.

There is also a risk that at some stage in the future a new owner will see that a 15A controller has been fitted and deduce it is acceptable to upgrade the solar panels to an array capable of delivering 15A into the battery (the input side will be an even higher current), without measuring the wiring.

Installing a lower value fuse will protect against this type of mistake. Not all owners are as diligent as you and failing to check the wiring is a suitable size is common problem.

 

[geem]

May I raise a concern about the common run of multiple cables or wires through the PVC tube.

Active DC generates a magnetic field around a wire, and this can induce a current in adjacent wires. Solar panels, especially small ones, can have quite quickly changing output (clouds and other shadows) and the resulting change in magnetic field can send a spurious signal into adjacent wires.

It would be worth knowing if the OP's other cables and wires in the PVC tube are carrying sensitive data or power.

All my data cables are screened. This is normal right?

 

[William_H]

I can not imagine current from a solar panel changing very rapidly as in microseconds to go from max to min. The least of your worries.
I know a PV cell can react fairly quickly. I have welding helmet with PV cell as sensor works pretty quickly and PV cells are used as sensor for photo flash repeaters. But total light level over a large panel is not going to change so quickly.
ol'will

 

[andsarkit]

All my data cables are screened. This is normal right?

Screened cables are good for data but probably not essential as differential signals over twisted pair are pretty good at rejecting interference. It was usual to only ground the screen at one end to avoid ground loop currents flowing in the screen but there is some debate on how necessary this is.
Low level analogue signals do require screening. The only analogue signals I can think of are tank level sensors which are not too critical if the needle flickers a bit.

 

[geem]

Screened cables are good for data but probably not essential as differential signals over twisted pair are pretty good at rejecting interference. It was usual to only ground the screen at one end to avoid ground loop currents flowing in the screen but there is some debate on how necessary this is.
Low level analogue signals do require screening. The only analogue signals I can think of are tank level sensors which are not too critical if the needle flickers a bit.

Raymarine data cable comes screened as standard. That's what we use. Radar cable is the same.