Solar Installation

[Bones]

I have taken delivery of a Steca PR1515 solar charge controller and an 85w panel. I will need 2 meters of cable from the panel to the controller and 1 meter of cable from the controller to the panel. The question is - what size cable?

 

The wires are screwed into the controller, and it looks like I need a ring attachment for the panel.

 

Also - how do I attach it to the batteries - do I wire them in permenantly and do they go via isolation switches or anything?

 

Many Thanks

Bones

[Keeping Up]

Based purely on general electrical knowledge and not any knowledge of your solar panels:

 

Since the panels have a maximum output of just over 7 Amps (12 volts x 7 Amps =84 Watts) I would suggest the use of 10-Amp cable (I'm not sure what that is in actual diameter but someone else here will know). There would be no noticeable voltage drop over the distance you suggest.

 

As a minimum you should protect things with a fuse, typically an in-line fuse rated at 10 Amps, as close to the battery end of the cable as is practical. My preference is to connect it to the isolated (non-battery) side of your main isolator switch, but it does depend on whether you turn that switch off when you leave the boat in which case you should connect directly to the battery (via the in-line fuse of course)

 

Allan

[Gibbo]

I have taken delivery of a Steca PR1515 solar charge controller and an 85w panel. I will need 2 meters of cable from the panel to the controller and 1 meter of cable from the controller to the panel. The question is - what size cable?

 

The wires are screwed into the controller, and it looks like I need a ring attachment for the panel.

 

Also - how do I attach it to the batteries - do I wire them in permenantly and do they go via isolation switches or anything?

 

Many Thanks

Bones

 

1 metre from the controller to the batteries is 2 metres round trip.

 

Let's go for a maximum permissible volt drop of 0.1 volts (anything more and you're kinda ruining what the controller is trying to do). This calculates out to 2.52mm sq cable.

 

However if you use this for the whole set up you will also have some voltage drop between the panel and the controller. This is lost power.

 

So go for about 4mm sq cable to keep the losses low.

 

The panel is better wired directly to the batteries otherwise, when you leave the boat and switch the isolators off, the solar panel won't keep the batteries topped up.

 

You need a fuse (10 amp is about right) as close to the batteries as physically possible BUT not in the battery compartment if the batteries are wet open cell types as this creates an explosion hazard.

 

Gibbo

Edited December 10, 2006 by Gibbo

[Tony Brooks]

1 metre from the controller to the batteries is 2 metres round trip.

 

Let's go for a maximum permissible volt drop of 0.1 volts (anything more and you're kinda ruining what the controller is trying to do). This calculates out to 2.52mm sq cable.

 

However if you use this for the whole set up you will also have some voltage drop between the panel and the controller. This is lost power.

 

So go for about 4mm sq cable to keep the losses low.

 

 

snip

 

Gibbo

 

 

Although this is the 100% correct answer until we know exactly what the purpose of the panal is I am not sure the practical answer is as clear cut as this.

 

The thinest readily available cable that copmplies with the RCD is rated at about 13 amps, even 1mm sq 14/0.30 is rated at 9 amps so the cable is very unlikley to overheat, but it will cause voltdrop and on battery charging 0.1 volt is important.

 

Now, if the idea is just to keep the batteries fully charged (rather than to try to cover loads) it is worth considering the facts that voltdrop is proportional to current flow and the larger the cable the more difficult is is to hide on an existing boat. As the batteries are charged (be it at a slower rate because of excess voltdrop on thinner cables) the charging current will fall, thus the voltdrop will also fal, untill the point where zero current flow produces no voltdrop. I doubt things will ever get to quiet that point, but as the batteries become charged the voltdrop along the cables will fall and therefore a higher charging voltage becomes available.

 

I suspect that with a solar panel being used to keep the batteries fully charged the length of time the charging current is available will allow smaller cables to do a satisfactory job, unlike the engine charging system where the limited time allowed for battery charging makes any voltdrop highly undesirale.

 

Comment:-

[Bones]

thank you for all your comments, it sounds as though everyone is just about saying the same thing (GASP!)

 

So, I am looking at using 10Amp cable which if I need to worry about RCD means I will need to get 13Amp cable - which I am sure is much higher rated than that for the general electrical wiring in the cabin...!

 

Do I need to worry about RCD? I don't think so....

 

Has anyone else used a Kyovera panel and a steca controller? If so I would be interested to know whether one has to disconnect the solar panel/controller when the engine is running? The notes on the Steca say another charging source should not be used in conjunction with the panel - the alternator is obviously another charging source.

 

 

 

Although this is the 100% correct answer until we know exactly what the purpose of the panal is I am not sure the practical answer is as clear cut as this.

 

the panel will be used to keep the battery charged/topped up - in which way will this make the practical answer (panel, wire, controller, fuse batteries) less clear?

 

Thanks for your helps so far!

[chris w]

Bones

 

So, I am looking at using 10Amp cable which if I need to worry about RCD means I will need to get 13Amp cable - which I am sure is much higher rated than that for the general electrical wiring in the cabin...!

 

Do I need to worry about RCD? I don't think so....

As has been stated above the current carrying capacity is not a limiting factor here. The volts drop is the main criterion to consider. You need to go for the lowest volts drop cable in a practical size. Gibbo's suggestion of 4 sq mm is a good one IMHO. The diameter of the actual wire inside is only about 2mm and so is a small flexible cable.

 

Also the RCD is not involved in this piece of equipment, it is a DC run charging controller.

 

The notes on the Steca say another charging source should not be used in conjunction with the panel - the alternator is obviously another charging source.

The usual reason for stating this is that if you have a solar panel, say for example, a wind turbine, the controller will be reading the voltage from the turbine and not the true battery voltage and therefore the controller will not be giving the correct charge. You could set up a simple switching system to allow choice of solar, wind and alternator for instance. A warning though, you should never start the engine with the batteries disconnected from the alternator as this situation will almost certainly blow the output diodes in the alternator.

 

Chris

Edited December 11, 2006 by chris w

[Bones]

Could we talk in Amps here? I am sure mm2 is helpful as well so could we use both or just Amps as terminology?

 

what is 2mm or 2.52 mm cable in Amps please?

[chris w]

Bones

 

It's not as simple as that unfortunately and that's why no-one has expressed it that way.

 

For example a 2 sq mm cable would probably be rated at around 15 amps max. ie: over 15 amps the cable will get hotter than is recommended.

 

However, in this case with the solar charger, the current is only about 7 amps max so from a heating perspective a 1 sq mm cable would do.

 

But it's not heating effect , but volts drop that is important in this case and the volts drop is calculated from:

 

volts drop = (total cable length in metres) x (current) / (59 x cable area in sq mm)

 

So, rearranging, the selection of cable area size is = (total cable length in metres) x (current) / (59 x volts drop)

 

If we want no more than 0.1v drop then the minimum cable size would be around 2.5 sq mm as Gibbo calculated. He suggested increasing this to 4 sq mm to offset any cable losses between the panel and the controller.

 

Chris

[Bones]

2sqmm at around 15amps is what I was wanting to know! Thanks.

 

I know about voltage drop and to add that on.

 

 

Now I know that we are all talking about the same thing in different langauge it is quite easy for me to work it out. Keeping ups calculation was very helpful to know the basics for AMPs

 

The wiring sounds very straight forward, thanks for everyones help. I will shout when I get into trouble... or with further questions!

[chris w]

If it's helpful here's a table of volts drop due to cable length and maximum current due to heating effect for various cable cross sections. (Please note that maximum current specified can vary from manufacturer to manufacturer depending as to which BS spec etc they are complying - so take max current as a guide only). Volts drops will be accurate however for all copper cables.

 

 

 

Area____voltsdrop________max current (amps) due to heating effect

(sq mm)

 

 

0.5_______34.0___________3.5

1_________17.0___________7

2__________8.5__________14

3__________5.7__________21

4__________4.3__________28

6__________2.8__________42

8__________2.1__________56

10_________1.7__________70

16_________1.1_________112

25_________0.7_________175

40_________0.4_________280

60_________0.3_________420

 

volts drop is in millivolts per metre length per amp. (Note that length is TOTAL length ie: from the battery to the appliance and back to the battery). So to work out the actual volts drop for a cable, eg: 10 metres of 6 sq mm cable carrying 15 amps then multiply 2.8 x 10 x 15 = 420 millivolts = 0.42 volts drop. This would be the most you would want to drop to ensure a reliable installation. Note that the cable itself will handle over 40 amps from a heating perspective but would be the wrong cable at just 15 amps at more than 5 metres between the battery and the appliance (ie: 10m total length).

 

In pretty much all cases involving boat cable calculations, volts drop will always be the determining factor in cable size. If you are running close to maximum current then your volts drop will be too large anyway for a proper installation. Allow about 0.4 volts maximum drop for most appliances (3%). For battery charging 0.1 volts max is a good guide.

 

Chris

Edited December 11, 2006 by chris w

[Gibbo]

I hate doing this because certain suspicious cynics think I am somehow trying to promote something. Which is laughable and ridiculous.

 

Anyway, choosing cables can seem rather complicated to the uninitiated and the way it is tackled depends upon the installation. To cover the subject here would make the thread another one of those tedious ones. So......

 

http://www.smartgauge.co.uk/cable_type.html

 

There is one already done. Really written specially for boaters.

 

Gibbo

Edited December 11, 2006 by Gibbo