Voltage drop from wind genny

[changeman]

I am installing a wind genny on a cat and am getting conflicting advice about voltage drop.
The installation covers 2 starting and 2 domestic batteries so I have bought 3 voltage sensitive relays to divert the charge wher it is most required. The cable from the genny to the first VSR is approx 5.5m, from the VSR to the first battery is another 3m and the total circuit around 20m.
One school of thought is that I need 13mm sq cable min throughout to avoid voltage drop. The other that I don't have a problem, and should use the existing 10mm sq circuit.
What is the real solution?
I have the circuit ready to send by email, if anyone needs more info.

 

[sjohn_gibson]

There are tables that show the cable size to use depending on Voltage and length, unfortunately I don't have one at hand. However try windenergy.com/AIR-X_Marine_Owners_Manual.pdf - it describes the problem and enables you to select a cable size.

In Summary, 12v systems are vulnerable to voltage drop. Wind generator's output are relatively low. Hence you should do every thing possible to counteract voltage drop. In summary, go for the biggest dia cable you can afford.

I mounted my wind generator at the top of the mizen mast - the cables from it could hold the mast up!

 

[VicS]

I think you may have got your cable sizes wrong by a factor of 10.

1.0mm sq is normal for 10 amps and 1.25mm sq for 13 amps for mains use. To avoid voltage drop becoming significant on low voltage systems it is usual to go one size up from the mains voltage ratings (Amps = 10x the area is a useful but approx rule of thumb for the mains rating)

To answer your question we need to know the maximum output of the genny but within reason the volts drop will not be so important as it is when powering a nav light for example.

 

[davidbains]

I would have thought you'll lose more power in the VSR than the wiring.
I suspect two core 13amp domestic wire will be fine. The thickest available
You can always double up if necessary. In fact if you see four core cable
you can use that and double up in the same sheave.
The wind genny isnn't going to produce that many amps.Or at least more
noise than power!!

 

[Anonymous]

I presume that you are installing a regulator for the windgen (unless it is the little Foregen)? If so, a major issue is the volts drop across the regulator. The windgen mfr and regulator mfr (probably the same mfr?) should give you all the info you need. It isn't possible to give you any sensible reply, based on the information you have given us.

 

[LadyInBed]

www.tlc-direct.co.uk/Technical/Lighting/VoltageDrop.html may help with calculations

 

[bazobeleza]

Try here and downlowd the air x manual there are a lot of useful tips on wind gens generally and also voltage drop

www.windenergy.com/air_x_marine.htm

 

[charles_reed]

Almost impossible to hazard a guess, because it's likely the big drop will be in the "voltage sensitive relays". If conventional diodes you'll get about 8% drop - from 14.4 to 13 volts. My wind generator puts out about 15.3v unloaded in 20 knots which drops to 14.4v downstream of the regulator when the batteries are fully charged.
I feed it, together with the solar panel, straight into the main bus and don't regard them as a serious charging device with separate feeds to individual batteries.
the feeds from the two are 22mm2, because that's what I had available, rather than working out the minimum X-section.

 

[pvb]

Conflicting advice?

If you thought you had conflicting advice before, Chris, you're probably even more confused now! Thanks for sending your circuit diagram. Here are some suggestions.

Firstly, let's correct some misunderstandings about VSRs - they have zero voltage drop and very little power loss (the sensing circuit typically consumes only about 10-20mA).

Your Air-X wind generator can produce up to 500 watts (say 40A), so the wiring needs to be fairly substantial. Air-X recommend 6AWG (13 sq mm) for cable runs over 30 feet at average wind speeds of 12mph. On this basis, as your cable run is around 30ft, I'd suggest you use at least 13 sq mm cable through to the first battery. If you intend keeping the boat somewhere with higher average wind speeds, you should consider thicker cable (the Air-X manual gives guidance on this). If you're buying the cable in the UK, you may find that 15 or 16 sq mm is easier to find, and this of course would be OK instead of 13 sq mm. Remember to fuse the cable with a 50A fuse; MegaFuses are good and have the required slow-blow characteristics.

The VSR circuit linking the 2 start batteries presumably uses a bi-directional VSR. It ideally needs wiring capable of taking the starter current. It's an unlikely scenario, but if the starboard start battery failed and you started the port engine first, trying to start the starboard engine would draw a high current through the VSR circuit linking the start batteries.

Unless there are special reasons for keeping them separate, you'd be better off permanently combining the 2 domestic batteries, this could additionally simplify the wiring and switching. Also, 2x136Ah batteries is very little domestic capacity for an 11m cat unless you're incredibly frugal with power usage - it'd be well worth adding more batteries as a priority.

 

[changeman]

Re: Conflicting advice?

PVB

Distance around all of the batteries is closer to 20m.
I note your comment about 13sq mm to the first battery. Does this mean I can leave the existing 10sq mm in place for the rest of the run?

 

[pvb]

Re: Conflicting advice?

A good rule of thumb for calculating voltage drop is the TOT rule. “TOT” stands for “Treble One Twenty”, and is all you need to remember. The 3 ones and the twenty work as follows:-

* 1 amp going 1 metre along a 1 sq mm wire gives a 20 mV drop.

That’s your basic figure, and all you need do now is change the 20mV figure in proportion to the other changes. So, for example:-

* an increase in current increases the 20mV proportionally
* an increase in length increases the 20mV proportionally
* but an increase in wire size decreases the 20mV proportionally.

You need to calculate your wire sizes based on actual cable runs and acceptable voltage drops. Your diagram is rather complicated, though, and you might consider simplifying it, perhaps by getting rid of some switches.

 

[VicS]

Re: Conflicting advice?

Now that really is a serious charging device and explains the cable sizes. I hope each of the battery banks are several hundred amp-hours to make use of it.

What a pity we could not have all been informed of its output to begin with. It is much more relevant piece of information than that it is for a Cat.

I reckon you could put another VSR (and an inverter) on there and feed the power to the national grid once all the batteries are fully charged. /forums/images/graemlins/wink.gif