It would be the high side I think? (Remember I've never done thi before) so would be the main feed from the bus bar so would be whichever is the main feed on the relay. It would be easy to access regardless yes, but the idea is to make it safe and never blow a fuse rather than making it easy to access if you get me.

Now... Why would that fuse blow first?

I thought if the seats failed for whatever reason the problem would travel down the wire from the fault with the seat going to the fuse box and blow the fuse in the fuse box.

But a problem with the live feed wire from the bus bar to the fuse box would blow that wire.

The terminal 30 busbar (and whole cabin fusebox) is fused, it's a 110A strip-fuse on top of the battery.  But that's only sufficient to protect the heavy gauge wire from the battery box to the busbar/fusebox.  You want another fuse close to the busbar, to whatever you're powering off it, suited to the wires used that go to that thing.

The current will be flowing through the whole circuit, so all parts of it will experience the stress of the current, so either could fail if they are equal. Really you should only need one fuse on the whole circuit, as exceeding the fuse's rating should cause it to fail regardless of where in the circuit the failure happened. 

 

 

 

Given a short in the seat that caused it's resistance to be substantially reduced, the resistance drops but voltage stays the same. By ohms law, the current would rise. As the battery's resistance is low, and the wire resistance is also low, you'd create an almost resistance free path round the circuit, creating a very effective heating element of hot wires. To prevent this, the fuse melts at 15A, severing the whole circuit. You know the wires can handle that amount, and the relay can, and the seats can, so it's the weakest link and fails before anything else melts. Having two fuses on the same circuit just means that the one with the lowest rating fails first. With both the same rating, which fails first is pot luck as they are both going to see the current for similar amount of time, be in places with slightly different cooling/temperature, and also have slightly different manufacturing impurities.

 

As Wino mentions above, you DO need a fuse as the 110A one for the bus bar is too high to (literally) save your ass, but this will be the one you have added to the fuse box.

Edited 15 February, 201610 yr by wiredsoftware

Yes, but that fuse should be as close as possible to where the current is coming from (the busbar), so that there is as short a length as possible of unprotected wire.

 

Edit: If that new fuse is in the fusebox, this requirement is automatically met.

Edited 15 February, 201610 yr by Wino

 

Edit: updated diagram to put the relay AFTER the fuse, which minimises the length of circuit that is fused at the higher amperage.

 

Edited 15 February, 201610 yr by wiredsoftware

I understand that diagram and know where I would put all the wires, so if people think that's the best way to do it I will do that.

Note, the link you posted for the relay earlier is from Hong Kong, any links to uk ones so I don't have to wait 3 month for it

I'm in contact with kufatec that designed and supplied the loom to see what they suggest for the live feed

http://www.ebay.co.uk/itm/1X-12V-12Volt-30A-40A-Auto-Automotive-Relay-Socket-30-Amp-40-Amp-Relay-Wires-/201457308257?hash=item2ee7ca9261:g:l3oAAOSwgQ9V02IO

 

http://www.ebay.co.uk/itm/12V-Automotive-Changeover-Relay-with-Bracket-40A-5-Pin-Car-Bike-Van-/252154744278?hash=item3ab5980dd6:g:CSUAAOSwHnFVwl6L

Edited 15 February, 201610 yr by wiredsoftware

Will sort my seats out now as well then!

Good to hear things are moving on for yours as well

Do you have your invoice details etc to download the polo manual to show where the live feed should come from?

As far as I can tell Kufatec won't help me what so ever in fitting their product without an invoice number. They are also painfully slow to write a 5 word reply

Edited 15 February, 201610 yr by hutchysrs50

Awesome little thread which I've enjoyed reading!

 

 

Personally, I would do what was said earlier, i.e. take a feed off the heavy duty wire, fuse it with something higher than your 15A (that fused relay shown earlier looks good!). Then run the output from the relay to the fusebox like you have done in your guide, but pop a 15A fuse in. That way if your seats ever try to take more than 15A, the smaller fuse will blow earlier, and hence be easily accessible without having to take the lower steering wheel cover off (or wherever you hide the relay).

I've bought 2 inline fuses, 15a and 25a, depending which direction I go in.

For now I'd just want to hear back from kufatec. Annoyingly they show how to wire up heated seats for other car (wire the live feed from a relay behind the dash) they supply a set length wire so it must go somewhere, they also have a download link but won't let me see it!!!

Well... They have replied and now sent me the instructions for the Polo 9N! Only taken since Sunday but at least I have them.

Now... The instructions for the live feed say...

Extra red line (live) - fuse 32 (heated seat fuse) or 24 solder on one side , the other end of the red / black fuse 36 or if possible crimp .

Measured at the vehicle to safety with a meter if it is ignition plus .

The fuse is fused with 15 A.

This has been roughly translated on Google from German but I read it as..

Single 6" Live wire - connect to fuse 32 (heated seat fuse)

Solder other end to red/black wire from fuse 36 or crimp if possible.

Test the wire Safley to confirm it is a switched live.

Fit 15A fuse.

Now fuse 36 as it happens is for the fans! So the instructions say to fit it exactly as I have done with the red / black wire again like I took the feed from, so... I don't need to do anything then?

Edited 17 February, 201610 yr by hutchysrs50

Up to you, they seem to think that wiring will handle it. If you're not so sure, go the relay route, which will 100% definitely handle the current.

 

If you can check the wire gauge and core count to the fan fuse, you could look up the max amp rating for it (think I might have posted a link to a lookup table last week?). If it's within specification at (25+15=)40A, do the wiring as they say.

Edited 17 February, 201610 yr by wiredsoftware

As above really. Not sure I'd be inclined to trust that it will be safe on their say so!

Sent from my D5503 using Tapatalk

As above really. Not sure I'd be inclined to trust that it will be safe on their say so!

Sent from my D5503 using Tapatalk

 Although thats true, i wired them up that way confident it would be safe, i just douted myself not knowing what anyone els has done.

Going off their guideance, it seems many other people with the Fabia, Polo & Ibiza have wired them up the same way perfectly fine.

 

Up to you, they seem to think that wiring will handle it. If you're not so sure, go the relay route, which will 100% definitely handle the current.

 

If you can check the wire gauge and core count to the fan fuse, you could look up the max amp rating for it (think I might have posted a link to a lookup table last week?). If it's within specification at (25+15=)40A, do the wiring as they say.

 

Im more worried about wiring up the relay route to be honest, i feel better knowing thats what they tell people to do and being my first instinct to do so aswell.

 

---------------------

 

Back to my original wiring diagrame i drew then, if a fault did happen, would the fuse for the fans / seat just blow or would it cause a fire?

I had it in my head if a fault happend with the feed it would just burn and catch fire since its before the fuse, but technically.. its all in circuite so a fuse would still blow, right?

 

If not, how can i keep it how i have it now and make it safer?

As said i have two inline fuses sitting doing nothing now, 15A and 25A

Unusually, you're not actually concerned with what happens when things go wrong (a short in the seat or fan, as they will blow their own fuses), but what happens if things are too right 

 

OK, there's lots of ifs and buts, but there's two questions that need to be answered:

 

1) Can the wire before the fuses handle 40A?

2) Would it ever need to?

 

Number 1 could possibly be answered as I mentioned above by finding it's rating. The second question is more interesting.

 

Both fuses (15 and 25) will be chosen such that under normal operation they will not exceed that current. In practice this means that both should be working at well under their maximum level at all times. Another point to factor in is that one of the circuits is for the fans. Motors in general will often have an initial draw when they are turned on which is substantially more than their running current needs, and the current will rapidly drop lower. I've not checked these type of motors, but you may find that their normal current draw is a long way below the 25A maximum, giving you plenty of headroom to run the seats as well.

 

So there are two possible tests you can do - you can turn everything on and see if the wire gets too hot (the low tech option), or you can measure the current. As you need to measure more than 10A, you'll probably find your multimeter doesn't go high enough, so you'll need an alternative method. You can buy current measuring clip-on devices (clamp meters/), or you'd need to rig up a shunt resistor inline and measure the voltage drop and calculate the amperage from that, which is getting pretty technical if you're already feeling out of your depth with the relay option.

 

You could chance putting a 25A inline on the feed wire, hoping that as mentioned above you will always sneak below it. This will certainly keep you from harm, but MAY end up blowing more than you'd like (possibly in the scenario where the seats are already running and then you switch the fans on). Without measuring the current, it's a guessing game. It should though keep you out of fire's way, which is the safer option. As Skoda will have built the wiring well within tolerance, suspect you could use a 30A fuse in there if you found the 25A kept blowing.

 

You mentioned a fire, so it's worth thinking about what will happen if the wire gets over it's maximum current: the insulation of the wire will melt as the core heats up and drip off. Once this happens,it's quite likely that the now naked wire will touch some part of the chassis, creating a short. As the wire is relatively thin, the wire core itself will probably melt at this stage, but if not the 110A fuse should pop. While the wire is in it's heated and bare state, the danger of fire comes from it being in contact with materials under the dash that would easily ignite, such as dashboard trim and foam.

 

A final off the wall idea - choose another circuit instead of the fans. The chances of you using the fans at the same time as the seats are relatively high, on a cold morning you will operate both at the same time. There are however other circuits with 25A fuses that may be less likely to be used:

 

Fuse No. -> function

==================

42 (15A) -> cigarette lighter (do you smoke? if not, divert this to another purpose! If you're only using it for USB chargers, they only need 2A tops, USB 2 was only 0.5A in theory)

46/53/58/66 -> electric windows x4

57 -> trailer (not fitted?)

62 -> sunroof (not fitted?)

 

Bottom line: you're probably fine, but there's is a big gap between probably and certainly, and how far you cross that gap is up to you.

Edited 17 February, 201610 yr by wiredsoftware

Convinced now not to do any wiring on my seats after this thread lol

Oh and also, why the hell isn't this easier and just direct standard cables, more like plug in and just go? They do the kit for the Mk4's etc to do this. Why the hell not Skoda ??

 

Skoda seem to do it, fit from factory, why the hell has it got to be so complex ???

Thank you for the long detailed reply Peter!

I don't know how I can check and positively say what rating the wire is, I can glance and possibly measure but would never know for sure unless skodas wiring diagrams show the feed for the fans being at whatever rating it is.

You MIGHT be onto somthing, got me excited thinking I could use the cig lighter fuse, however I'm fairly sure it isn't a switched ignition live.

I have already done the low tech option you say I had both heated seats on the highest setting and the fans in highest speed, but only for five / six minutes and the wire was still cold.

I might still get my auto electrician to check it out then and ask if they can measure the current etc and see if they think the wire is suitable.

And sorry to scare you Ant. I've confidently wired up the full wiring loom apart from the live feed, which at the time I was over the moon with. It's just after when I sat down I was like.... "Wait a minute, if that wire is for 25A and the seats are 15A that's a total of 40A, that could be bad!"

And now I'm stuck at this

Will check wiring diagrams in a bit and tell you what the fan wire sizes are, for comparison.

Important and relevant edit: just realised that above this snapshot, the 4mm² wire changes to 2.5mm² on its way to the fusebox, and is 2.5 before the fuse as well, so you really need to tap off wherever that pre-fuse 2.5mm² wire comes from rather than anywhere further down that wire. See post #49 instead of this diagram.

 

 

Here's one I annotated earlier for someone, wire (cross-sectional area) sizes in mm²  inset into each wire above the colour.  Incoming from the fuse at the top, E9 is the speed switch, N24 the resistor pack, V2 the cabin fan itself:

 

 

 

I'd imagine the wires Kufatec use have similar insulation thickness to Skoda wires but not necessarily; so same outside diameter doesn't 100% mean same wire cross-sectional area, but probably close.

Edited 18 February, 201610 yr by Wino

(thanks for the PM too, looks to me like they're recommending this option too)

 

I reckon you're fine, no harm in wiring in a 25 or 30A fuse inline too if you wish, just to be on the safe side. 

 

I'd hope that your auto sparky had a clamp meter or similar, if so they can put this to bed once and for all

Edited 18 February, 201610 yr by wiredsoftware

Slightly 'bigger picture' snapshot with ideal tapping in point at orange arrow.

 

 

Not sure where that A34 junction physically is, maybe at the back of the relay panel?

Been resolving most of this via pm's so thinking about keeping it as it is and adding an inline fuse from the main feed just before it goes into the fuse box and splits off for the seats also, so the feed is protected, it might blow but then... thats the idea.