Inverter Wiring

[philsimo]

Hi all,

 

I`m going to be fitting a Victron 3000 inverter soon and could do with a little advise regarding cable size.

 

Reading the Victron manual they advise using 90mm^​2 cable for runs up to 5 mtrs and 120mm^​2 for runs between 5 and 10 mtrs,

my run is between 5 and 6 mtrs.

 

At present I have 70mm^​2 cable, as the inverter has 2+ and 2- terminals would it be possible to run another 70mm² cable along side the original giving a total area of 140mm ^​2​ ?

 

If so I presume both positive cables would need to go through the same battery isolator but be individually fused, at present there is a 250 amp fuse fitted, so I would need to fit a similar one in the new cable.

 

Any advise would be much appreciated.

Phil

 

 

[WotEver]

... I presume both positive cables would need to go through the same battery isolator...

Yes.

 

... but be individually fused...

Nope, no need. Terminate both cables together onto the megafuse. It'll protect both of them.

 

Tony

[philsimo]

Thanks Tony, Victron advise a 400 amp megafuse does that sound about right?

[WotEver]

Thanks Tony, Victron advise a 400 amp megafuse does that sound about right?

Yep.

[WotEver]

Thanks Tony, Victron advise a 400 amp megafuse does that sound about right?

Well, I say yes but when pairing cables this is where it gets interesting. As you can guess I've been thinking about this.

 

Ideally, a 70mm² cable should be fused at 200A. Because you're pairing them, a 400A fuse is fine should the inverter suddenly develop a massive internal short. However, should just one of the cables come adrift at the inverter end and short to the case then it would be glowing pretty well before a 400A fuse were to blow. Yes, this is unlikely, but you should be aware of it. Fuses are to protect the cable, not the circuit, and while 400A is fine for a pair it's not fine should the pair get separated.

 

So.., ideally a better solution would be two 200A megafuses, one for each cable.

 

Tony

[philsimo]

Tony,

just one last question, well two actually, am i right in thinking that these fuses should be at the battery end of the cable run?

 

also, is there any need to fuse near the inverter as had been done with the old combi installation?

 

many thanks Phil

[Robbo]

Tony,

just one last question, well two actually, am i right in thinking that these fuses should be at the battery end of the cable run?

 

also, is there any need to fuse near the inverter as had been done with the old combi installation?

 

many thanks Phil

They should be at the battery end yes as the batteries can supply more Amps than the cable can take. The combi fuse was internal and presumably to protect its internals. Edited July 27, 2016 by Robbo

[Sea Dog]

Sound advice above should it be essential that you use doubled up cables, but it's not really necessary, is it? You're fitting a good quality inverter and them skimping on the installation and, whilst you know what's going on, there's potential for future confusion. For the extra few quid the 5m 120mm cables would cost over and above the second 70mm runs and the two fuse sets, why not do the job right?

 

Heck, you'll even get a set of 70mm jump leads for free!

[WotEver]

I agree with Robbo and SeaDog

[by'eck]

Hi all,

 

I`m going to be fitting a Victron 3000 inverter soon and could do with a little advise regarding cable size.

 

Reading the Victron manual they advise using 90mm^​2 cable for runs up to 5 mtrs and 120mm^​2 for runs between 5 and 10 mtrs,

my run is between 5 and 6 mtrs.

 

At present I have 70mm^​2 cable, as the inverter has 2+ and 2- terminals would it be possible to run another 70mm² cable along side the original giving a total area of 140mm ^​2​ ?

 

If so I presume both positive cables would need to go through the same battery isolator but be individually fused, at present there is a 250 amp fuse fitted, so I would need to fit a similar one in the new cable.

 

Any advise would be much appreciated.

Phil

 

 

 

If at all possible I would aim to reduce the cable run from battery to inverter. A 12 metre return run is really too long assuming you were quoting distance between. I would also do the job properly and source a singe run of the correct gauge cable for each leg. You can see yourself the issues that can arise otherwise - think simple but robust with heavy current cabling.

 

Please also ignore comments that the fuse is only there to protect the cable. It most certainly isn't. Thinking should be to choose fuse to protect equipment then cable size and connections to minimise volt drop for given cable run at max load. A 400 amp fuse should be ok in your case.

 

I can qualify above when a 500 amp fuse blew feeding an accidentally overloaded 3500 watt inverter. The 70mm2 cables didn't get remotely warm but the inverter did Without doubt it saved my Combi which is still working fine three years later. Ditto 600 amp fuse on my fouled bowthruster although it had thermal overload protection as well.

[WotEver]

 

Please also ignore comments that the fuse is only there to protect the cable. It most certainly isn't.

This is bad advice and is incorrect. The fuse to inverter cables most certainly is there to protect the cable. Any competently designed Inverter will have internal fuses to protect itself, the fuses are solely to protect the cabling.

 

Tony

[Alan de Enfield]

 

 

 

Please also ignore comments that the fuse is only there to protect the cable. It most certainly isn't. Thinking should be to choose fuse to protect equipment then cable size and connections to minimise volt drop for given cable run at max load. A 400 amp fuse should be ok in your case.

 

 

 

 

 

This is bad advice and is incorrect. The fuse to inverter cables most certainly is there to protect the cable. Any competently designed Inverter will have internal fuses to protect itself, the fuses are solely to protect the cabling.

 

Tony

 

 

Ding-Ding

End of round one.

 

No wonder there is confusion around electrical installations when two "experts" are so diametrically opposed,

[WotEver]

Sorry, but you can't refer to someone as an 'expert' if he believes that circuits should be fused to protect the equipment.

 

First link found at random - see the first Q&A:

http://www.xantrex.com/documents/Tech-Doctor/marine/Tech%20Doctor-_4.pdf

[by'eck]

Sorry, but you can't refer to someone as an 'expert' if he believes that circuits should be fused to protect the equipment.

 

First link found at random - see the first Q&A:

http://www.xantrex.com/documents/Tech-Doctor/marine/Tech%20Doctor-_4.pdf

 

I didn't say that the fuse wasn't to protect the cable as well, more that it's value should be chosen to protect the equipment. By default then the cable will also be well protected assuming an adequate gauge has been employed. Your linked document highlights the importance of correct cable choice.

 

There are many examples where a manufacturer chooses to recommend an external user provided fuse in the fitting instructions. In other cases it's included in the fitting kit but still external to allow easy replacement. I would be amazed if you are not aware of this already. If the manufacturer has chosen to include and internal fuse then fine the cable can be fused to protect it. Ditto if the cable is feeding a distribution network with a range of equipment.

 

Furthermore I provided clear examples where my comments regarding fusing to protect equipment were upheld with particular reference to the topic, but which you seem to have ignored.

[WotEver]

When a piece of equipment is supplied with an external fuse, like a car radio, that fuse is provided to protect the equipment. You will note that it is at the equipment end of the supply cable.

 

The feed to that piece of equipment will be fused at the battery end. That fuse is most certainly to protect the cable.

 

Any circuit, and a high-current load such as an inverter is no different, is fused at the battery end for one reason only and that is to protect the cable. There are no exclusions to this - that is the purpose of the fuse.

 

If you wish to protect the equipment externally then it should be fused at the equipment end of the cable as in my car radio example above.

 

If an overloaded combi doesn't shut down of its own accord (or blow internal fuses) then it is a poorly designed piece of equipment.

 

Tony

[by'eck]

When a piece of equipment is supplied with an external fuse, like a car radio, that fuse is provided to protect the equipment. You will note that it is at the equipment end of the supply cable.

 

If the fuse is located close to the equipment then clearly it won't protect the larger part of the cable run. Both can be protected by locating fuse or breaker of similar value at the distribution panel though. Isn't this how most are wired?

 

The feed to that piece of equipment will be fused at the battery end. That fuse is most certainly to protect the cable.

 

Any circuit, and a high-current load such as an inverter is no different, is fused at the battery end for one reason only and that is to protect the cable. There are no exclusions to this - that is the purpose of the fuse.

 

No in answer to both of those points in bold, it can protect both, see above

 

If you wish to protect the equipment externally then it should be fused at the equipment end of the cable as in my car radio example above.

 

Why specifically, again referring to above?

 

If an overloaded combi doesn't shut down of its own accord (or blow internal fuses) then it is a poorly designed piece of equipment.

 

Well I'll come clean on this one and say its a Sterling Combi. The estimated temporary load was probably in the order of 4kW which had already tripped the 16 amp shore supply when the inverter took over. It could be argued that the equipment was rugged enough to take the short term abuse, and as said its still working fine. TBH I was surprised it didn't shut down although it was on Charles Sterling's recommendation that I fitted the 500 amp fuse, which I remember considering a little high at the time.

 

Tony

 

To sum up I will say that in general most DC equipment isn't fused internally. Like it or not, but I accept this on two points - it keeps the price down - it allows easy accessible external fusing without the complexity of effective double fusing.

 

Finally another example where equipment fusing saved me a lot of money. My Sealand Vacuflush pump with very low mechanical gearing temporarily stalled (you can guess why ) although its normal current draw was only 8 amps it tripped the 16 amp dedicated panel breaker and consequently lived for another day. A new motor was listed at well over £100!

 

I've already admitted there are situations where only the cable fusing needs to be considered, when there is further dedicated fusing further down the line, but please don't argue against the hard facts I've provided.

 

FWIW in over 20 years of cruising covering nearly 60,000 nm I've never had a cable burn out. I have witnessed a catastrophic melt down on a near brand new £750,000 Oyster yacht though, so am most certainly aware of the importance of cable fusing allied to adequate gauge for the task.

[WotEver]

It is incorrect to state that "the fuse should be chosen first to protect the equipment..." It is bad advice and will lead to poor installations.

 

What I have written in the past is...

In all cases the fuse should be sized to match the maximum current capacity of the cable it is feeding, regardless of what equipment that cable is powering. The only exception to this rule would be in the case of a dedicated feed to a single piece of equipment where the fuse can be smaller than the cable's capacity in order to also protect the equipment being powered.

[nicknorman]

I think the two contenders both know the issues and are only seperated by semantics!

 

Both agree that the fuse must not be greater the the maximum rating of the circuit. Of course the circuit primarily consists of the cable but also perhaps such things as the isolator switch. It's not hard to envisage very fat cable chosen thus to constrain voltage drop on a long run, having a rating greater than the isolator feeding it. The fuse might thus be chosen to match the isolator's rating.

 

For something like a Combi there will typically be a manufacturer's recommended fuse size which, presuming the cabling is in accordance with the manufacturer's instructions, will be less than the cable rating. If nothing is specified I'd choose the fuse to suit the cable's rating. A Combi will have internal protection and may well be designed to allow substantial short term overload on a time constant that doesn't match the fuse's. For example our Mastervolt Combi recommends a fuse size of 250A whilst stating that the peak current is 400A. A 250A fuse doesn't blow as soon as the current gets to 251A!

 

However consider a different scenario: a boat has 2.5mm^2 wiring to some halogen lights. These are then replaced with LEDs. Should the fusing be left to match the now over-rated cable, or might it be prudent to fit a smaller fuse more in keeping with the expected max current from the LEDs? The latter, I suggest.

 

So the question "does the fuse protect only the cabling / circuit, or the cabling/ circuit and the equipment?" doesn't have a binary answer. It depends.

 

Oh and just to be clear, "protect" in the context of the equipment doesn't mean protect from damage, it means (help to) protect from causing overheating/fire.

 

Finally on the subject of dualled cables and 1 vs 2 fuses, Tony is right, however being pragmatic about it, let's say there were two cables rated at 200A and one 400A fuse, if one cable disconnects and shorts, you are likely to have a lot more than 400A flowing and the fuse will blow rapidly. You would have to envisage some failure mode where the fault current was miraculously limited to 400A before a dangerous situation arose. Rather unlikely.

 

Well except that if the cable disconnects and doesn't short, the other cable takes all the current and if the inverter is working hard, this might overheat the remaining cable. Of course this problem is not fixed by dual fusing of the +ve wires since it applies equally to the -ve wires. So to cater for these scenarios you would need 4 fuses. Or maybe you will die from something else before you have installed all of them! I suggest that good initial installations (proper crimps etc) and protecting against chafing is a more pragmatic solution than fusing every visible wire just in case!

[philsimo]

Well I`ve decided to go with the larger cable, it seems to make sense to keep things as simple as possible.

 

Really appreciate the discussion and replies, Many thanks to all.

 

Phil

[Sea Dog]

Well I`ve decided to go with the larger cable, it seems to make sense to keep things as simple as possible.

 

Really appreciate the discussion and replies, Many thanks to all.

 

Phil

 

Sensible choice - and you've restored the peace, it seems!