Fuse size

[Drayke]

I’m just fitting a Victron Phoenix Inverter 12v 2000va, I can’t find on the destructions what rating of fuse to add between batteries and inverter, so can anyone help me with the fuse size please.

Many thanks.

Fred.

[WotEver]

300A would be fine in the absence of any instructions. 

[Chewbacka]

1 hour ago, WotEver said:

300A would be fine in the absence of any instructions. 

So long as the supply cable between the batteries and inverter is at least 70mm2.  

[MtB]

On 25/08/2017 at 23:45, Chewbacka said:

So long as the supply cable between the batteries and inverter is at least 70mm2.  

 

Surely that depends on the length of the cables.

[WotEver]

8 hours ago, Chewbacka said:

So long as the supply cable between the batteries and inverter is at least 70mm2.  

Pretty certain that either 70 or 90 is recommended by Victron in the manual from memory. 

7 hours ago, Mike the Boilerman said:

 

Surely that depends on the length of the cables.

I think Chewy was making the point that a chunky fuse requires chunky cables. 

[nicknorman]

7 hours ago, Mike the Boilerman said:

 

Surely that depends on the length of the cables.

No, it doesn't.

[Detling]

Cables have a maximum safe current, based on temperature rise so different when in open air, trunking and bundled etc. This current is less than that required to melt the insulation or catch fire. The fuse should be less than this safe current, but may be more than the device will take. So a 60 watt headlight will take 5 amps when lit but due to the length of cables to the bow will probably wired in cables with a safe current rating of 25 amps, you can use any fuse above 6 amps up to 25 amps. The fuse is to prevent the cable overloading and catching fire, not to protect the bulb.

 

[WotEver]

A 2kVA inverter will draw around 200A at full chat. A 200A fuse would die from thermal cycling frequently enough to be a nuisance. 400A is overkill. 300A is a good compromise. 

That was my thinking. 

[MtB]

On 26/08/2017 at 09:08, WotEver said:

A 2kVA inverter will draw around 200A at full chat. A 200A fuse would die from thermal cycling frequently enough to be a nuisance. 400A is overkill. 300A is a good compromise. 

That was my thinking. 

 

A further point is that a fuse does not necessarily blow when carrying its rated current particularly for short periods. Blowing is not a precise science. A particular combination of current and time is necessary and fuses are rated by time to blow and well as current. Small fuses are anyway. 

[mango]

I would go for 200A unless the inverter is used on full power.  The fuse can always be replaced with a 300A one if it fails.  

[WotEver]

2 minutes ago, mango said:

I would go for 200A unless the inverter is used on full power.  The fuse can always be replaced with a 300A one if it fails.  

I find it difficult if not impossible to imagine a scenario which would blow a 200A fuse that wouldn't blow a 300A fuse. Therefore what's the point in under-rating the fuse and causing certain failure down the line?

For their 3kVA inverters Victron suggest a 400A fuse, not a 300A one. 

[nicknorman]

3 minutes ago, mango said:

I would go for 200A unless the inverter is used on full power.  The fuse can always be replaced with a 300A one if it fails.  

Why? Why deliberately put yourself in a position where the fuse may blow if you use the device to its full capability. And don't forget it will likely have a significant overload capability. What is it you are trying to achieve by under-speccing the fuse?

[alan_fincher]

24 minutes ago, mango said:

I would go for 200A unless the inverter is used on full power.  The fuse can always be replaced with a 300A one if it fails.  

I wouldn't! 

(Why?)

[mango]

1 hour ago, nicknorman said:

Why? Why deliberately put yourself in a position where the fuse may blow if you use the device to its full capability. And don't forget it will likely have a significant overload capability. What is it you are trying to achieve by under-speccing the fuse?

How often does a 200A fuse blow on a 2kVA inverter in normal use?  Each to their own.

[Drayke]

Thanks for all the replies. I am changing the inverter from a Sterling 3000va to the Victron 200va, so the cabling will be ok as I’m intending to use to use this to power the new inverter. As to the fuse I think I will go with the 300amp.

 

Thanks again for the replies.

Fred.                                                                                                                                                                                             

P.S As per usual there is always a difference of opinion on what is the correct way of doing things.

[alan_fincher]

1 hour ago, mango said:

How often does a 200A fuse blow on a 2kVA inverter in normal use?  Each to their own.

Define "normal use".

2KW provided by 12 volts needs 167 Amps at 100% efficiency.

As few inverters apparently better 90% efficiency, that means more like 185A is possible in practice.

That is too close to 200A to make that a sensible fuse value, particularly if the inverter is designed to be able to cope with over 2KW for a brief surge when something exceeds it.

As has been said, there are few circumstances where a 200A fuse will blow when you want it to, where a 300A will not.  On the other hand it is not hard to come up with scenarios where the 200A will blow where you don't want it to, but the 300A will not.

Fred is making the correct choice by using 300A in my view.

 

[Detling]

My victron has a fuse in it, I have fitted another near the battery as suggested by the bss man but the cable is 1.5 meters long so a bit of an overkill. He who must be obeyed is always right.

[mango]

2 minutes ago, Detling said:

My victron has a fuse in it, I have fitted another near the battery as suggested by the bss man but the cable is 1.5 meters long so a bit of an overkill. He who must be obeyed is always right.

The fuse in the Victron would not offer protection if their was a short-circuit between the battery and inverter.

[mango]

42 minutes ago, alan_fincher said:

Define "normal use".

2KW provided by 12 volts needs 167 Amps at 100% efficiency.

As few inverters apparently better 90% efficiency, that means more like 185A is possible in practice.

That is too close to 200A to make that a sensible fuse value, particularly if the inverter is designed to be able to cope with over 2KW for a brief surge when something exceeds it.

As has been said, there are few circumstances where a 200A fuse will blow when you want it to, where a 300A will not.  On the other hand it is not hard to come up with scenarios where the 200A will blow where you don't want it to, but the 300A will not.

Fred is making the correct choice by using 300A in my view.

 

I wonder how often a 200A fuse would blow in practice.  I have spare megafuses, which are readily available, but if I recall correctly, Victron use a type that is more difficult to get hold of.