Inverter fuse/earth

[Lee bay]

hi
2 questions 1) what size in line fuse should I use for a 1500w(continuous) 3000w(peak)on the 12v input?
2) where should the earth wire be connected?
Cheers

 

[DBK]

I would fit a 250A Mega fuse. The current figure with these fuses is their working current, they blow at twice that - 250A is 3000W.

The earth is an old chestnut on here. You will be perfectly safe not fitting one in my view. You could fit instead a latching RCD but again opinions differ on these too.

 

[Raul]

35mm2 cable-150A fuse and double pole RCD.

 

[MichaelT]

I've got a 2000w inverter with 250a mega fuse. No earth.

 

[Lee bay]

Thanks for the replys 250A fuse orderd earth lead in the bin.

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[Raul]

I've got a 2000w inverter with 250a mega fuse. No earth.

To protect what?

 

[Bandos]

What? Should you not put 12v Negative to ground/vehicle chassis

 

[MichaelT]

To protect what?

Sorry?

 

[AdrianChen]

Thanks for the replys 250A fuse orderd earth lead in the bin.

What size is the cable? 2.5mm²-120mm² Fuse should be to protect the cable I thought?

 

[Richard n Ann]

Sorry?

I think he means what size cable are you protecting with the fuse

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[MichaelT]

I think he means what size cable are you protecting with the fuse

50mm2

 

[berni109]

You need a fuse that will protect both the battery and the cable between the battery and the inverter.
What is the maximum discharge rate of your habitation battery and what size cable connects both together.

 

[Raul]

I fused my 2kva inverter with a 150A fuse via a 70mm2 cable, to 700ah lithium.

The values you find on line, or retailers, it will be max wire ampacity. What they don't tell you, is, that rating is only valid IF cable is in free air, or, on a metal perforated tray, away from wall and cables spaced out of each other by one diameter minimum.
For example a 35mm2 max is 181A, free air, now same cable in trunking/ conduit on a wall with 55% empty trunking/conduit, the rating is 125A.
And if, that conduit passes through an insulated wall, the rating will be 99A.
This values are for PVC thermosetting and thermoplastic PVC 70degC.
All taken from IET BS7671 18th edition amendment 3. You can find all tables/values on IET website, On Site Guide appendixF table F4 (i).

Now figure a fuse, that will blow in time within cable parameters, taking in consideration the prospective fault current of the source: battery. The battery can give you CCA fault current amps for lead, or at least 5C for lithium. These are approximate, there is no tabulated values for such sources, or at-least I haven't come across them; data-sheet from battery manufacturer can give you a tabulated figure, or if you have the equipment measure the Zs.

Sticking a large fuse may not be very smart as it will offer no protection, but will sustain a overload for to long; it only takes 65w to start a fire.

 

[Lee bay]

Thanks for your reply Raul probley need to look at cable as well as fuse rating for the cable, not sure of the cable size as it was supplied with the inverter looks about 35mm2, short run about 300mm to battery

 

[Raul]

If it is 35, and so short, 150A will be plenty for any surge. Surges above nominal are 0.1-0.4s and overload to 5s, so you want a fuse that blows after a overload of 5s max. If fuse is too big the overload duration extends into hundreds of seconds, no good for the cable.

Most importantly buy a reputable name fuse, no name fuses do not work well, sometimes never blows on time.

You never mentioned anything about your battery; please tell what you got.

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[autorouter]

2) where should the earth wire be connected?

The earth wire should be connected to the chassis. That's the same way the EHU earth is connected. The question is what to do with the neutral wire.

If you are only running one appliance from the inverter output, you can do nothing, leave the neutral isolated from the chassis. It will behave like the output of an isolation transformer, which is an alternative safety arrangement used in high-risk areas like industrial washing areas and bathroom shaver sockets.

If the inverter output goes to several sockets, for different appliances, then you should connect the neutral to the earth. That's what is done in a domestic setup, either at the consumer unit or the substation.

If you have an EHU inlet, then the neutral will already be connected to the earth at or before the hookup post. It's not a good idea to have an second earth-neutral connection inside the motorhome. The usual arrangement is to have a relay that automatically disconnects the motorhome earth-neutral link when EHU is plugged in. That's how for example Victron do the earth connection.

 

[berni109]

autorouter does that not depend on which brand of inverter you have. Some claim to have the equivalent of an RCD built in. Other don't tell you anything and leave you to sort it out.
If it's centre tapped 115v live and 115v nuteral then as you say.
Everything I have read says DO NOT have 2 earth to nuteral bonds in any system

 

[Lee bay]

If it is 35, and so short, 150A will be plenty for any surge. Surges above nominal are 0.1-0.4s and overload to 5s, so you want a fuse that blows after a overload of 5s max. If fuse is too big the overload duration extends into hundreds of seconds, no good for the cable.

Most importantly buy a reputable name fuse, no name fuses do not work well, sometimes never blows on time.

You never mentioned anything about your battery; please tell what you got.

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