# Inverter blowing a fuse

#### cliffandger

LIFE MEMBER
We (Cliff) has installed a 1500/3000w inverter. It didn't come with a fuse and worked fine when he fitted it. He has since put a 40amp fuse in and I tried the same hairdryer test and it blew the fuse. Seeing the inverter hasn't got a fuse does that mean we don't need one?

I think I have asked the correct question, but Cliff is as we speak under the van trying to work out what is wrong, so please excuse me if it's not quite as clear a question as it should be!

Any suggestions welcome.

Cheers

#### scotjimland

##### Free Member
Yes, it should have a fuse in the 12v supply cable.. and rated to the max load the cable can carry .. do you know what size the cable is ?

For the moment we will assume the cable is big enough to carry the maximum load of the inverter. It may not be.

the formula for calculating the current in Amps is Watts div by Volts... A = W/V

To calculated the fuse size divide the watts by the volts .. ie for 1500 watt div by 12v = 125A

and for 3000 watt the fuse size would be 250A

Your 40A fuse can only supply 40A x 12v = 480 watt

What is the power (in watts) of the hairdryer ? Dived that by 12 for the Amps it's pulling.

Hope that helps and explains why your fuse is blowing ..

Last edited:
OP

#### cliffandger

LIFE MEMBER
Thanks for that. We rang an inverter company and they have said for 1500/3000w it should be 125A. So Cliff has just gone off to get both 125A and 250A fuses, and if the 125A blows he'll put in the 250A.

All this for a hairdryer!

#### hilldweller

LIFE MEMBER
All this for a hairdryer!

They could be motorhomes number one problem.

#### Gizmouk

We (Cliff) has installed a 1500/3000w inverter. It didn't come with a fuse and worked fine when he fitted it. He has since put a 40amp fuse in and I tried the same hairdryer test and it blew the fuse. Seeing the inverter hasn't got a fuse does that mean we don't need one?

I think I have asked the correct question, but Cliff is as we speak under the van trying to work out what is wrong, so please excuse me if it's not quite as clear a question as it should be!

Any suggestions welcome.

Cheers

40A wont last long

3000W @ 12v will require a 250A fuse.

However, you'll also need to check the cable sizing - 250A @ 12V would require 70mm CSA double insulated (291A rating).

What size leisure batteries do you have ?

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