That's about fuses

[duk2n]

Hi folks,

I had this afternoon a discussion with a colleague about what fuse he would need for a 12v power socket where he plans to plug a 110 watt air pump.

My maths for this are 110w / 12v = 9.16 amp, so he would want a 10 amp fuse.

Then he comes saying that he's going to use an ATO/ATC fuse and they are rated 32vdc, so he bets for a 4 amp ATO fuse ( 110w / 32v = 3.44 )...

I told him to plug that 4 amp fuse to see how it will blow, but unfortunatelly we didn't have one. So we tried with a 7.5 amp fuse and... well, it didn't blow!

Now I'm confused. I still think I'm right and what he needs is a 10 amp fuse, it doesn't matter it is rated 32vdc. In fact, I believe 32vdc is a common automotive standard.

Maybe the 7.5 fuse we used is very slow or it is a faulty one, or my maths are wrong?

Opinions, please

 

[bo7o]

Did you measure the current? I would think the fuse does not really know about the voltage... The voltage drop across the fuse will just be some small number based on its small resistance. So I would guess it is a slow blow fuse, or the current is a little lower than the 9 amps, or a combination of the two??
See http://www.bcae1.com/fuses.htm
Section on fuse blow times supports what you see...

 

[670cc]

Hi folks,

I had this afternoon a discussion with a colleague about what fuse he would need for a 12v power socket where he plans to plug a 110 watt air pump.

My maths for this are 110w / 12v = 9.16 amp, so he would want a 10 amp fuse.

Then he comes saying that he's going to use an ATO/ATC fuse and they are rated 32vdc, so he bets for a 4 amp ATO fuse ( 110w / 32v = 3.44 )...

I told him to plug that 4 amp fuse to see how it will blow, but unfortunatelly we didn't have one. So we tried with a 7.5 amp fuse and... well, it didn't blow!

Now I'm confused. I still think I'm right and what he needs is a 10 amp fuse, it doesn't matter it is rated 32vdc. In fact, I believe 32vdc is a common automotive standard.

Maybe the 7.5 fuse we used is very slow or it is a faulty one, or my maths are wrong?

Opinions, please

The fuse is rated to safely work on a 32 volt circuit. That's the safety rating and does not change the math on what current the fuse will open at. So, your math is correct.

One should also consider the maximum current rating of the 12 volt socket. A cheap socket might only be rated for 5 amps, but a better quality socket may be good for 10 amps or more. Remember, the fuse is there to protect the wiring and the socket from exceeding their ratings. The fuse choice is not really based on what you intend to plug into it, unless you just want to overprotect the circuit knowing that the full rated current will never be needed.

Another thing: The wiring to the socket can drop some of your voltage if the wire is undersized, so you're not really working with 12 volts, rather something less. On the other hand, if you plug in your device with the engine running and have healthy wire sizes, you may actually be seeing nearly 14.5 volts at the device.

Using a multimeter to check the current and voltage during device operation is the best way to see what's going on.

 

[FrankQC]

Two things to keep in mind:

1- Car voltages are often calculated @ 13.8 V, so 110 W / 13.8 V = 7.97 A

2- That 110 W must be fully loaded. So if the pump has a maximum pressure of, let's say 60 PSI, then the max 110 W must be consumed near that 60 PSI. The lower the pump motor is loaded, the lower it will drain power.

The best way to validate this is to connect an amp (current) meter.

 

[duk2n]

Guys, thanks for all the info.

I didn't plug my multimeter because it has the typical 10 amp maximum and I was afraid to spoil it. Anyway, it was a combination of the pump not pulling at 100% and a slow 7.5 amp fuse. Today we tried a 4 amp fuse and it melted down about 30 seconds after we pushed the pump to its limits. A good 7.5 amp fuse doesn't blow in the same scenario (we keep the load for 60 seconds) but it turns hot. So the maths were correct, a 10 amp fuse is the right one, as far as wires (16 AWG) and the socket (rated for a maximum of 20 amps) are ok.

bo70, thanks for the link, good info there

 

[Billy Budd]

As someone who sells auto fuses, PLEASE buy major brand fuses and stay away from the Wang Bang Ho Chinese fuses. They have been recalled numerous times and have burned a bunch of vehicles to the ground. You get what you pay for.

 

[duk2n]

As someone who sells auto fuses, PLEASE buy major brand fuses and stay away from the Wang Bang Ho Chinese fuses. They have been recalled numerous times and have burned a bunch of vehicles to the ground. You get what you pay for.

You said you sell auto fuses. I'm looking for low profile mini fuse holders, the same kind of fuses the NC700X has, so I have not to carry different spare fuses, but can't find them. Do you know where to look for them?

 

[Billy Budd]

They are called a LMIN for the low profile. You may have to go to an auto dealership. I don't know what kind of auto parts stores you have.