Fuses, Current, Watts, What What What!

Submitted: Tuesday, Jun 26, 2018 at 20:27
ThreadID: 136903 Views:2293 Replies:6 FollowUps:3
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Dear All

Firstly. I have no electrical qualifications at all, however I do understand the relationships between Volts, Amps, Watts and Resistance.

We are going to UK soon and hiring a car. So I thought it would be useful to make a double adaptor for the 12V outlet, to be able to run the Garmin and charge the IPad and IPhone at the same time. See photo. total current draw at 12VDC about 8A so I thought a 10A fuse would be about right.

When putting the components together I noticed that the fuse in the male plug was marked 250VAC 3A. So I initially thought that’s 750Watts .. why so for a 12V plug? That is equivalent to 60 A in a 12V circuit. Didn’t make any sense to me at all. So off I went on a web search. I didn’t find anything scientific, from a reputable organisation to explain the relationship between V,I,P and fuses.

What I did find was a lot of comments on various forums. This one made some sense to me.

“ If you think of electricity as a flow of electrons through a conductor (eg. copper) the voltage (V) can be thought of as a measure of how much energy each electron is carrying (its potential). The current is the number of electrons passing through the circuit and is measured in Amps (1 Amp = approx 6.25X10 to the power 18 electrons per second!). The power of an electrical devise is measured in Watts (W) and is quite simply calculated by W = VxA which is easy to understand if you consider that each electron gives up the amount of energy it is carrying to the device as it flows through it. The ability of a given conductor (eg. copper) to carry a current is dependent on its dimensions, the thin wire in a fuse will heat up, due to the resistance (another story!), as the amount of current (No. of electrons per second) increases until it reaches its melting point. It is the number of electrons flowing, not the amount of energy each is carrying, that results in the heating effect. So it is easy to see that the rating of a fuse has everything to do with current, but is unrelated to voltage.”

So the question is: Will a 10A fuse rated at 250VAC blow at a current exceeding 10A in a 12VDC circuit?

Looking forward to hearing from those of you that know.

Cheers John

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Reply By: Shmang - Tuesday, Jun 26, 2018 at 20:39

Tuesday, Jun 26, 2018 at 20:39
Short answer is yes.
Just have to worry about the amperage rating mate, 10A is 10A regardless of voltage.
The voltage rating relates to the maximum voltage the fuse will still work as intended, e.g. at voltages above 250V the fuse won't be able to break the circuit effectively.
Enjoy your trip to the UK
Regards Chris

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Reply By: Member - nick b boab - Tuesday, Jun 26, 2018 at 20:43

Tuesday, Jun 26, 2018 at 20:43
This link from John & Val's blog may shine some light ??

Cheers Nick b

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Reply By: Nomadic Navara - Tuesday, Jun 26, 2018 at 20:49

Tuesday, Jun 26, 2018 at 20:49
John, fuses just act on the current flowing through them. It is just the current that is flowing through them that will heat them up and melt them if the current is too high. The voltage is not important unless the supply current exceeds the rated voltage of the fuse.

The voltage rating of the fuse is there to indicate the maximum voltage of the circuit. If you use the fuse in a circuit that is higher than the operating voltage then you are likely to get arcing over in the fuse after it blows due to the excessive operating voltage.

To put it another way, if the voltage in the circuit is not higher than the fuse rating then the fuse will blow if the current flowing through it exceeds the rated current. This applies no matter what the voltage in the circuit is. Voltage/current relationships or Ohms law do not apply in the main.

Hope that is clear to you.
Retired radio and electronics technician

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Follow Up By: Member - J&A&KK - Wednesday, Jun 27, 2018 at 12:04

Wednesday, Jun 27, 2018 at 12:04
Many thanks Chris, Nick and Peter.

I have learnt something again. Never stops even in older age.

It has taken a while, for the fact that a fuse will blow at the same current irrespective of voltage and wattage, to sink in. I had always believed that Ohms Law was in play with fuse ratings. I now understand the voltage rating on a fuse is to do with its capability of not “arcing” at its rated voltage. Not well stated I know.

Cheers John
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Reply By: RMD - Tuesday, Jun 26, 2018 at 22:54

Tuesday, Jun 26, 2018 at 22:54
Although the fuse size will run the 8Amps if every charger is plugged in at start up time, the instantaneous current draw may be initially more than 10 amps and blow the fuse. Best to check that operation before departure.

A ciggy plug working at relatively high current draw possibly will cause the end ot the giggy plug to become hot in the base, it is only a point contact after all. If it gets hot it will burn/melt the plug. Just 4 amps running a Waeco fridge heats them to extinction if the contact ability deteriorates and resistance is present.

If the Garmin and the chargers are plugged in when the engine is started, most vehicles will use a CanBus system and the start electrical noise/interference from the chargers may be detected by the vehicle electrics and it may go into error mode or some functions of the vehicle might not work properly.

A Nav system has been known to prevent Mazda CX7 audio system and phone functions. Plugged in, after engine has started and all was normal. Similar may occur with Euro cars.
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Follow Up By: Member - J&A&KK - Wednesday, Jun 27, 2018 at 12:22

Wednesday, Jun 27, 2018 at 12:22
Thank you RMD for the advice.

It’s highly unlikely that all three devices will be plugged in at the same time. So generally the current draw will be in the order of 4-5 amps with both the Garmin and IPad connected.

However I take your advice on the heat issue and the potential for CanBus interference at startup.

We experienced some strange issues, with phone Bluetooth connection and audio, on our last driving holiday in Europe. Our hire car was a Ford Kuga (not recommended) and the problem only occurred on start up, when the Romoss lithium battery pack AND the IPad, were plugged into the vehicle USB ports. It took me days to finally work out that the the issue only occurred if those devices were plugged in prior to starting the car. We use exactly the same devices in the 2014 Prado here in Aus and have never had a problem.

Cheers John

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Reply By: Hoyks - Wednesday, Jun 27, 2018 at 06:17

Wednesday, Jun 27, 2018 at 06:17
The vehicle ciggy plug will also have a 10A fuse hidden under the dash somewhere, so it is likely to go before the one in the plug winks out.
AnswerID: 619810

Follow Up By: qldcamper - Wednesday, Jun 27, 2018 at 10:42

Wednesday, Jun 27, 2018 at 10:42
original lighter type sockets normally have a thermal link as well as the fuse so if a high resistance is present in the connection the heat will(should) blow this link before any serious heat is generated. But as the sockets normally switch off with accessories you will be in the car and smell any heat damage occurring.
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Reply By: Jaroca Removals & Storage - Wednesday, Jun 27, 2018 at 18:50

Wednesday, Jun 27, 2018 at 18:50
This post has been read by the moderation team and has been moderated due to a breach of The Advertising/Self-Promotion Rule .

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