Determining Fuse Size
The question of determining fuse size has been a common one lately so we decided to make a quick write up on choosing the correct fuse size. One common misconception about choosing the correct fuse size is that it’s dependent the load of the circuit. Actually, the load of the circuit should have nothing to do with choosing a fuse size. The fuse size should be based on the SMALLEST wire (largest gage number) in the circuit. Here is how to correctly choose the right fuse size for your circuit.
1) Determine the wire gage you already have by locating it on the package or measuring it, you can also look at the topic this was posted in on Oznium forums for a step by step guide to finding the amperages for common products at the Oznium store.
2) Use the following table to determine the maximum current for whatever wire gage is being used.
3) Take the maximum current value obtained from the table and find the largest fuse you can find that still falls within the limitations. DO NOT EXCEED THE VALUES ON THIS TABLE! Common automotive blade-style fuses exist at 5A-20A in 5A Increments. Ex: 5A,10A,15A,20A
Determining the Total Amperage of your Circuit
So you just bought your stuff at Oznium and are getting ready to plan your installation while USPS gets it to your door. One of the first questions to ask when planning your installation is what size wire to use, which will later determine what fuse to use.
Don’t worry if you’re lost, you’re at Oznium, we’re helpful here.
Current is measured in Amperes, abbreviated to Amps or just the letter A. Because of the low-current nature of the products at Oznium, most of the products, and the table that I’ve developed, have current listed in milliamperes or mA for short.
1A is equal to 1000mA
To find the total amperage in your specific installation, refer to the table below. Find the items that you are installing and their current requirements. Add the values and divide by 1000 to get your total current in Amps. You can this use this value in the table above to determine the minimum wire size required.
Here’s an example.
Say you bought a cold cathode kit for each side of the dash (2 Transformers), 5 superflux LED’s for your vents, and a 4.7″ Flexible LED strip for your center console.
If you want to put all of these on a single circuit, you’ll need to know the current. Based on the table above, each transformer draws 700mA, each Superflux LED draws 80mA, and the LED strip draws 80mA
If you add up (700*2)+(80*5)+(80*1) you would get 1880mA total.
Divide this by 1000 to come up with 1.88A.
Put 1.88A into the top table in this post. That table tells you that you should have no smaller than 21 gauge wire for your circuit.
Personally I would go with the 20 gauge wire and a 2.5A fuse.
Hopefully this guide helps you install all the products here at Oznium quickly and most importantly, safely.
I’d like to thank PwrRngr for his help in working on this guide, as well as the general help that he does on the boards.
Anyone that needs additional information or has specific or more complex installations, please don’t hesitate to sign up and search, then feel free to post in the Installation area if you still have questions.
If I’ve missed anything or left anything out, please correct me either in this thread or through PM, and I will correct the tables.
The only thing I’d change is your comment on the fusing.
Having spoken to several electrical engineers, the suggested MAXIMUM fuse rating is no MORE than 50% of the maximum current load a wire can handle.
So, for example, your chart lists 12 gauge wire’s max amperage as 18.6A – that means that you should never use larger than a 10A fuse TOPS (9.3 to be exact, but that’s not a standard fuse size) – less is even safer. This will assure that, in the event of a short, the fast-acting fuse pops well in advance of the wiring insulation even beginning to warm up, protecting you from a fire.
Also, when choosing a fuse, remember that in a DC circuit, using a fuse too close to the expected current load will in effect create a bottleneck in the current (resistance point), which can stress the electrical system, possibly leading to damage to the battery or other sensitive components.
According to my multiple engineer sources, the best thing to do is calculate the expected draw across the circuit and then multiply that number times 1.5 to determine the fuse to be used.
So, for example, let’s say you want to add two 72″ flex strips to your underbody. Each flex strip rates 1200ma (1.2A). 1.2A X2 = 2.4A X 1.5 =3.6A, rounded up to 4A. Since a 4A fuse might be hard to come by, a 5A fuse will be fine. Just be sure to use wire that can handle at least 10 amps (twice the max value of the fuse), to be sure the wire can handle the load safely.
Clear as mud?
Expected accessory(s) draw X (times) 1.5 = suggested fuse size for the circuit.
Fuse rating X 2 = MINIMUM current rating of the THINNEST piece of wire in your circuit. Thicker wires are always OK, as thicker wire can tolerate more current load.
I’m just relating what 3 different electrical engineers have told me about not only the safety-factor of the DC wiring you should use in a car, but also how that wire affects the entirety of the electrical system. I have no idea of these same strictures apply in AC wiring (used in houses).
I’ve used these very conservative figures in all the wiring on my (very, very computerized) MINI and I’ve not had a blip despite all the lights I added, and when the MINI certified techs looked the car over (the last time I had it in for scheduled service before crashing it), they complimented me on the thoroughness of the set up.
In contrast, I’ve heard literally dozens of horror stories form people that used either a fuse or related chassis wiring rated too close to the draw of their accessories, and who ended up with smoking wiring or, worse, an electrical fire. Bottom line is that if you always err on the side of being very conservative, you won’t likely have unexpected surprises down the road. The final choice of what parts to use is, or course, every individual person’s choice – it’s your car payment.
Me, I’m leaving myself PLENTY of overhead for safety reasons.
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