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How to Determine the Fuse/Wire Size for YOUR Project

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mx107marlin

Joined: Aug 12 2007
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Post Thu Jan 24, 2008 11:42 pm

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 itís 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.

User posted image

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.

User posted image

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.
Tdawgthegreatest

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Post Fri Jan 25, 2008 12:27 am

Very nice. and very helpful icon_smile.gif
PwrRngr

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Post Fri Jan 25, 2008 8:30 am

sweet, nice write-up. I think it should be a sticky
ImagoX

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Post Fri Jan 25, 2008 12:54 pm

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?

Just remember:

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.
PwrRngr

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Post Fri Jan 25, 2008 1:54 pm

ImagoX wrote:
Just remember:

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 going to flat out disagree with you on saying the wire should be able to carry 3 times the load. A factor of safety of 3, that's outrageous and unheard of. I'm not saying you can not do it your way, it's just WAY overkill.

If you don't believe me look at the wiring in your house wiring. It has wire running much further than 15 feet (after that wire gauge should get thicker depending on length) and is almost dead on to this chart. Actually, I have 10awg wire running to my stove which is on a 30A breaker. Also, for a 100W speaker I don't see anyone running 10awg speaker wire. If you want to further discuss, PM me so MX107's thread doesn't get jacked.


Last edited by PwrRngr on Fri Jan 25, 2008 1:55 pm, edited 1 time in total
ImagoX

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Post Fri Jan 25, 2008 2:05 pm

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.
Passions

Joined: Jan 12 2008
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Post Fri Feb 01, 2008 11:54 pm

Anyone know what the interior lighting stock wiring on a car is?

I looked at most cars and I believe it is 22 gauge for the interior lighting wires.

I think using a set of cold cathodes, 700mA, I can get away without using a relay. Anyone agree?
robbie

Joined: Apr 23 2006
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Post Mon Feb 04, 2008 12:39 am

Ok so mx_marlin what you are saying is that if i am running 18 gauge wire to my fuses in the fuse block that i should use a 5amp fuse correct? If that is the case try and let me know via pm so i get it easier because as it stands i have a pack of 10amp but if i need to take them back and get 5 amp i will thanks man by the way i will be running 5 switches off of 4 fuses in the block, would this be safe and if it is whats the most practical setup. the switches are as follows.

1.Inferno
2.LED strips (7 9.5 inch) (2 4.5 inch) (1 28 inch)
3.Single LEDS (18:)
4.Strobes in the headlights and Foglights
5.3 cathodes in the engine bay
blue one

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Post Fri Feb 29, 2008 8:23 am

great info.
ImagoX

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Post Mon Mar 03, 2008 8:25 pm

Damn this thread is handy - I've referenced it like 3 times just this week. MODs - howza 'bout givin' a bruther a sticky?
mx107marlin

Joined: Aug 12 2007
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Post Mon Mar 03, 2008 9:49 pm

Haha... thanks... I feel like I've contributed to the forums now biglaugh.gif
ImagoX

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Post Mon Mar 03, 2008 10:08 pm

Gratz, Marlin - now you're "Oz l33t". Have a ducky on me --> ozzy_change.gif
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