Most people get caught up on fuse wire size but once you get the hang of it, it shouldn’t cause too many headaches. This is an in-depth fuse tutorial explaining everything you need to know about fuses and how to size a fuse. We’ll go into all the relevant details and what it all means so you’ll go from novice to pro in no time at all.
Why do you need a fuse?
The main function of a fuse is to protect your wiring, but in order to do so, you’ll have to get the right fuse wire size from the start, too low and it’ll blow, too high and you’ll end up doing damage to the whole circuit!
Things could get really ugly really quickly, so to avoid all that mess you’ll need to ensure that you get the right size fuse for the job every time. For your fuse to do its job correctly and protect your wires, it should be rated about 1.1 to 1.5 times the rated current value. It’s also a great idea to get an ATC fuse holder to protect and mount your fuse.
One common misconception about choosing the correct fuse size is that it’s dependent on 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.
How to calculate fuse rating
For those of you who want to get right down to business, let’s not waste any more time, here’s how you’d go about calculating fuse size correctly in 3 simple steps:
- Determine the wire gage you already have by locating it on the package or simply by measuring it.
- Use the following table to determine the maximum current for whatever wire gage is being used.
- 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.
eg. 1A is equal to 1000mA
To find the total amperage in your specific installation, refer to the table below.
Find the items that you’re installing and their current requirements. Add the values and divide by 1000 to get your total current in Amps. You can use this value in the fuse size chart 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
Add it all up..
(700*2)+(80*5)+(80*1) = 1880mA in total.
Then..
1880mA / 1000 = 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.
If I’ve missed anything or left anything out, please correct me through PM, and I will correct the tables.
What is fuse rating?
You’ll usually find the fuse rating on the side of your fuse, which will be given in amps. Fuse rating is the amount of current needed for the fuse to blow or break. When this happens, it stops the electrical power from flowing through the electrical circuit.
Why are fuses rated?
The fuse rating is a valuable piece of information because it helps you to protect your electrical circuit and therefore should never be neglected. Every electrical circuit will need a different amount of electrical current, what is just right for one electrical circuit may be too much or too little for another. Do the right thing and protect your circuit.
Hopefully this guide helps you to install all the products here at Oznium quickly and most importantly, safely.
Anyone who needs additional information or has specific or more complex installations, feel free to get in touch or post a question below.
Read common questions and answers on ATC Fuse for LEDs
I have a question what size fuse in my car car should I run with a 150 W LED high beam and low beam Just want to know what size I should be running to get the most out of my LED headlights whatever information you can give me that would be helpful thank you
I’d suggest a 15 to 20 amp fuse
Amperage is determined by W/V, so if the wattage is 150 and you are running it on 12 volts, it is 150/12. Or 12.5 amps, so I wouldn’t use over a 15 amp fuse.
Watts ÷ volts x 1.2 = fuse size in amps required as a general rule.
Living off the grid, as I do, in the Mohave desert, groping joe has forced me to go to an electric refrigerator and freezer. I have fried several solar controllers and now have 1060 watts with 400 watts of wind turbine and six 29 series dc batteries.
Yesterday I connected my 1500 watt freezer inverter to a 400 amp relay set to come on 13.0 volts. This morning it blew a 25 amp fuse but but held on a 30 amp. I may have to use a circuit breaker instead of a fuse. Your thoughts please.
If I understand correctly, a 1500W inverter has the POTENTIAL to draw 115A from a 13V supply. a 400W load will ask a little over 30A from the 13V supply, not allowing for efficiency losses. Therefore any irregularity of the load circuit could cause a spike in the inverter draw to kill even a 30A fuse. This may be mitigated by using a delay type 30AT fuse, but really the wiring will be tested if it is not specified for the 30+A.
So if I need a in wire glass fuse that has 12v- 0.1a what is the size should buy
I recently bought:
• Flush Mount LED Bolt Cool White, Aluminum (1W), 11 mm (4 pairs) = don’t require fuses.
• Marine LED Navigation Light – 112.5 Degree Boat Nav (1 pair: green & red) = don’t require a fuse.
• 12v 10ah lithium batteries (2).
But my 4-pin (12v, 30a) rocker switches suggest 20a fuses. All my wire gauges are 22AWG. Your advice is “ For your fuse to do its job correctly and protect your wires, it should be rated about 1.1 to 1.5 times the rated current value.” and 22AWG is max rated at 1.84. Do I have the wrong rocker switches? I’ve already bought the switches and fabricated into my boat so I really like to use them.
Your detailed advice and instruction is greatly appreciated. Thank you in advance!
Firstly, thank you for your purchase of our LEDs for your boat.
Both the LEDs and rocker switches do not require fuses. They will light up and function just fine without a fuse. Your switches will also function just fine without a fuse.
That being said, it is best practice to install a fuse on your circuit.
The fuse will protect your wiring in case of a short circuit.
If you don’t have a fuse, and there’s a short, your wiring could melt and cause a fire.
If you do have a fuse, and there’s a short, the fuse will blow before letting too much current through your wires.
You can disregard the recommended fuse size of 20a for your rocker switches.
To calculate the best fuse size, you must first figure out how much current your LEDs draw.
The rocker switches themselves don’t draw any current (except for maybe a negligible amount if there’s an LED inside).
Your LED bolts draw about 37 mA @ 12VDC, so 4 of them draw 37 x 4 = 148 mA
Your LED navigation lights draw about 125 mA @ 12VDC
148 + 125 = 273 mA
273 mA = 0.273 amps
You’ll be fine with a 0.5 amp or 1.0 amp fuse.
Without knowing more detail about your rocker switches, I can’t say whether you have the wrong ones, but it sounds like they are OK.
Thanks Phil for your wonderful and detailed explantions! It’s been very confusing because everyone has a different take, so I really appreciate from manufacturer’s answers. So these fuses should be wired from the battery lines? And not from the devices?
You’re most welcome 🙂 Yes, it can be quite confusing, but I’m happy to help.
You should put the fuse(s) as close to the battery as possible on the positive wire.
For this application, you only really need one fuse.
You should not run ANY SIZE fuse for your 150 watt LED or to “get the most out of it”!!!! You install a fuse to PROTECT the WIRE!!!!!!
That means FIRST you determine what size WIRE you need based on the Length of the wire run to the 150 watt lights. If there is some pre installed factory wire and you have changed the lights. The FUSE size for that Factory wire has already been determined and should NOT be increased! If the wire shorts out and has too large a fuse installed it could melt the insulation right off the copper conductor and get so hot that it could start something on fire! If I misunderstood what you were asking I apologize. I just think it’s important that NO ONE thinks a fuse size is rated only for the loads it it supplying power to. That is only one part of the considerations of properly fuse protecting a circuit.
The proper size is 15.
I’m not sure if I agree 100% with the way that calculations are being done here. For one, a 12V battery isn’t always going to be at 12V. I’ve seen battery voltages on vehicles fluctuate between 10V-13V. So if you take the worse case low battery scenario (10V), then I think that you would actually get a 15A draw RMS. but then your LEDs will draw a little higher when you first switch them on (Peak vs RMS), so if you multiply by (square root of 2 or ~ 1.41 rough estimate for peak) then you actually get about 17.1A… So if you’re blowing 15A fuses with a 150W LED setup, then I think it would be better to try a 20A fuse with wire and relay sizing to match.
The following formula will give you size fuse you need. ( watts divided by volts times 1.25 )
So Norman Keene if i want power a 150 watt small heater and I divide watts into volts in my scenario 12 volt my math comes out to 15.625 amp fuse ?would I use a 15 amp fuse or go to 20amp fuse.
With your calculations, 15.6 amps. Always go to the next higher fuse. I generally go to higher fuses when my amps are 1/2 to 1 amp below fuse being used. Surges can cause fuse to blow.
I tend to agree but that start up in rush current isn’t usually considered as fuses tend to have a tolerance for start up current. Voltage drop in a 12 volt circuit is more relevant & larger cables than required is always a prudent choice ( then fuse to suit that larger size )
What size DC fuse should I use to power a 400 watt DC to AC inverter?
Assuming you’re powering it with 12V, you should use a 35A fuse.
And 35 amp capable cable.
I am installing a 20 amp max marine waterproof 12VDC socket above my rear bumper for accessories such as a tire inflator (15 amps). The cable run from the battery is approximately 15 feet.
1). I want to ensure no power loss to plan on running a 10AWG wire. Is that too much, too low, or about right?
2) Does the inline fuse go near the socket or the battery? MIght install separate fuse box in the engine bay, so can add more accessories later.
10AWG wire will be just perfect for your application. It’s best to install the fuse as close to your battery as reasonably possible. This way, if there’s a short in the wire or something draws too much power, the fuse will blow before the wire has a chance to overheat, melt, and start a fire.
My question is
How we check fuse rating ok,
Like if our fuse is 40A and on the fuse have written 40A by manufacturer but how we check it is 40A
I think may be that fuse 50 and 30 A so plese provide answer
To check whether a fuse blows at a certain current, you need to run different currents through it until it blows.
I have a similar situation as a previous poster. I want to install an additional 12VDC 15A outlet in my car. Besides wiring to the car battery, I’m to tap in my under-dash fuse box’s rear power outlet using a Add-A-Circuit fuse tap. I believe it’s rated for 10A but many people on the internet have use 15A fuses. Anyway, the pigtail with crimp is usually a 16AWG wire. I’m also going to be using 2 15A fuses with the Add-A-Circuit tap.
My question is what is the proper gauge wire to use if the cable run is about 6′-7′ from tapped 15A fuse to my center console, between my from seats. I’ve read, prior to finding this website, that the wire gauges people used ranged from 12AWG-18AWG, According to your chart, I should be using a 12AWG, which a lot of people say it’s overkill.
The inquiry I get from other people is what is the new outlet going to be used for. I just say it’s going to be used the same as the other 2 factory installed use. e.g. phone charging, small fridge.
The outlet covers say it’s a 12VDC 120W Max. I’m kinda rusty on my electronic principles but given P=VI > P=12 x 15 > 180W. confused. TIA for your quick reply.
Hey Noland, If the outlet cover says 12VDC 120W max, then that means you shouldn’t plug in devices that consume more than 10 amps.
So instead of a 15A fuse, you might want to use a 10A fuse.
Because the length of the cable run is relatively short at less than 10 feet, you’d be safe using a 16 gauge wire.
I have a customer who has a Chinese made track hoe. He needs a 65 amp fuse. I can’t find the actual fuse he needs. Will a 65 amp fuse cause a problem in a 60 amp fuse holder with 8 ga. wire? He has to use 65 amp fuse.
It should be OK to use a 65 amp fuse.
So Im running a Fuse Relay Block that has 6 relays those 6 relays will power 6 different LED Light Bars for off road.. I calculated all 6 of those Led’s Watts.. lets say 1500 watts together. The Fuse Relay Holder has a Positive and Negative side. What Fuse do I use to from the BATTERY TO THE POSTIVE SIDE OF THE FUSE RELAY BLOCK? Thanks 🙏🏽
If your LED lights bars draw a total of 1500 watts, then I suggest a 150 amp fuse.
Calculation: 1500W / 12V = 125 Amps, but you should add a bit of a buffer (about 20% more), so a 150 amp fuse is perfect.
So I am trying to connect 2 x 55w lights on my car, would I add the two together and do the math that way? First time build so I am trying to make sure I don’t do any shortcuts. Any recommendations for wire size would be appreciated as well.
2 x 55 watt lights on a 12V system will draw about 9 amps.
I suggest 14 gauge wire, and a 15 amp fuse.
Thanks!
So im going to fit 4 x 3.6w halo led lights on my car. Can you recommend what size wire and fuse please
4 x 3.6 watt lights on a 12V system will draw about 1.2 amps.
I suggest 18 to 20 gauge wire, and a 2 amp fuse.
I want to run 2 x 400w LED lights. What size of wire and fuse should I use please?
Ive just had an electritian use 12 gauge wire on a 12v set up on a RV im building. I have calculated the amps for the fuses so thats all o (i think) but is the 12 guage wire a problem. Thanks
It depends how much current (how many amps) you’re running through the wire.
most appliances im running are under 10 amps
12 volt
Eighteen LED lights, .5 watts each, 24 AWG, plus two additional led lights .5 watts 22 AWG
Using two flasher relays, half the lights on each for a sign.
A rocker switch for each half (10 led lights, plus flasher)
At times only one side will be lit, other times both at once.
Can use a single 1 amp fuse?
Can 18AWG wire be used to run them in series?
Thanks
If you have 20 x 0.5 watt lights, that means your total power draw is 10 watts.
10 watts at 12 volts is equivalent to 0.833 amps, so yes, a single 1 amp fuse should be fine.
You can use 18 gauge wire.
It doesn’t make sense to wire them in series. You should wire them in parallel.
I have a 120 VAC 60 HZ 12 W transformer with an output of 15.95 VDC 8.4 output to charge a battery that has 13.2 V battery. I would like to know what size inline fuse to use to protect the charger from over heating burning out the transformer if a shorted battery is plug in.
I have a 22 inch 96W light bar with a relay wire harness With an in-line fuse and an on/off switch. It has now burned up 2 different relay blocks. Any idea why it’s doing that, and how to prevent that? I have the red wire hooked the + side of the battery and the black wire grounded to the frame under the hood.
I’m trying to add a 12 volt plug to my truck bed. I’d like it to be capable of drawing atleast 10 amps. Was thinking of using 10 or 12 gauge wire but my concern was the connection between the add a fuse wire(which all seem to be 16 awg) and the 10 gauge wire. Will this be an issue since the wire will go from 16 down to 10 right out of the circuit?
I am installing a pair of OZ Amber Mini Trail Lights on my motorcycle. Each lamp is 10 watts. Run is less than 5 feet. From this blog, I assume that on a 12v system drawing 20 watts is equal to 1.666 amps. Is 18 to 20 gauge wire, and a 2 amp fuse correct? Thank you.
Im runing 2- 30 Watts led spot lights on 20 awg wire directy from 12volt car battery what size of fuse do i need.
Thanks.
Have a question.
A 12v domestic R solar electric has three 10w lamps and 30w black and white television.Determine the;
a) Size of a fuse required.
b) Amperes hour consumed if the daily use in 3 hours(assume system losses are 15%)
I bought three 1 watt screw led amber lights from you and wired with an add a fuse to my fog lights. What size fuse should I use for the led
Each light draws about 37 mA @ 12vdc, so thats a total of 111 mA (0.111 A)
A 0.5 amp or 1.0 amp fuse will work fine.
I’m installing 4 small auxiliary lights on my Jeep as rock lights. Wire size 22 gauge. Each is .05a @ 12v. Going to rocker switch. Do I use a 20amp fuse? And what size relay? I have a 40 amp relay, is that ok! Thanks
0.05a @ 12v sounds quite low. Please double check the actual current draw. Perhaps you mean each one draws 0.5a @ 12v?
If each light draws 0.05a, for a total of 0.2a, you can use a 1 amp fuse.
If each light draws 0.5a, for a total of 2 amps, you can use a 3 to 5 amp fuse.
A 40 amp relay will work fine.
Thank you very much. They are .05a each x4.position pods by alpena.I will use 1amp fuse.
I have installed a 12A 150W light bar installed. It’s hooked up to a switch box with 7.5A fuse… I’ve used it for about 5 mins and it hasn’t blown. According to this, I should use a 12.5A fuse minimum. Is my math correct? Why hasn’t my current 7.5A fuse blown yet?
Its possible your light bar draws less than 150 watts of power. 150W could be the maximum theoretical power, but the manufacturer may be driving the LEDs with less current. One way to find out the actual current draw is with a multimeter / amp-meter.
The voltage of car’s battery fluctuates from about 11.5 to 14 volts, depending on whether the car is running or not.
So at 11.5v, 150W will draw about 13.04 amps
And at 14v, 150W will draw about 10.7 amps
If we assume your “150W” light bar only draws 100W (it might even draw less!):
At 14v, 100W will draw about 7.14 amps (and your 7.5A fuse might not blow).
There’s no harm in keeping the 7.5A fuse. If it blows, you can always replace it with a higher rating (I’d suggest about 15A).
I was thinking about adding a 6 space fuse panel to my truck for small amp accessories. My thought was an 80 amp split circuit relay to power the accessories when key is on the length of the run is no longer than 4 ft. The relay I was looking at has size 12 awg wire. My question is, is this possible and if so what size fuse should I use to protect the circuit from the battery and what is the smallest amperage should I use the close the relay
I am planning to install Hazard Lights on a Classic car. I have obtained a perfect wiring diagram to achieve this. This diagram shows an in-line fuse to protect the circuit. The wire required should be able to carry a current to flash 4 lights each containing 21W bulbs. I plan to make any wire connections using 6.3mm male and female spade crimp connectors.
What gauge wire should I use and what size fuse should I use for this wiring?
Thanks in advance for any help you can give me.
I bought an ARB air compressor which I’ve mounted under the hood of my jeep. I used the supplied wiring harness which has a built-in fuse ( close to 12 volt battery), and also a pre-wired relay (plug & play ). The remaining harness ends in the cab at the supplied rocker switch. There is a thin red/yellow wire (18-20 gauge) attached at that end of the harness (close to rocker switch) which the instructions indicate to connect to an ignition source so that the compressor can only be switched “on” while engine is “on” which is what I want. I want to connect the wire to the cig. lighter in the fuse box under the hood. The existing cig. lighter fuse is a 20 amp. I have a fuse tap to use here, but wondering what size fuse to use.
I have a slightly different application. I plan to use a Watersnake trolling motor on my 17.5′ Grumman canoe. For the sake of balance, my 105 AH battery (heavy) will be mounted toward the front, about 13′ linear away from the transom. By the time I route wire along the gunwales and transom, that will be more like 18′ of wire. My motor draws 15A max at 12V. I have calculated that I should use 4 gauge wire (Amcor or equivalent) to be conservative and minimize voltage drop. I am strugging with choosing a fuse and holder. I would think 20A would be an appropriate value, but all of the fuse holders I have found use on the order of 14 ga wire to connect. Isn’t that going to reduce the the capacity of the entire circuit to what it would have been if I used 14 ga wire for the full length? Or doesn’t the calculation work that way? Also, in this application, my understanding is that the fuse is intended to protect the motor as much or more than the wiring. I think that concept is also the basis for the USCG fusing requirements, although that could be incorrect. Sorry for my ignorance, I have a decent working knowledge of 110V AC household wiring, but I’m finding the crossover of that knowledge to 12V DC power electrics to be lamentably lacking.
Great table, thanks for the article.
I fully understand the relationship between amps/volts/wire-size/fuses/etc., and this gives me the table needed for DC/auto applications. What I don’t know is the rating of the fuse box itself – does that work the same way as a household break panel?
This could be off topic, but if I use a fuse tap with an existing 5 amp fuse, can I put in my own, say 20 amp fuse (assuming the fuse tap is rated for 20+ amps) and still use the existing 5 amp fuse for the existing circuit?
I have two whip lights. Wiring diagram says use a relay if you use a rocker switch with a 10 amp fuse. What gauge wire should I use?
I’m wiring a 30 amp 3 prong toggle switch to power my 1200 watt car audio amp. I want to put a fuse in between the switch and the 12v power source. What size fuse would be appropriate for 12awg wire? Thanks
I have a small fan in my bedroom that stopped working. The instructions say to not use more than a 5 amp fuse. I put a 2.5 amp fuse and the fan still doesn’t work. I used the 2.5 amp fuse because my other small fans use them and I thought that since the directions said not MORE than a 5 amp fuse that the 2.5 amp fuse would work. Does this mean my fan is kaput, or should I order 5 amp fuses? Thank you for any advice.
Hi, I have a model railroad my DCC will be 3.5 A, so I was wondering what size in-line fuse I should use to protect my DCC control system and my train I have 14 gauge wire runners my bus duct hoping you can give me a couple pointers
I bought a panel with 2 USB chargers and a 12V cigarette lighter jack. The USB chargers are 5V @ 2.1A each. When I’m charging my laptop it draws 4.6A through the jack. There is also a small led display showing voltage and a rocker switch. The whole thing comes with a built in 10A fuse but the wiring is only 18AWG. If I’m using all the ports at once it blows a 5A fuse. I’m going to need to rewire the whole panel with larger wires aren’t I?
I’m running a 6awg power wire ftom 12v battery to a power accessories block to run accessories lick lights and such the the same thing for the grounds. It’s about 5ft each of 6awg. What size fuse do I need?
In need of some help, currently in the process of sending power to a relay from an ignition fuse tap to power an accessory fuse block, what size fuse should I need to power the relay? The relay will be on a 12v circuit with 1.8 watts at 80ohms resistance.( Relay is a 4 pin 120amp rating )
I hv 80 pound solenoids to pop the doors on my 1992 GMC Sonoma. The other day my driver’s solenoid got stuck & started to smoke. I rushed to remove the fuse that I installed that came with the kit. It was 25 amps. Y didn’t the fuse blow? Instead it started to fry the Circuit. After I removed the fuse, that didn’t ever blow, I could see how the inside of the fuse holder that 1 side of it had been fried some. Can you comment on this for me please?