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What percentage increase is needed when it comes to using the correct fuse?
Posted by: OttawaCorvairGuy ()
Date: October 30, 2017 06:10PM

What percentage increase is needed when it comes to using the correct fuse?

Examples:

If I have an item on a single circuit that takes

1. a 9.16Amps to operate said item and with a 50% increase it would be 13.74Amps so I would round up to the next fuse size of 15A

2. Another item takes 10amps to operate and with a 50% increase would be 15A fuse?

How do I know if that is enough or the correct way to fuse each new circuit when one is rewiring their "project" car?


Thank you,
Tony

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: MattNall ()
Date: October 30, 2017 06:14PM

You basically have it!!

Incandescent lights and Motors have a "Surge" when first energized.. and the 50% covers that..usually..





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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: OttawaCorvairGuy ()
Date: October 30, 2017 06:18PM

MattNall Wrote:
-------------------------------------------------------
> You basically have it!!
>
> Incandescent lights and Motors have a "Surge"
> when first energized.. and the 50% covers
> that..usually..


So are you saying that 50% increase is the rule of thumb "normal"?

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: MattNall ()
Date: October 30, 2017 06:21PM

Pretty much so..





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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: Frank DuVal ()
Date: October 30, 2017 08:15PM

Remember, the fuse protects the wiring, not the object! Along with how much current is drawn, also important is how long it is on. Say a 10 amp load is wired with 16 awg wire. That is at the limit, but if the load just runs every now and then for a minute, you will probably be fine. But if it ran at 10 amps for 23 hours every day, wire might get warm. Not dangerous, just warm. I squared R losses, Ohm's law again.

In the NEC, for most multi load circuits, you design for 80% of the wire/breaker/fuse rating. i.e. 15 amp circuit, feeds lights and known loads (fans, etc). You should limit the known loads to 120 x 15 x .8 = 1440 watts. I know, this is a simple "DC" Ohm's law, but it is close with resistive loads. Now if you know a vacuum cleaer, portable lights,etc are possible to be plugged in (and they always are) do not load to 80%!

Frank DuVal

Fredericksburg, VA

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: MattNall ()
Date: October 30, 2017 08:44PM

Best pictorial around...





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Edited 2 time(s). Last edit at 10/31/2017 10:46AM by MattNall.

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: OttawaCorvairGuy ()
Date: October 31, 2017 05:50AM

Hey Frank and everyone else,

1. I just want to make sure that the FUSE I chose to use, for each individual circuit, is the best choice to protect the entire circuit.

2. That the Amperage will NOT be too high for that circuit - Allowing it to blew at the RIGHT time to save the item, the wires. In other words the entire circuit.

FYI - There is roughly 50 to 62 feet, if not more, of 16ga wire operating 19 Clearance, Park, Marker and Tail lights on one circuit.
This does not include the POWER wire running another 26 plus feet to the light switch.

I need, want, and plan to rewire the entire vehicle.

NOTE:

Because the runs are so long and the battery is so far away to begin with.
In most cases I plan to move up one wire size per wire or circuit.
Example: This lighting system currently uses 16ga - so I will replace it with 14ga.

I am also dividing the current circuit in half. To shorten the run, reduce the load / resistance.
There will be a front and a rear circuit. Putting them on their own Fuse.

BACK TO MY QUESTION - Is there a rule of thumb - a list or chart to go by?
Is a 50% increase a good rule of thumb? (Matt says it is)
I would rather be too low, with a fuse size, than way too high and burn the circuit up before a fuse blows.


We know that 14ga wire is good for a 20A fuse.
However, if I have 7 lights at .24Amps per bulb for a sub-total of 1.68 and 2 more lights at .59Amps for a sub-total of 1.18 or a grand total of 2.86Amps on that 14ga wire / circuit.

The above mentioned would be 4.29Amps with am increase of 50% or a 5A fuse.
One could even go with a 10A fuse if they find out a 5A fuse blows too easily - due to any surge.

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: vairmech ()
Date: October 31, 2017 06:06AM

Here is what I posted elsewhere to the same question.

If you "over wire" then you need to compromise. The best thing to do is look at any automobile and see what they have done. I just looked at my truck wiring and they have a 10A fuse for EVERY tail light and parking light bulb.
I have included my truck fuse block. If you look at the stop lamps that is a 25A fuse!
There is a lot to think about when you are designing a wiring harness. First you need the design, then you need what you are controlling and then you need to figure amp draw for each item on that circuit. With that you then need to figure wire size and the voltage drop for the length, size appropriately, and then what fuse you need to protect the WIRING! You need to do this in that order or you will get lost.
Look at this chart, it will take a lot of the guess work out of the wiring!
http://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: cnicol ()
Date: October 31, 2017 06:54AM

@Ken: That's a great chart! I've been using GM's "Best Practices for Upfitters" guide but the Offroaders chart is much simpler.

@Tony: You mentioned "the best fuse to protect the device". Here's another thought: When a device starts using more than normal current, it's already toast! The idea is to protect the wiring. Your "50% above normal" works for me provided that number is well within the wire's capability.

Craig N. Coeur d'Alene ID.
66 Black Monza 4dr, 4.2L V8 49k
61 Seamist Jade Rampside 140 PG
60 Monza coupe (sold, sniff sniff)
66 Sprint Corsa convt - First car! Re-purchased 43 years later
2+2 gnatsuM 5691

+17 Tons of parts

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: cepak ()
Date: October 31, 2017 07:24AM

In 1996 I believe I added a Crane optical ignition and a high output 12 Volt coil
to my 1968 VW Beetle. The controller unit was mounted on the firewall and the
optical sensor in the distributor. I never thought about how much current was
going to be drawn by the controller unit on the original wiring harness. It was
a 50 mile one way commute, and smoke started pouring in from the dash. The fuse
never blew. When I opened the trunk, the wiring was on fire. I was able to pull
the main 12 volt wire and beat the fire out with my shirt. Then I realized how
close the fuse block is to the gas tank in VW Beetles and thought, WOW this could
have been worse. Needless to say I got towed home and had to replace the whole
wiring harness ($99 from JC Whitney catalog).


Tom Cepak
DFW, TX

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: Demon-Xanth ()
Date: October 31, 2017 07:43AM

You can use a chart like this to get an idea of where your fuses should be based on the wiring, remembering that with curves and bends that a wire can be as long as 30' in our cars:


'vairless. But not forever.
Cars need to drive to be alive. One that never leaves the garage may be perfect, but dead.

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: OttawaCorvairGuy ()
Date: October 31, 2017 12:02PM

Thank you Craig,

BTW - I repeatedly said circuit. (I may have also said device somewhere)
But to me the circuit means the entire system. Which to me means the wires, as they are part of the circuit and complete the loop / circuit.

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: didget69 ()
Date: October 31, 2017 07:20PM



bnc

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: OttawaCorvairGuy ()
Date: November 01, 2017 04:47AM

ha ha ha






Too bad you can't read - fuse replacement is not the issue...

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Re: What percentage increase is needed when it comes to using the correct fuse?
Posted by: owen ()
Date: November 01, 2017 01:56PM

Thanks for that chart [Demon-Xanth], most of what I've seen before tops out at 25'.

The wires in an UltraVan are much longer than 30'. When I replaced the taillight wires I measured 37' from the turn signal switch to the right taillight. Fuse to the brake switch to the turn signal switch is probably another 15', not to mention ~30' from the battery to the fuse.

Owen



Edited 1 time(s). Last edit at 11/01/2017 01:57PM by owen.

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