It doesn't work this way; the device draws the power (current) it needs, the wiring supplying that power needs to be of adequate size to be able to supply it without burning up.

Think of it this way... the typical home toilet uses a 3/8" plumbing connection to receive water to fill the tank. If the pipes in the wall are smaller than 3/8" in diameter, the tank will be slow to fill.

Most home plumbing in the bathroom area (at least in my experience) is 1/2" in diameter, this should be adequate to feed the toilet and a certain number of other outlets (faucets, other toilets, whatever) down the line (elsewhere in the bathroom/house) BUT ONLY TO A POINT, the half-inch pipe can flow only so much (the equivalent would be current in electricity, measured in amps).

If the pipes in the wall are, say 1" in diameter, the toilet isn't going to fill any faster, it can draw only what it can draw through a 3/8" tube.

So, having an oversize wire won't supply more power than the coil can normally use... an undersized wire will overheat as the device is still going to draw what it needs.

~~

Fuses are to protect the wiring, a fuse will blow if more current than it can handle is being requested (say, from a shorted circuit or failed component).

 

Good description.

Sorry for the late response, but we are on vacation. In fact, we still are and I won't be able to get my head around this for a few more days.

 

Chris makes a good analogy.
A device will not be harmed by too much current, only too much voltage. (potential)
You wouldn't want to plug a 12V device into a 120V circuit but we use all kinds of things in our home like a 3A drill in 15A circuit all day every day.

Skip White sells inexpensive HEI's on Ebay by the thousands. I've installed a few of those...
They seem reliable if you follow the instructions but have big problems with spark scatter and misfire if you starve them for power.

If you search for "ready to run distributor" MSD comes up, and Summit -does- sell them.
More money than I would pay for an HEI, but people buy stuff for the name all the time.

 

Sigh. Please read what I typed fellas. I was worried that the 30 AMP FUSE is too bloody big...I have been doing aircraft and automotive electrical work since 1994, I am well aware of the fact that bigger is better with regard to wiring...arbitrarily choosing a fuse size...not so much.

Chris, Gary, and Ardwrk, totally agree with almost everything you are saying...just pointing out that your explanation is pointed the wrong way.

The 1 thing I disagree with is that the fuse is to protect the component...unless you are running wiring that is just barely adequate to needs, then the fuse is to protect the wiring. Since I usually go heavier than needed, the widgit the wire is connecting will be crispy critters long before the wiring melts....if the fuse is too big.

ArdWrk, Thanks for the tip, I will look Mr. White up! I know MSD makes HEI's, what I meant was that you kept referring to an MSD, and I was wondering what I missed, since I did not see it in the post anywhere. The 2 brands of HEI I have run to this point are DRAGON FIRE and DEMOTOR...DF was from Ebay, Demotor from Amazon.

Cheers, and Happy Labor Day Folks!

 

Absolutely correct on that, fuses and CB's are sized to protect the wiring NOT the device. It does no good if you have a short in the wiring and the fuse is so big that wiring goes red hot and melts before the fuse blows or the CB trips. That is how fires get started. You can under size a fuse or CB for a given wire size to protect a device no issue, and usually even then the limiting factor for the fuse or CB size is the device's internal wiring.

The minimum wire size you should use for a 30 amp fuse at 12V is 8 gauge AWG. Also as the length of a run increases wire size must go up. The reason for this is, as the length of the run increases, volt drop increases, thus increasing amp draw. Increasing wire size reduces the volt drop.

For example for 30 amps at 12V if the run is 15' and under the minimum wire size is 8 gauge AWG, if it is over 15' the minimum wire size 6 gauge AWG.

Under sizing wiring for a given CB or fuse is a sure fire way to start a fire if you get a short in the wiring.

I included a chart form the engineering hand book for sizing wire at 12V hope this helps.

 

I already said that I am quoting the instructions from the manufacturer of the units I've used.
(and that I do not know what MSD calls for)

As for wire gauge and derating it, as above...
We *all know* that every gram of copper the manufacturers can pare out of the harness they will.
(just look at the headlight switch wiring in these trucks)
10 gauge solid or 12Ga stranded is 30A rated to 105C in every book I've ever read.

 

As for wire gauge and derating it, as above...
We *all know* that every gram of copper the manufacturers can pare out of the harness they will.
(just look at the headlight switch wiring in these trucks)
10 gauge solid or 12Ga stranded is 30A rated to 105C in every book I've ever read.[/QUOTE]

For AC 120V 30A 10 gauge is correct. NEC and CEC Electrical Code states 10 Gauge regardless if it is solid or stranded for 30A.

Some well regarded sources state that you can use 18 gauge for short runs for 30A at 12V in automotive which is ridiculous since the max load capacity of 18 gauge is 19 amps in a chassis application.


You can't always believe everything you read.


Also many charts and references do not state if the ratings are for open air or chassis (enclosed space) wiring. If they do not they can not be trusted to be accurate.
Automotive is considered chassis wiring.
10 AWG is the absolute minimum you should use for chassis wiring for 12V at 30 amps and even then only for short runs 3-4 feet after that you should be stepping up to an 8 gauge. In open air you can go as low as 16 for runs 2 feet or shorter. It is not just the carrying capacity of the wire but also the environment it is installed in.
The maximum ampacity for a #12 in a chassis application is 30amps there is no safety margin using a #12. #10 is rated for 40 amps.


Wire sizing has been up rated for 12V in Automotive applications since these trucks were built. This why we rarely see burned and crisped wires and connectors in newer vehicles as the older guidelines were marginal at best.


You will never have an issue over sizing a wire. But you certainly will by under sizing it.

 

So, I stated the manufacturers recommended installation for the HEI style units that I have installed is 10Ga fused at 30A.
All replies agree that you can't run one off the existing coil+ wire.

I'm not sure how there is any question or problem with that.

 

Thanks Gary, I am sure I am just over looking something stupid. Enjoy the rest of the trip.

 

Figured I would post a more recent picture of her. I was able to drive her around some today, transmission shifts out like it should and seems to run well. I'll get the oil changed tomorrow and transmission fluid with filter as well.




Took her to the fire station to wash the funk off of her.

 

I give up. Ok, almost.

ArdWrk, I already noted that I was glad your instructions stated to use a 30 Amp fuse...mine had no instructions on the size of the fuse to use. I am not questioning it in the slightest. What I was saying, is that PRIOR to your post, I was mildly concerned that I had used too big of a fuse when I wired my system up. All I had handy at the time was a 30 Amp fuse, so I used it. ie, I will not be changing the fuse out for a smaller one..

Matthew, I will comment this. If you oversize your wiring, then your fuse protects the component, not the wiring, since the component will fail before the wiring, IF the wiring is heavier than the bare minimum required.

Hope y'all had a wonderful Labor Day Weekend!

 

Maverick,

She looks pretty dang good!

 

Thank you, looks better without all the green funk mildew and so forth. Still have a long way to go but she is getting there.

 

 

David - What are you trying to say with the last post?