Help me understand auto electric please!!!

[litevette]

As I do different projects on my car, I keep noticing the major need to rewire some things. Add to that how I read on here about you folks modernizing your electrical systems and you can understand why I want to tackle this. I would greatly appreciate any and all help in understanding the following questions, and hope that you take into consideration that I know NOTHING about wiring, electricity, wire guages, amperage, etc. Please speak as if you are trying to explain to a child! Oh, and by the way, if you don't have all of the answers that's o.k. Help on tthe things you can please!

1. If I rewire something, and want to use relays (I've read on here that they are good to use to avoid "voltage drops" to the item being powered), where would the fuse go? Before the relay? After?

2. In the above relay situation, what powers the relay? If for instance I wanted to add relays to my lighting system, would the power from the headlight switch now just power the relay? Would I need to fuse between the headlight switch and the relay also?

(for questions 1 & 2, I've pasted below diagrams I made. Which is correct if either one is? A or B?)

3. What size or guage of wire needs to go to the various components? Should I just look at the old wire size feeding the component and match it in size? Are there certain wire sizes that are universal for say guages, headlights, horn, fan, etc?

4. If I use terminal blocks to feed various components, and there are two compnents running off the terminal block, would the wire feeding power to the block have to be double in size of the wires feeding the two components? For example: The two components need 10 guage wire each, would the wire feeding the terminal need to be 5 guage? ( I was told that the higher the number the smaller the wire, so I'm geussing that a 5 guage is double the size of a 10 guage?) SEE FIGURE C

5. Can a switch, a relay and the component all share the same ground source? Any reason why not? SEE FIGURE D

6. Anywhere you don't want or need to use a relay? Could I use one everywhere? (not that I would).

7. What size fuse for what component? How do you determine this?

[TnBlkC230WZ]

Lots of questions.

1. Relays are meant to handle high current loads while having a small current switch.

2. Voltage drop is a function of load in amps on X size wire over Y distance. The longer the wire run, the larger the wire needs to be to maintain a specified voltage level. You can run 60 amps on a 16 guage wire for 3 inches with minimal voltage drop, but try to do if for 10 feet and you'll start a fire.

3. Relays are good for headlights with factory harnesses because the factory feeds the entire 60s and 70s era cars with a 10 gauge wire. This allows lots of voltage drop as the electrical load in the car goes up. My factory harness had 11 volts at the headlights.

You could use a relay to power everthing. Modern cars use lots of relays. When to use a relay is dictated by your wiring design. There really are no hard rules around their use. They are cheap and provide lots of flexibiltiy. Anything that is controlled by a low amp switch will need one.

4. The size wire feeding your fuse block is dictated by the over all load. You need to list what will be on any given wire and figure the current draw of each item. Add them up and you and use that to figure your wire size.

You need to look at this two ways, max load and nominal load. You may want to limit voltage drop to one volt nominally, but can allow a two volt drop at max load. Let's say you have power windows. Their draw has to be figured into the max load, but they are used infrequently for short periods so they don't really impact the nominal draw.

Custom wiring harnesses are great because you can build each circuit to meet the demands of your car. My critical cirucits such as head lights and fuel pump are built for less than 1/2 volt drop from the end of the sense wire. All other circuits have less than a 1 volt drop. Aftermarket panels are good because they handle higher loads but don't taylor each circuit based on load, length and wire size.

5. Fuses protect wiring, not devices. It is ok to use a smaller fuse if the device permits, but you never install a fuse that is larger than what the wiring can handle.

In theory, you can place the fuse anywhere in the circuit, even the ground wire, and be protected. I recommend and usually do place the fuse as close to the power source as practical. The power source is usually the altenator or battery. The reason is if a power wire gets chaffed and goes to ground, it will be protected.

6. Yes, the switch, relay and device can share the same ground source. If you gound everything to the chassis, they do share the same ground.

[litevette]

Thanks so much for the excellent response. I think I need to consult with you some as I go along. Is that O.K.? I'm currenty involved in some other projects on it, but hope to start to tackle the wiring soon. I just want to make sure I get it correct, make it dependable and not have to do it more than once!

[nullshine]

One thing to remember about wire gauges:

The gauge sizes do not increase proportionally, a 5 gauge wire has approximately 3 times the cross sectional area (current carrying ability) as a 10 gauge. Therefore, two 10 gauge wires do not equal a 5 gauge.

For cross-sectional area (typically measured in circular mils):
For every gauge size down (larger size) multiply by 1.26.
For every gauge size up (smaller size) divide by 1.26.

If you know that a 10 gauge wire will safely handle 10 amps over a certain distance, then a 9 gauge should handle around 12.6.

Here's a handy chart:
http://www.powerwerx.com/wiregauge.asp

Also remember that you won't typically find any odd numbered gauge sizes being supplied, just even.

Sorry if it's too advanced, but later on you may find it useful.
Calculating resistance:
R = K x L / CMA

Calculating voltage drops:
Voltage drop = K x I x L / CMA

R = Resistance in ohms.
K = Ohms/mil-foot (use 10.4 for copper, 17 for aluminum, which isn't common.)
I = "Intensity". In otherwords, amperage.
L = Length in feet
CMA = Circular mil area, as listed on the chart above.

Resistance in a 30 ft. piece of 10 gauge:
10.4 x 30 / 10,383 = 0.0301 ohms
Voltage drop in a 30 ft. piece of 10 gauge with 15 amps through it:
10.4 x 15 x 30 / 10,383 = 0.451 volts

These are all at temperatures of 20 degrees Celsius(68 Fahrenheit). At higher temperatures the resistances and therefore voltage drops will be slightly higher, but very little.

[TnBlkC230WZ]

Thanks so much for the excellent response. I think I need to consult with you some as I go along. Is that O.K.? I'm currenty involved in some other projects on it, but hope to start to tackle the wiring soon. I just want to make sure I get it correct, make it dependable and not have to do it more than once!

Sure, You can post your questions. If I don't get to it, someone will.

[LSx_88_Ciera]

To add to the already excellent replies:
If you don't know the proper size fuse for a component you can figure it by checking the resistance (ohms Ω) and then use ohms law to find the amperage draw.

 E
I*R
E=Volts, I=Amps, R=Ohms

Since we know that we are dealing with 12v we can find the amperage requirements with ohms.
For example:
a fan that reads .5Ω
12v/.5Ω=24A

A few things to know about circuit types: (see pic below)
Series circuit - Voltage and resistance are additive and amperage is constant.
Example:12V each bulb is 1Ω giving a circuit total of 2Ω (1+1) 6A (12v/2Ω)
If you read voltage after the first bulb you will get a reading of 6v (voltage additive) after the second bulb it will read 0v. This assumes a perfect wire with 0 resistance (hypothetical). No matter where you read amperage will be the same.

Parallel circuit - Voltage is constant, Amperage is additive and resistance is reciprocal (formula below).
Example:12V each bulb is 1Ω giving a circuit total resistance of .5Ω 24A (12v/.5Ω) each bulb will consume 12A (additive)

 Reciprocal 
(don't forget when adding fraction you have to find lowest common denominator)
1   +   1      2  invert  Rx
R1      R2    Rx           2

1   +   1      2  invert  1
1Ω     1Ω      1          2

Reciprocals confuses some people if you need/want a better explanation let me know.



Not all wires are created equal the finer the individual strands of wire are the better/higher current they are capable of handling. This is because the higher strand count has more surface area and electricity travels over the wire not through it.

[6'9"Witha69]

www.madelectrical.com

[crustysack]

just reading this as a NON electrical savy person( but not stupid) where should relays be placed?/ I would assume closer to the fuse block but does it even matter (my electrical ignorance) I have rewired my car with a painless chassis harness are there any critical area i would need to put relay in?Headlights,fuel pump,cooling fan,power windows,power seats, etc?? I have a 65 GTO with an 02 LS1 and as previously stated my electrical experience is small.

[Twentyover]

just reading this as a NON electrical savy person( but not stupid) where should relays be placed?/ I would assume closer to the fuse block but does it even matter (my electrical ignorance).

Doesn't matter, as long as the power feed to the relay is fused near the source (like at a terminal block). For instace, in one car I have the headlight relays in the headlight nacelles.

I have rewired my car with a painless chassis harness are there any critical area i would need to put relay in?Headlights,fuel pump,cooling fan,power windows,power seats, etc??

Any high load feature.mI run headlight, HVAC system, Engine cooling fans on mine. If I had power windows and seats,, i'd power them through relays

[litevette]

I think this could end up being a pretty good thread for those who are just starting to get into the electrical part of their cars and who desire more knowledge. Thanks to all who share their knowledge of this here. I also did a little searching on the net. Here are some links to some stuff I found that I beleive can also be of interest:

http://autorepair.about.com/cs/troub.../aa063001a.htm
How to use digital multimeters. Also, look at the bottom for links to "How to do basic trouble shooting".

http://www.autoshop101.com/autoshop16.html
This includes:
Electrical Fundamentals Electrical Circuits Control Devices Circuit Protection Variable Resistors Understanding Relays battery Basics Battery Services Understanding Alernators Understanding Logic Gates

http://www.nxtbook.com/nxtbooks/waytek/catalog223/#/6
Took this one off a thread here. These guys have every freaking electrical related connector, relay, tool, etc. you could want for. Also has some good explanations of how some of the stuff is to be used.

[JRouche]

This is a good thread. Car electrical systems can be intimidating for alot of folks.

For my car I use a relay for just about all the circuits. It may seem over kill, but as stated, relays are inexpensive and they do two functions.

One of the functions is to switch the high current loads (lights, fans, compressors, door lock solenoids, window lifts and any other higher current loads) remotely. And I say remotely because they are secondary switches. The primary switch is usually in the passengers compartment. Like a light switch, window lift or door lock switch. And the primary switch can be in the engine compartment for say a cooling fan.

A prime example of a remotely switched system that ALL cars have is the starter solenoid. The starter solenoid is just a big relay. Takes the low current of the key switch and turns the solenoid (big relay) on and off. And that relay switches the very high current for the starter on or off.

I like relays primarily for the safety issue. A small wire will carry the current for say the headlights. But if you replace the lamps with some newer modern lamps they will pull more current than the old lamps of years gone past. Brighter lamps mean a lower electrical resistance and more current. More current through the same wire means more heat. The wire will get warm, sometimes hot. The old covering on the wires may not withstand the additional heat. So it can fail then you get a short and possible fire if the fuse doesnt stop the additional current.

Now secondarily. I like relays for the reduced voltage drop on the lines. Specifically for lighting and fans.

I like relays for lighting because it stops the dimmed lighting issue. The lamp will see full battery voltage which allows it to pull all the current it needs to be as bright as you want it to be. When you run the headlights through the dash switch without a relay you are dropping alot of voltage on the lines and switch. All those electrical conductors act as resistors in the circuit. Each of them, the small diameter wire, the switch act as small little resistors in the electrical path. And a resistor in the circuit will drop a certain amount of voltage reducing the voltage that the lamp will see and consequently reduce its ability to pull the needed current it needs to glow at its brightest. The lamp needs all of the voltage from the battery to be able to pull the high current it needs. If you have dropped some of the voltage on the wires and switch then the lamp has a lower voltage and wont pull the high current it needs to shine as it was designed to.

Same with a cooling fan. It wont spin up to its designed speed due to a reduced voltage.

A relay can be put anywhere in the circuit. As long as it has the full battery energy on the source terminals.

Now the fuse blocks. They should be as close to the battery as possible. Because every inch or foot away from the battery they get is an unprotected line, the big, hot unprotected line that carries the current from the battery to the fuse block. That line is unprotected. Its a very "hot" line and unprotected. So you need to keep it short or very protected like some major shielding. You dont ever want that line to short. It is unprotected (not fused) and can burn the car down.

Yikes!!! I could go on and on. Taking about how I did my electrical system. But that would take days.

Summary... Relay the heck out of it. Use fuse blocks, I like them. For wire gauge?? Too many links to rehash that. Go large is all I can say. Its not that much more weight, these aren't airplanes. JR

[crustysack]

so for my headlight(pardon my ignorance) which are already wired up with no relay, I place the relay in between the positive wire coming from the headlight switch and the light and then I run another pos wire directly to the battery from the relay?? so the switch gives the relay the signal to turn on and the relay then draws direct from the batt?? if that is correct do i put another fuse between the bat and relay, and what relays should I get (brand,part#) thanks

[TnBlkC230WZ]

so for my headlight(pardon my ignorance) which are already wired up with no relay, I place the relay in between the positive wire coming from the headlight switch and the light and then I run another pos wire directly to the battery from the relay?? so the switch gives the relay the signal to turn on and the relay then draws direct from the batt?? if that is correct do i put another fuse between the bat and relay, and what relays should I get (brand,part#) thanks

You are correct. You definitely need a fuse. 25 amps on a 12 or 14 guage wire should be fine. For headlights, the cheaper Bosch and Tyco relays are fine. You can even run a micro relay so they need less space. The wire from the battery will go to pin 30 on both relays. The old head light wires will go to pin 85 of the high and low relay, pin 86 goes to ground and pin 87 will go to the high or low beam lights.

For higher loads, I only use Omron or Durakool relays.

[crustysack]

WOW I'm learning so I should relay the lights,the fuel pump,power seats,power windows and cooling fans,does that sound correct? can i also run single power wires and then split them for the windows and seats?? or do I need to run individual wires for every relay??

[TnBlkC230WZ]

WOW I'm learning so I should relay the lights,the fuel pump,power seats,power windows and cooling fans,does that sound correct? can i also run single power wires and then split them for the windows and seats?? or do I need to run individual wires for every relay??

You do catch on quick. The seats and the windows can use the same wire. Just make sure the wire and fuse can handle both.

[litevette]

I'm still following and learning too, so others stick yer .02$ in too! (If you've got something good to say that is!)

[crustysack]

soooo how about I run a 4 gauge wire from the battery (+)to fuse block (less than 2 feet)with a 50 amp for my ignition system( 02 LS1) then 8 gauge wire to that relay,(about 3 feet) 30 amp fuses for seats, windows,fuel pump, and 1 for each of the cooling fans all with 12 gauge wire for the 30 amps ( the longest run will be to fuel pump about 8 ft)- does this sound ok???