Automotive electrical question..

[Junkman]

I need to do some repairs to my 1963 Chrysler, where the original ambulance body builder had added lights and wiring. My problem is that I am not certain how I can establish the amount of current draw of each circuit. Presently, there are 20 amp fuses protecting 14 gauge plastic coated wire. If I apply the rule of thumb of household wiring, that a 20 amp fuse protects a 12 gauge wire, and a 15 amp fuse protects a 14 gauge wire, I realize that the fuse it too large. How can I accurately determine the load on each of these circuits and make sure that none of them or drawing too much current for the wire size. I don't want to gamble that this car is going to go up in smoke because of the incorrect fuse.
As a side note, I have found that the modern day replacement glass fuses that are made in Taiwan or China, are worthless. They constantly blow where a USA Buss brand fuse will not, even though both are the same amperage. Got to find me some new old stock fuses at a good price.

 

[Gary S]

In looking at original car wiring diagrams, I see that the factory did sometimes use some #14 wire and fuse it at 20 amps. Remember that car wire is stranded wire which has higher current carrying capabilities than solid wire does. I'd think that if the factories did this, you can too.
The best say to determine circuit load will be to put an ammeter inline with the circuit with all accessories running. That should give you the total load. Once you know that, you can wire and fuse it the way you want.

 

[rinny_tin_tin]

I need to do some repairs to my 1963 Chrysler, where the original ambulance body builder had added lights and wiring. My problem is that I am not certain how I can establish the amount of current draw of each circuit. Presently, there are 20 amp fuses protecting 14 gauge plastic coated wire. If I apply the rule of thumb of household wiring, that a 20 amp fuse protects a 12 gauge wire, and a 15 amp fuse protects a 14 gauge wire, I realize that the fuse it too large. How can I accurately determine the load on each of these circuits and make sure that none of them or drawing too much current for the wire size. I don't want to gamble that this car is going to go up in smoke because of the incorrect fuse.
As a side note, I have found that the modern day replacement glass fuses that are made in Taiwan or China, are worthless. They constantly blow where a USA Buss brand fuse will not, even though both are the same amperage. Got to find me some new old stock fuses at a good price.

Its more than the conductor cross-section that determines ampacity-- Insulation for example will impact current rating of a conductor. However, even if you were to apply the 1999 NEC Table 310-16, 90 degree C Cu 14 AWG cable is rated for 25A, while table 310-18 shows TYpe Z @ 34A. It depends on insulation type. You may want to see if SAE has standards that covers automotive conductors. Also, fuses trip much faster than CB's
- so it doesn't sound like you are in dangerous territory.

 

[jjkrjh]

Junk, after reading you post, I couldn't tell if you were questioning wire size or how to figure out if it was going to let the smoke out. Might just be me being up past my bedtime.
I would first take some amp. readings. Most multimeters only have a 10 amp. capacity. The clamp accessories are generally pricey. I found a clamp on DC amp/volt meter at Sears for $29.00 that actually has been very accurate. Couldn't find it on their site but it is similar to this one.

http://www.sears.com/shc/s/p_10153_12605_03482369000P?vName=Tools&cName=Electrical+Shop

Once you know how much is being drawn, you could make repairs as required.

 

[ddawg16]

The amp specs for AC (buildings) takes into account that you are running a lot longer distances than in a car. The specs are basically based on acceptable voltage drop. The shorter the run, the less the drop.

In a car (truck), the wire lengths are a lot less than a house....so the voltage drop is a lot less...I think you will be ok.

The following Link gives some examples of max current for wire size and chassis vs transmission. It also shows the max freq for each wire size.

This wire size link has some actual #'s associated with dc wiring for autos.

Regarding skin effect....not really and issue for cars....DC....quoting from the above link:

Skin effect is an effect that the electricity in high frequencies does not use the whole condictor area. High frequencies tend to use only the outer parts of the conductor. The higher the frequency, the less of the wire diameter is used and higher the losses. Skin effect must be taken care in high frequency coil designs.

 

[Torque1st]

Some data from deep within my accumulated files...

For automotive use:
AWG-Fuse
18-15
16-20
14-30
12-40
10-50
8-70
6-100

Similar to the NEC do not load any wire beyond 75% of the capacity listed above.

Personally I like to use about 50% of the listed fuse ratings which coincidently brings the wire ampacity down to about the NEC rating.

 

[fourfeathers]

We used one of these the other day. Maybe it'd help? Not sure if they have them for glass fuses though. http://www.sjdiscounttools.com/esi306b.html