circuit breakers

hey guys had a question. i got a 1965 mustang that a guy brought to me and it has a 120amp circuit breaker feeding the sound system. The system is fed by 2 8awg wires coming from the breaker. im wondering if there is a better way to do this. I dont remember 8awg being able to handle that much power. but i guess he really like circuit breakers so i was wondering if anyone could point me in the right direction to get a decent replacement. thanks guys

Do you mean the wire or the CB?

IMO a CB is better because it's quicker & cheaper than replacing blown fuses.

If you're worried about voltage drop, I wouldn't since you're getting ~0.12V drop per meter at 100A with 2x 8G and that probably outweighs any CB or fuse drop.
If it's a magnetically tripped CB then you (should) have the lowest protection voltage drop possible.

Trip times depend on the overload and trip profile which depend on the fuse and CB, but they'll be irrelevant as far as the cable protection is concerned.

I'd use heavier wiring if the load was around 100A (it shouldn't be any higher for 120 protection and cabling), but I design based on a maximum allowable voltage drop (typically below 0.5V) as opposed to wiring tables which are often a MAXIMUM wire capability (for safety etc) or may allow (eg) a 3V drop.

Based on this chart:

https://www.the12volt.com/info/recwirsz.asp

an 8awg wire can only safely handle 100A. Therefore, the maximum fuse / breaker size should be no more then 100A per 8awg wire.

If you have two 8awg wires connected to one breaker that breaker should only be 100A max - anything over could lead to a fire in the vehicle.

If you are running two cables ideally they should have their own circuit protection device - sharing an overload device is never a good idea. Take for instance, if the two cables are paralleled and one cable comes loose, you are now pulling all the current through one 8 awg wire that can't physically handle the power. When a wire can't handle the power it heats up until it melts the insulation and anything around it.

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Kevin Pierson

Thanks KP - I totally neglected the one of two bad connection scenario.
What's worse is that I thought 8G to be ~70A rated for chassis wiring (maybe from the powerstream.com AWG tables?)!

I honestly have no idea what the wires will safely conduct which is why I always make sure I link to my source!

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Kevin Pierson

I'm a big fan of traceability, and normally I do the same.
But ATM I'm posting from a "dormant" netbook because my PC & I are having some disagreements eg, HE won't let me login to the12volt.
Hence none of my usual links but I found the powerstream table easy enough.

I know others hereon had reasons for NOT using the powerstream tables that I occasionally referred to,. But for me - in practice - it makes no difference since I use resistance per length (per gauge) to decide what gauge and the various tables agree on those figures.

I have never used "wire capacity" tables other than as a check to not exceed cable ratings (ie - for litigious reasons), but I have always been well below tabled ratings - sometimes even for "free air" wiring (...and AC ratings).
Obviously my voltage drop allowances are more demanding than what the tables suggest - noting that most tables are max current ratings ie, for safety as opposed to good performance ratings (which obviously is length dependent).     

It does amuse me when the car audio forums suggest a cable gauge based on table ratings - totally ignoring the relatively long lengths to car boots/trunks etc.
In most other cases it's rarely an issue - eg, engine bay & cabin, or lighter loads.