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I am starting to wire up my 78 restomod with an aftermarket harness, 100amp alt ( for now, might go a bit more as the budget allows) and have 2 questions. I am planning on putting a breaker ( instead of a fuse) on the feed line coming from the alt to the battery at the battery end. ANy rule of thumb about how large a breaker. For example, 100 amp alt, is 150 amp breaker OK, not enough? Same question regarding the aux power block I am putting at the front of the car for the fans, elec headlight motors and a few other things. The feed line will go from the battery ( not the starter) to the block and I want to protect all of it at the battery. Add up the amp requirements and use that for the breaker, or a little more? Thanks.
Typically a breaker or fuse is used to protect the wire not the items being run.
So the size of the breaker is going to be dependent on the size of the wire you are running to the accessories being run.
Ie, if electric fans are being used and they pull 30 amps max at a start up then a 10 gauge wire may be run and 30 amp breaker used. If the wire shorts out between your power source ( battery) and the accessories then it won’t turn into a source of fire and will trip the breaker.
Add up the amp requirements and use that for the breaker, or a little more? Thanks.
At least 1.5x more. size fuses or breakers too close and they'll likely burn instead of tripping and you really don't want to be building a fire hazard into the car.
Last edited by lionelhutz; May 21, 2018 at 11:04 AM.
At least 1.5x more. size fuses or breakers too close and they'll likely burn instead of tripping and you really don't want to be building a fire hazard into the car.
The circuit breaker has to be capable of breaking the max fault current that can be generated in the circuit and in this instance that is the max current that can flow from the battery (this information is available from the battery manufacturer). So you have to size the circuit breaker not only on its running current rating but also its short circuit current capacity which is normally expressed as a kA rating (kilo amperes). Example a typical domestic circuit breaker will have a short circuit rating of between 1.5 to 6 kA or 1500 to 6000 Amps. I wouldn't use any automotive circuit breaker that is not clearly marked with a kA rating.
6 guage stranded wire is rated to 100 amps for the 9-10 ft run. So I also changed to a resettable circuit breaker which did not exceed the capabilities of the wiring (what I trying to protect) so I'm using this:
T Tocas 80 Amp Circuit Breaker Trolling with Manual Reset, 12V- 48V DC, Waterproof (80A)
The only thing I am concerned about in this 'circuit breaker' installation idea is where it is being installed.
Something about it being in the CLOSED battery box area just does not sit well with me. Others can feel about it as they like.
YES..I am also aware that the one chosen seems to be a sealed unit due to being waterproof.
No one needs to come in and tell me what GM put back in that area due to I know they have a fuel pump relay close by on the 1982's...and the ECM. The junction block is not an issue due to it can not spark. And the fusible link that comes of that wiring on a some 1980's is protected by the covering when it burns out.
But...like I wrote..it raises a concern....or at least I have to worry about it because if I was doing this I would HAVE TO be concerned due to being liable.
the first thing i wondered was why 150amp breaker was
chosen for a 100amp system.
like said, wires are protected is the idea behind fuse/breaker.
oh well.
Thanks for all the replies. Let me see if I can hit them all. I understand that the breaker is designed to protect the wire, not the accesories ( which have their own fuses). I haven't decided wether my feed to the battery or the aux power feed from the battery will run under the car or through the car to the engine compartment. My thought is to protect against any shorts that might happen due to me putting a screw into the wire or any other unforseen happening, like an accident. When I removed the old harness I was shocked at how many melted connectors, cracked wires and all kinds of nasty dangerous connections were there. So I decided to protect as much of this system as I can, and granted it might be overkill, but as the firetrucks arrive at the house I don't want to think if I had only added one more fuse or breaker...My feed to the battery will be an 8 or a 6 ga. My feed to the aux power box will be a 6 or a 4. I haven't finished adding all the accesories up yet. My intention is to protect the battery from shorting out through the alternator wire. I would think 100 amp alternator would need more than 100 amp breaker, but I am not an electrician which is why I am asking. 1.5% more seems like a reasonable number. I will do the calculations once I pick the wire ga. . Same for the aux feed, 1.5% more than the max draw, which would account for surges, fans kicking on etc.. That will determine wire size which will determine breaker size. As for mounting locations, I plan on leaving the battery box with only the battery in it. I plan on using the small center compartment for the breakers, ecu and some relays for the car interior. I am also using a Ron Francis wiring kit which has a drop down fuse box, mounted with a magnet anywhere you can fit it. When time comes to work on fuses etc, just pull it down and lay it on the floor. No more hitting the t tops with my old feet while my head under the dash. Thanks to you all for the feedback. I will follow up and maybe get some pix when I get this all done.
As a test I once ran about 500A through 10' or so length of T90 just to see what would happen. It took minutes to start melting the insulation and the wire didn't glow or anything silly like that. I personally wouldn't be concerned with using a 150-200A breaker on a 6 gauge piece of wire in a car, especially if you use quality wire with even better insulation than that T90 had.
Originally Posted by Haggisbash
The circuit breaker has to be capable of breaking the max fault current that can be generated in the circuit and in this instance that is the max current that can flow from the battery (this information is available from the battery manufacturer). So you have to size the circuit breaker not only on its running current rating but also its short circuit current capacity which is normally expressed as a kA rating (kilo amperes). Example a typical domestic circuit breaker will have a short circuit rating of between 1.5 to 6 kA or 1500 to 6000 Amps. I wouldn't use any automotive circuit breaker that is not clearly marked with a kA rating.
This has nothing to do with what I posted, but it is another good point. You probably should include some of the circuit resistance to get a realistic short-circuit current. After all, even 6" of wire from the battery terminal to the circuit breaker will have a significant impact on the short circuit current.
The melting I'm referring to is not if a short occurs. Run a fuse or circuit breakers right around their rated current and they might burn instead of properly clearing.
If you add all the power requirements and I stall a breaker that big, when something shorts it will melt down and fry anyway. Each wire has it's own load capacity. You put 5 10 amp wires on 1 50 amp breaker you have effectively no circuit breaker. Now, do you want a 100 amp breaker between alt and battery? Diodes usually pop open, but they could concievably short out, so that is not a terrible idea.
But...like I wrote..it raises a concern....or at least I have to worry about it because if I was doing this I would HAVE TO be concerned due to being liable.
DUB
Hi DUB,
You made me re-think the circuit breaker in the battery compartment. So I started reading more. Here is a good article for marine use but I think most of it applies to our corvettes:
DC circuit breakers may have to break very high amperages. In such situations, the points inside the breaker may arc over, and may fuse together. The ability of a circuit breaker to safely handle such a situation is its Ampere Interrupt Capacity (AIC) rating.
With DC circuits, the over-current protection is always placed in the positive side. Circuit protection should be connected as close as possible to the source of power.
Fuses, circuit breakers, and switches should not be installed in battery compartments because of the risk of corrosion coupled with the potential presence of explosive gasses.
ABYC E-11 recommends that each ungrounded conductor connected to a battery, battery charger, alternator, or other charging source, shall be provided with overcurrent protection within a distance of seven inches (175mm) of the point of connection to the DC electrical system or to the battery.
So I guess the breaker in the battery compartment is out. I don't like the idea of a breaker mounted under the car. I could just use the next size or two up in fusible links. What are others doing for high amperage protection when upgrading their wiring to handle larger alternators ?
I am glad that you researched this out and found this information.
SO..my GUT feeling was correct. Glad to know I can still count on that in regards to the initial location you chose for your circuit breaker being in the battery box NOT being the BEST location for it. Something about a potential spark and battery gases just did not sit well with me.
And not that it matters but I am a kinda 'over kill' kinda guy. IF a 6 gauge wire will work but it is on the edge of being at the limits. I go one gauge size larger wire myself.
I am glad that you researched this out and found this information.
SO..my GUT feeling was correct. Glad to know I can still count on that in regards to the initial location you chose for your circuit breaker being in the battery box NOT being the BEST location for it. Something about a potential spark and battery gases just did not sit well with me.
And not that it matters but I am a kinda 'over kill' kinda guy. IF a 6 gauge wire will work but it is on the edge of being at the limits. I go one gauge size larger wire myself.
DUB
Like they say, go with your gut feeling...So with my setup, 6 guage wiring is more than enough to handle 100 amps over the 9-10' run and I know that fusible links are typically 4 guage sizes smaller to protect the wiring, so I think I will just use the 10 guage fusible link:
This will protect the wiring correctly, keep a breaker out of the battery compartment, and still fulfill the requirements of being close enough to the source.
And not that it matters but I am a kinda 'over kill' kinda guy. IF a 6 gauge wire will work but it is on the edge of being at the limits. I go one gauge size larger wire myself.
DUB
No way- you into overkill!!! I know I'm not!!!
The old rule of thumb- if the wire is run in the engine compartment- thanks to the temperature- go one gauge bigger.
Note- Amperage ratings for the wires are at 70ºF AND not bundled.
I talked to one of my friends who is a very senior Hardware engineer who has done a lot of high amperage wiring. He told me to can the idea of a circuit breaker and instead change to a AGM battery (which doesn't gas off and requires no venting unless overcharged, i.e. above 14.4v, Note: Optima 78 batteries have a vent hose you can run outside the compartment) and go to a fuse such as:
He also said if using 6 guage which is rated 100-125 amps, the fuse should be above the wire rating and only blow if the wire has a short. He said the fuse can now go back in the battery box.
Anybody agree or disagree ?
Richard
Last edited by lakerider57; May 22, 2018 at 12:48 PM.
Choice of battery design...location of the wire to keep it away from heat...which we ALL know is not good for wiring...the gauge size...using a fuse, fusible link or circuit breaker...the choices and options are...what they are.
SO..I know me and I would go one gauge larger on the wire...due to possible RFUTURE wiring additions. Along with the fact that depending where it is routed...and if it going to be in some split-loon or not..... I feel better if it was larger.. Gosh almighty..it is not a big deal to go ne size larger is it??? Or am I missing something...OR...keep it as planned.
I KNOW I m not the wiz-kid on everything electrical...but I do it the way that has always worked for me BUT I am also willing to consider other peoples ideas and principles IF they seem to mesh with what I feel in my gut is right. Because I am the one who has his @ss on the line when I work on his stuff an I am liable.
circuit breaker protection
