Simple question. I'm wanting to protect the nearly 15 feet of battery cable to the starter with an appropriate fuse. Is 300A adequate for starter plus engine harness power? Is 400A needed?

The engine is a 5.3L iron block gen III truck engine with a stock starter. The engine harness power is coming from the post on the starter as well. So engine power needs to pass through this cable and fuse as well as starter current.

The restomod project I'm working on has the battery relocated in the trunk. It was in the spare tire well, on the right side, just behind the rear axle. It's been pushed even further back when the spare tire well was removed to clear a path for the dual 2.5" exhaust tailpipes.

I'm hoping to use a Z case fuse at the battery terminal. The holders for those are available up to 400A, although 300A ones are more readily available. If more amperage is needed, I could go to a Mega Fuse type.

 

Putting numbers to my dilema, I can go with this fuse holder from LittleFuse, available on Mouser for $23.76,

LittleFuse 0FHZ211Z

plus a couple of LittleFuse ZCase 400A fuses (one to use, plus a spare) for $13.80 each.

That's all rated for 400A max current. The total with shipping is $59.35


Or, if 300A is adequate, this from amazon is available for $21.95, for the holder with the fuse. For an apples to apples comparison, add $10.95 for a spare 300A fuse from the same Amazon source.

If I need 400A, then the first option is acceptable. But if 300A is enough, it's tough to justify nearly three times twice the cost.

 

If the fuses protect the wire, then they should be sized according to the wire size, not the load current.

Otherwise, when the worst-case scenario occurs (such as, the wire shorts to ground, like it chafes through or something), the fuse won't blow to protect it.

Of course they have to be big enough not to blow from normal load current either, but then the wire has to be big enough for that; then the fuse should be selected.

Pretty sure you'll find the peak starter current to be something in the 100 - 150 A range. From a copper wire table, #0 wire as an example, has very close to 0.1 ohms per 100', or, 0.0001 per foot; at 150A, that would give you about 2.25 W of heat dissipation per foot at normal full-load current. The amount of heat that would damage the wire needs to be determined, then the fuse chosen so that it would blow before enough current could be allowed to flow long enough for that to happen.

Other power demands like lights or fans or whatever shouldn't matter; long-term, the alternator will be supplying those loads, not the battery. Their current will only be passing through the batt cable if the alt dies, or if you run them alot while the engine isn't running.

 

Thanks for the reply, @RB04Av

My concerns are two fold here. The fuse must be high enough amperage to supply power to the starter plus enough power to run the PCM, ignition, fuel injectors, and everything needed to start the engine, plus any accessories that are turned on and not on the "off when starting" power. The wiring system in this car is a mess, and a lot of stuff that should automatically go off when the starter is activated doesn't power down. That's another issue for another day.

The fuse must also be small enough that if the big battery cable shorts out, the fuse blows before the insulation on the wire catches fire or melts, and before the water/acid in the battery boils and the battery bursts. This should protect against shorts from chafing, shorts from collision damage, or the odd mistake by the mechanic changing the starter without remembering to disconnect the battery because it's out of site, out of mind back in the trunk under a floor panel.

I get all that, and we're probably about on the same page here.

Your post lead me down a rabbit hole that sprained my brain. I'm seeing conflicting information on the amperage capacity of the 0AWG (1/0AWG) cable I'm using. It's SAE J1157 rated/certified, and it's commonly used as welding cable, battery cables, inverter power cables, and in some battery backup systems for solar arrays and wind power systems.

Welding supply places say it's good for up to 350A at a total circuit length of 100 ft. at duty cycles up to 60% (and ambient temperatures up to 105°F/40°C). I've got about 15ft of the stuff, so I'm well within the length there. The duty cycle on the starter approaches zero (a few seconds on, a half hour or more before another burst), so I'm way under the 60% duty cycle. After the engine starts, charge current will be under 145A (rated alternator output) in the other direction, and well under the starter (plus ...) draw, but that current will be present as long as the engine's running.

But, other sites list 248A as the maximum continuous current for "chassis wiring" and 150A to 170A for the maximum continuous current for "power transmission." I'm pretty sure neither of those apply to my situation.

I also know that on modern (recent model year) GM vehicles, GM uses a 400A fuse on the starter cable (often 2AWG or even 4AWG, although usually significantly shorter lengths than I have). So presumably a short on a smaller cable is enough to blow a 400A fuse, or why bother with that?

So I'm pretty sure that the starter load won't burn up my 15 foot long 1/0 cable. The voltage drop should be well under a half volt (just guessing). And I'm pretty sure a short circuit along that cable will blow a 300A fuse before the cable will reach the melting point of the insulation or the battery contents boil and explode. For the other direction, for charging current, this should get the battery up to voltage much more reliably than the 2AWG/8AWG wiring that's there now. I'm upgrading from 2AWG to 1/0 AWG for the run to the starter, and from 8AWG to 4AWG for the run from the alternator to the starter.

My big question was whether the load (starter plus engine harness power plus potentially some accessories) would blow a 300A fuse. It sounds like your best guess-timate is it probably shouldn't. I was thinking the same thing, but not entirely certain. Even if your guess is low, and the starter pulls 200A, that leaves 100A for the rest of the stuff that needs power to get the thing running and the alternator spun up.

 

Peak starting motor current varies WIDELY with ambient temperature, oil viscosity, dynamic compression ratio ( cranking compression) as well as mechanical condition of engine and the starter MOTOR itself.
A warm V8 smog motor with a PMGR ( reduction gear ) starter with 5W30 can roll over with as low 150 amps A high compression, stock cam motor in cold weather with heavy dinosaur oil can pull 600, 700 or more amps for a short instant until the crank shaft gets up to speed.... .

Generally; STARTER MOTOR CIRCUITS ARE NOT FUSED.
A master disconnect switch is reasonable, but any fuse you might select, sufficient to protect the starter cable from melting will add UN-Acceptable resistance to the starting circuit.
It is NOT done. There is no rational reason to fuse a starting motor circuit.

 

I've never seen...ever...there be a fuse to the starter.

I highly doubt 400a would be enough and fail to see any reason to have one.

 

^^^^^^^THIS^^^^^^^
The ONLY thing between the power source (battery, relay) and the starter should be one healthy cable. THAT'S IT.

 

Thank you both for your input. But really, HAVE YOU LOOKED AT A NEW CAR OR TRUCK LATELY???

Every new car on the showroom floor at the local Chevy dealership has a fuse for the starter. So does every vehicle at the local Chrysler/Jeep/Dodge/Ram dealership. Fuses on all circuits (including the starter) are now required by Federal Safety Standards for all new vehicles sold in the USA. It's been that way for a few years now.

For "hot rodders" moving the battery to an alternate location, protecting the longer battery cable(s) with fuse(s) is just a really good idea™.

I ended up using the 300A fuse setup from Amazon, but my spare fuse is from LittleFuse. It turns out that the LittleFuse Z Case fuses for 8mm posts are specifically designed to be "starter fuses." They aren't quite as slow as a "slow blow" fuse, but they don't blow instantly from overcurrent either. The 300A "starter fuse" can pass over 500A for up to 10 seconds, and 900A for a full second without blowing. The Current-Time curves for the "starter fuses" from 300A to 500A are shown here:



The full data sheet for those fuses is at https://www.littelfuse.com/media?res...mega-datasheet

As for "any fuse you might select, sufficient to protect the starter cable from melting will add UN-Acceptable resistance to the starting circuit," the resistance for the 300A starter fuse is 0.13mΩ (0.00013Ω) according to the data sheet. That's about the same resistance as adding 1 foot to the length of the 1/0 pure copper cable I'm using. Not really significant.

Wow. You really wouldn't like this car's new setup.

Battery terminal -- Battery fuse block -- 300A Starter fuse -- 2 ft x 1/0 pure copper cable -- pass through junction block with 3/8 copper threaded rod -- 11 ft x 1/0 pure copper cable -- 3/8 junction block post on firewall -- 1.5 ft x 2 gauge pure copper cable -- starter.

Even with all that, it's performing acceptably. When battery is fully charged (12.8V at the battery terminals), I see 11.7V at the battery posts while cranking (internal resistance of the battery causing that drop) and 11.3V at the starter post. I'm losing about 1/3rd the power in the cable system as I'm losing inside the battery itself. I consider that excellent performance. The thing cranks fast and starts just fine.

 

Your observed 11.3 volts AT the starter under ideal conditions with a fully charged battery in SUMMER is very far from your stated " Excellent performance"
You have custom built a marginal starting system.
Expect failure to crank in cold weather, and potential sluggish operation during heat soak, HOT engine conditions.

 

For good reasons, and I'm obviously not the only one.

 

Starters have a huge range of current draw depending on starting conditions. It's impractical to try to use a fuse or circuit breaker. And the resistance of those devices would lower cranking speed quite a lot. So then how do engineers protect unprotected circuits? The answer is they take great care in cable routing and clipping to protect the cable, and use SGX cable with really tough insulation. Basically, it's safety through mechanical protection.

Starting battery cables are not sized for current because the current draw is way above what the cables are rated for continuously. Cables are sized by allowable voltage drop so that starter can perform to spec. General guideline is resistance of cables and all connections in entire starting circuit should not exceed 0.0017 Ohm for 12V systems. That's a general guideline to help people succeed. In rare cases the cable might be upsized due to high temperature rise over time, but that's usually satisfied by default when following the resistance guideline.

There is a real hard fast requirement though that the starting system resistance (including battery) not be too small, or else the starter will be damaged by the in-rush current and torque during start up. That's all engineering tech spec stuff that is specific to each starter motor and not public info. But from a practical standpoint, you'll be fine as long as you use batteries with stock CCA ratings. One of the big mistakes people do is grab the highest CCA battery that will fit in the car.

 

To the bold stuff, I had all of those issues, plus another issue (battery wasn't fully charging when driving) before I installed the fused (larger) starter power wires.

Before, I was getting 10.4V at the starter while cranking. So I'm almost a full volt better than what was in the car before, plus I don't have to worry about the starter power cable chafing at a grommet, and I've got fuse protection on the whole system.

In my experience in the past, anything over 11V measured at the starter is usually good enough to crank. You are correct that right now, late summer/early autumn is close to "best case" for starting and charging systems. However, you should also note, my battery voltage drop (measured at the battery terminals, not at the starter) is most of the voltage drop now, and that voltage drop seems to indicate a possibly weak battery. Before any real cold weather returns (we don't get really cold in here GA), I do plan to get the battery tested and make sure it's up to the task. It may be somewhat "damaged" from the prior issues with the charging system and starting system.

The car has been on the road for a couple weeks now with the fused and upgraded power wiring. The car owner/driver is very happy with it and reports that all of the starting issues have been resolved (for now). I'll be keeping an eye on things, and I'll definitely get the battery out and get it tested soon. If it's weak, it'll get a appropriate battery for the truck that the engine/drive train came from.

 

Fuses on starting cables is a hot topic in the world of Pro Touring, where the battery usually finds it way to the trunk for weight. Same applies to race cars, or performance builds in general. I totally get the discussion. The positive cable in the event of an accident, that found itself being cut or pinched, would short out and the fuse becomes the battery in that case. Without a doubt this would cause a fire and you’d likely have a car burnt to the ground. The ongoing argument has always been however…if the battery cable has been compromised to the point of damage, the vehicle is likely cut in half or bent so badly that it’s a total loss anyway. This of course IF you have ran the cable in a safe manner utilizing grommets and proper strapping. Common sense really goes a long way here. I have no problems with fusing the cable myself, but I find it unnecessary if you’ve done your homework. I make my living with electricity so I understand the ins and outs of this discussion. While most automakers are fusing the power cable, not all of them are. Even some that have the battery in the rear.