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Old 10-30-2014, 12:56 PM   #1
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Fuse for house battery setup?

My 2005 E350 has the factory aux battery installed and I am planning on wiring it up for my house system. I ordered up a distribution block and plan to install a couple extra 12v aux outlets and some led lights for the interior. The biggest draw will be a fridge that will be plugged into one of the aux outlets. I also plan to buy a small inverter to plug into one of the outlets as necessary to charge a laptop or power other small/light duty things.

My plan is to run 4ga wire from the aux battery into the rear of the cabin and put the distribution block in the passenger rear corner (compartment where the jack and accessesories are). The distribution block will obviously have fuses to each of the power points, but do I also need a fuse between the block and battery? I'm guessing yes, but want to make sure. I'm thinking a 50amp fuse will be sufficient for my setup. If I should go more or less, please advise.

Also, does anyone have any good leads on some LED light fixtures? Im looking for something low profile and bright.
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Old 10-30-2014, 06:32 PM   #2
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Re: Fuse for house battery setup?

Yes you need one. Not sure a 50 amp is the rout size. My 120v to 12v converter/distribution box has a 30 amp fuse for the battery.
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Old 10-30-2014, 07:35 PM   #3
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Re: Fuse for house battery setup?

I'm no electrical expert, but this is what I have done (mostly on boats):

1) The first step I take is to size the cable. I do that by first figuring out the round trip distance of the wire (you have to count both ways in DC), then the amperage that it needs to tolerate, and also take into account the permissible voltage drop of what I'm powering (usually either 10% or 3% on boats depending on the item, but generally the less the better). There are then tables that show what size cable you need.

2) Once the cable is sized, then the fuse is sized to protect the cable (so figure out what it would take to "start the cable on fire" and then protect at a lower amperage).

This way you don't end up with either larger than necessary cable (expensive and can be a pain to run), or a too small one. Or a fuse that is too large to protect the cable, or too small to withstand the load(s).

I would have a fuse as close to the battery as possible (or even on it, see Blue Sea MBRF). That said, I actually don't like a lot of individual wires on my battery terminals, so I run a cable to a positive bus and go from there with anything that needs to run "to the battery post" (cable(s) are still fused though). On the negative side, in my case, cable goes to shunt for battery monitor and then to negative bus (so again, no "collection" of cables on the battery post).
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Old 10-30-2014, 08:19 PM   #4
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Re: Fuse for house battery setup?

I went to a website that has a calculator to size the cable. I could get away with 8ga wire, but I figure I would run 4ga to be safe. It seems that most wires nowadays aren't true to their size and run a little small.

The biggest amp draw will be a fridge (to be purchased at a later date) which would spike at around 5-6amps at startup. The only other item that might draw more than a couple amps would be a small power inverter used to charge a laptop or something. I imagine that wont be more than 4amps. So if everything were to be on and drawing max amps, I would guess that it would be right around 10 or 11 amps. Does that mean I should fuse the 4ga wire at around 30 amps?

I'm not sure how to calculate "start the cable on fire". But since I'm not pulling high amps, I'm thinking this shouldn't be too much of a concern. Each power point will be fused at the distribution block as well, so hopefully that will be a first line of defense against anything major.
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Old 10-31-2014, 01:07 PM   #5
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Re: Fuse for house battery setup?

I like the circuit wizard on the Blue Sea Site, they also have a Phone App that is handy, and for the record wire size has not been getting smaller, other than better insulation's may make it appears that wire has been getting smaller.
http://circuitwizard.bluesea.com/
If you mouse over the individual items the help will give you a good description, of what it is, so I won’t go into details there.
  • • Circuit Voltage - is pretty obvious, we are working on 12 Volt systems
    • Load Current – amount of current used by the load
    • Length of conductor – This is the total length of the circuit, so add both the Positive and Negative wire , in many cases on vans we will see the ground wire go to the frame. In the van I usually assume that the length is at least twice the positive run. So if we run 10 ft of positive cable and 10 ft of negative cable, the total length would be 20 ft
    • Allowable Voltage drop – This will have more effect on the wire size then load in many cases. A standard is 3% if you are wiring for a critical load and main circuit. 10% for other loads
    • Type of load, Variable loads are ones that may supply motors, fuse panels , 12 volt receptacles
    • Wire Insulation Temperature Rating- Better wire – Better rating, this may also allow you to use a higher gauge wire. In most cases if you choose Marine Wire, it will be 105C, if you go with off the shelve auto primary wire you may see 60C
    Additional Derating Factors.
    • Duration – length of time current is passing through load. I would use this if I know I have a pretty heavy current draw. A 12 volt air compressor may be good candidate here.
    • Terminated on Fuse – in most cases I answer yes
    • In Conduit or Sheath – running wire in conduit or bundles of power cables together will creat more heat.
DC Circuit Protection Selection
  • • Battery (CCA)Cold Cranking Amps- A lifeline group 31 will have a CCA of 600 amps at 0C, a 4D will have 1100 Amps at 0C, you should be able to find CCA specification for your battery
    • Main or Branch- A main would be considered a wire going from the battery to the fuse box or fuse panel. If you direct wire a load to the battery, it could be considered a main circuit. The Branch would be from your fuse panel to load
You need to remember that this will produce fusing requirements so that you don’t burn the wire, or use it as the fuse. The individual components that you are wiring to may require a smaller fuse for protection. This also does not take into account any charging devices to the battery, that would be a different requirement.



Ok here we fill it out and, I put down 20 Amp Load, and 20 feet of circuit, 3 % voltage drop (this is a main circuit), fixed load, you sound like you will have receptacles downstream. I also choose 105 temp rating, this would be marine wire, of a good Automotive wire. I also added the additional derating of being terminated on a fuse.
Now let’s look at the output of the calculator. The recommended wire is AWG 8 or 8 gauge, this size is really derived from the voltage drop of 3% requirement, If I change that to 10% I would get Voltage drop requirement of 14 AWG. In either case the voltage drop is the determining factor in the wire size chosen.
Circuit Wizard output with voltage drop at 3%

Circuit Wizard output with voltage drop changed to 10%

Circuit Wizard output with Voltage Loss 3% and insulation temp rating at 60 C.


Now let's look at the fuse or breaker suggestions . Note The Minimum AIC is derived from the battery I chose, which in this case was a group 31 with a CCA of 600 at O C.


Hope this helps some, feel free to follow up with questions.

-greg
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Old 10-31-2014, 01:18 PM   #6
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Re: Fuse for house battery setup?

Our van has a circuit breaker mounted right next to the house battery (not a fuse). The main purpose of the circuit breaker is to trip in case the main cable shorts to the frame. A short like that could cause a fire and a lot of damage. So the breaker or fuse just needs to be sized based on the amp capacity of the cable itself (and of the distribution block). And it needs to be mounted close to the battery in order to protect the cable.

I would recommend sizing the main fuse and the distribution block for more amperage than you think you need, just to account for future additions.
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Old 10-31-2014, 01:39 PM   #7
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Re: Fuse for house battery setup?

Greg,

Thank you for that thorough explanation of everything! Pretty much everything you entered to derive the 8ga wire is correct. I was planning on using 4ga wire just to be safe, but I really don't ever plan on running anything more than what I described. I'll probably head out to a local stereo shop and get some good copper wire and get the ball rolling. I still need to order up a fuse holder and a fuse for the main line, but now that I know what I need, I can do that.

Jin
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Old 10-31-2014, 03:22 PM   #8
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Re: Fuse for house battery setup?

Go plug your data in to the circuit wizard, the fuse / Circuit breaker recommendation does not show all of the Blue Sea data, most of their breakers were below that. For the Main circuit .I concur with dhally a reset-able breaker would be the preferred choice, and yes closer to the battery is better. It would not hurt to design for extra capacity, from the battery to the fuse panel. They key thing for the breaker is that it is of a sufficient value to protect the cable It must be lower value capacity of the cable, and it must have a high enough AIC value to protect itself from the battery. If you would put the 1100 CCA rating for the same circuit, you will notice that some of the fuses/breakers are not recommended because their AIC is to low.


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