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Electrical Discussion of preventative/corrective maintenance and other technical issues regarding your coach's electrical system.

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  #1  
Old 11-08-2009
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JackandLiz JackandLiz is offline
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Default HOW MUCH CURRENT?

Howdy.
Does anyone how much current an 8V92 starter draws? Does anyone how much current a 12.5 KW Power Tech generator starter draws?

I need this information to determine what size fuses I need to install. Where could I purchase such fuses?


TIA,
Jack
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  #2  
Old 11-08-2009
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Jack I just spoke with Lucas Willemse about yours and others comments and he's going to read them and post later today.
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  #3  
Old 11-08-2009
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Default HOW MUCH CURRENT?

Jack:
I know there will be more discussion on this, but this should help a little.

As per Tom Warner's post on "What did we learn from Randy's Barbecued Bird"
First to clarify the issue, fuses are installed to protect a specific device, over a specific wire size. You cannot fuse a cable higher then its ability to carry a specific current (amps). In the case of the 800 amp fuse it is to protect the BATTERIES which in turn could cause a severe hotspot that could cause a fire if you dont catch it in time due to the batteries high current capability.

Tom, the part in bold, you cannot say it any clearer.

As far as the 800 amp protecting the batteries - totally disagree.
If the batteries were to become a hotspot, short out internally or something fall on them, or whatever, the fuse will do absolutely nothing.
In order for a fuse to blow, it has to have current flowing through it.
The only way current can flow through that fuse is from the cable side.

On another thread, which I will post this as well, someone wants to know what size of fuses to install.
Fuses come in many varieties, slo blow, normal, quick blow etc.
On a starter, if the starter is rated for X amps, then the fuse can generally be 125% larger. In some cases, you can put in a smaller fuse and change it to a slo blow, which in turn allows larger currents to flow for a specified period of time.
(Normal fuse will generally allow a slight over amperage, as a quick blow is generally rated for the specific current with no time delay)
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Old 11-08-2009
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This is developing into an extremely interesting thread with great possibilities for membership info.
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  #5  
Old 11-09-2009
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Default HOW MUCH CURRENT?

Jack:, although this does not answer your question, I too am looking for the answers you asked about. I am putting this topic over here instead of under Travel Tips.


To all: I am concerned that maybe the wrong information is being given. As someone has mentioned, what works for me might not work for you. I would not want anyone to feel they are "covered" with a false sense of security.
Personally, I do not think that an 800 amp fuse does much good. I need more information regarding "HER" coach so I can do more calculations to determine what I will be using.


Fuse – what does it do? It protects the circuit. The circuit includes both the wire and the equipment ahead of it.
When designing a circuit, there are numerous things to take into effect:
  • Device or devices being used
  • Length of cables required
  • Voltage required, in our case it could be either 12volt or 120 volt.
  • Service duty of the equipment being used
  • Type of cable being used
  • Location of cable being used, temp, enclosed etc

Location of Fuse: Generally the location of the fuse in a negative grounded system is on the positive side. Two comments “If the fuse is in the negative side and it blows you now will have voltage trying to get back to the battery negative./ If the fuse is on the negative (ground) side some where there maybe a ground path that can lead to current flow.”
Both of these statements are false. Once the fuse has opened, there is no more current paths, so voltage/current cannot go anywhere.

Cables – this is one area in which we can almost agree. Chaffing is probably the biggest problem that we can experience in our coach, we must all pay due diligence in when we move the cables that they are secured again. We should also check for where the cables go.
When we talk about the cables shorting to ground, it is very unlikely that a cable will find a “HARD” ground. Generally, through chaffing, the shorting to ground is gradual and could carry many hundreds of amps, yet not high enough to trip high fuse ratings like 800 or 1000 amps. A 4/0 cable is rated for 300 amps in free air, this is a far cry from the 800 or 1000 fuses. As you can see, if the 4/0 cable had a current of 500 amps, say a starter stuck in he “on” position, the cable would definitely heat up and possibly melt but not even come close to popping the fuse.

  • TW cables in Free Air Raceway
  • #2 140 amps 100 amps
  • 2/0 225 amps 145 amps
  • 4/0 300 amps 195 amps

Battery Protection: The battery protection is going to be defined in two ways:
  • Protection from harmful or excessive currents, such as shorts. The fuses will blow if properly spec’s thus saving the battery from damage.
  • Batteries themselves – NO PROTECTION from internal shorts or circulating currents of any kind.

Cutoff Switch: I agree with putting the cutout switch on the NEG side of the battery, but not for the reasons that was specified. The switch could fail in the NEG or POS side and the coach would have power to all. My reason for putting the cutoff switch in the NEG side, is for safety. Remember, the first thing that is to be done when we work on the batteries is to disconnect the NEG side, so a cutoff switch would make this real easy.
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  #6  
Old 11-09-2009
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I agree, Bill. Your "cable" comments are correct. A 500 amp draw could certainly start a fire with no response from an 800 amp fuse. My FC is a coach that is relatively easy to inspect for main cable damage, that would be the best way to avoid a fire. A coach like "Matchmaker" would be difficult to inspect with any confidence that all area's were well looked at.
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  #7  
Old 11-09-2009
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Hi Bill,


Thanks for all your imput on wiring would you mind taking a minute and defining what "TW cables in Free Air Raceway" means from your post above.
  • TW cables in Free Air Raceway
  • #2 140 amps 100 amps
  • 2/0 225 amps 145 amps
  • 4/0 300 amps 195 amps
Thanks Denny
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  #8  
Old 11-09-2009
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Jack and Liz- I did not fuse the starter- only the house wiring. You can use a much smaller (and more sensitive) fuse that way. I also replaced much of the engine compartment wiring with welding cable- it has a much thicker hide than the stuff that BB used and is easier to bend and shape, hence it does not rub as hard against bulkheads. To further protect the starter cable, I wrapped it in the asphalt-impregnated wire loom that BB used in our coaches. That stuff is nearly impossible to cut. Rather than going to a single stud (like BBird had it wired) The cables from my battery bank go to a large, copper bus bar with 4 studs- one to the starter (not fused), one to the jumper cable stud (not fused), and one to the house circuits, chargers and alternator (fused).

The 4th cable goes to a class-T fuse block like this-



To properly size the fuse, I recommend turning on all 12V circuits, lights, etc... with the chargers off and also running the inverter with the ice maker and microwave. Then measure your total 12V load on the battery bank with a clamp-style ammeter on the negative battery cable. I'd guess that you'll have a load of around 350 amps with all of that stuff on.

All of my batteries are linked- except the genset battery (which has it's own disconnect and fuse). The battery bank is switched with a large disconnect switch rated for 900 amps continuous duty and an engine cranking load of 2000 amps (like this one)-



If you'll be at Quartzsite, there will be plenty of people there to take a look at what you have and give some pointers. I have an older chassis and this is what has worked for me over the past 2 years. Separate battery banks may complicate things a little bit. One last thing that I'll add is that I used 4/0 welding cable to supply power from the batteries to the bus bar and then from the bus bar to the starter and from the bus bar to the house circuit distribution point where the DC shunt is located.
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  #9  
Old 11-09-2009
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Default HOW MUCH CURRENT?

TW cables in Free Air Raceway
#2 140 amps 100 amps
2/0 225 amps 145 amps
4/0 300 amps 195 amps


Free air - means a single wire by itself, not like what we have
Raceway - multiple wires together

A raceway can be a pipe, a wire tray or like what we have in our birds along the top or bottom.

Shane has some nice equipment, but like he says, he is mainly protecting the house system. He also has a nice switch for linking batteries etc together.
I will be posting some other options shortly for linking batteries and I will find out what the starter current draws are for the 8v92.

Each coach has to be different, some have more electrical items than others, and not all coaches will be protected 100 percent.
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  #10  
Old 11-10-2009
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Default HOW MUCH CURRENT?

Shane:
You have some good points.

Since you are using a 4 post main contact place, you could fuse the starter wire as well, but the other points you make are real good things.

What I am trying to find is the current draw for the starter motor for both the 3208 and 8V92 starter motors. Any ideas??
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