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post #1 of 11 Old 11-09-2010 Thread Starter
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Cable size for fuse block?

Got a question about the cable size to the 150 amp fuse block.

Am ready to purchase the two cables, positive and negative for the 150 amp fuse block.

The fuse block is 14’ to the neg buss and then it is a couple feet to the battery. So say it is 17’ to the battery negative side.
Neg 17’
The positive cable is 11 ½ feet and a fey to the battery from the buss so say 15’.

Neg 17’
Pos 15’

At genuinedealz.com on the calculator for wire size it says “one way”.

Genuinedealz - Technical - Calculators

17’

12 VDC 1 phase

3 percent drop

= 4/0 cable!

So that would mean I would take the longest run the neg at 17’ and calculate for a 3 percent drop, as I am on the cautious side. This fuse block will handle the VHF, SSB, bilge alarm, propane detector and some other devices.

So at that length 17’ and 3 percent drop I get 4/0 cable. That seems real big.

The fuse block only has a Ό stud for hooking a cable to.

If I go to a 10 percent drop for non critical stuff I get 2 awg.

So my question is what am I doing wrong if anything?

Does the fuse block really need to handle 150 amps?

Should I be adding both wires together?

There is a 125 amp fuse for the fuse block then the fuse block.



Last edited by SanDiegoChip; 11-09-2010 at 10:15 AM. Reason: pictures did not show
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post #2 of 11 Old 11-09-2010
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The fuse is sized to protect the wire, so you need to chose the appropriate wire size for the loads it supplies and then size the fuse to protect the wire.

Voltage drop calculations are done with the entire round trip of the wire, from the positive source, to the load, and back to the negative ground.
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post #3 of 11 Old 11-09-2010
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Chip,

Forgive me if I'm missing something here, but it seems to me that 150 amps is overkill. Have you totaled up the number of amps all of the connected devices actually use? The SSB seems to be your biggest load, and that's probably going to be around 30 amps (check your documentation.) The VHF is likely less than 10 amps, and the others are probably also in the low single digits.



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Originally Posted by SanDiegoChip View Post
Got a question about the cable size to the 150 amp fuse block.

Am ready to purchase the two cables, positive and negative for the 150 amp fuse block.

The fuse block is 14’ to the neg buss and then it is a couple feet to the battery. So say it is 17’ to the battery negative side.
Neg 17’
The positive cable is 11 ½ feet and a fey to the battery from the buss so say 15’.

Neg 17’
Pos 15’

At genuinedealz.com on the calculator for wire size it says “one way”.

Genuinedealz - Technical - Calculators

17’

12 VDC 1 phase

3 percent drop

= 4/0 cable!

So that would mean I would take the longest run the neg at 17’ and calculate for a 3 percent drop, as I am on the cautious side. This fuse block will handle the VHF, SSB, bilge alarm, propane detector and some other devices.

So at that length 17’ and 3 percent drop I get 4/0 cable. That seems real big.

The fuse block only has a Ό stud for hooking a cable to.

If I go to a 10 percent drop for non critical stuff I get 2 awg.

So my question is what am I doing wrong if anything?

Does the fuse block really need to handle 150 amps?

Should I be adding both wires together?

There is a 125 amp fuse for the fuse block then the fuse block.


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post #4 of 11 Old 11-09-2010 Thread Starter
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more questions

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Originally Posted by PorFin View Post
Chip,

Forgive me if I'm missing something here, but it seems to me that 150 amps is overkill. Have you totaled up the number of amps all of the connected devices actually use? The SSB seems to be your biggest load, and that's probably going to be around 30 amps (check your documentation.) The VHF is likely less than 10 amps, and the others are probably also in the low single digits.
OK that seems like what I am looking for. This is what I need to understand.
It would be 17 + 15 so 32 feet round trip.
32’
Do I size the wire to the total fuse block or to the size of the loads?
It seems to the size of the loads.
There are 12 – 30 amp fuse slots. I do not know of any that will be 30 amps. Like you said maybe the SSB which we do not have yet.
So if we have say a bilge alarm (I forget the amps) but say 10.
Then say the propane detector say 10.
Then say the VHF at say 10.
Then thermostat for the new refer we are getting say 10.
OK then just to make it easy for me say 10 on all of the 12 fuses.
Then I would say ok that is a 120 amps, darn that seems a lot too.
Ok say 10 on 6 fuses and 5 on 6 fuses.
That is 60 plus 30 amps on the fuses.
So 90 amps. None is critical so say 10 percent drop on 90 amps?
This seem right?

So I could go with 3 AWG for 32’ and get a 9.73 percent voltage drop.
I can probably drop these numbers down further.

Another question:
In the chain and anchor locker we have 3 LED lights all fused together to a 5 amp fuse.
That seems ok. It was a suggested way to go rather than fuse them separately.
Can I then following that example put two or three items to one fuse if the all together add up to the 5 amps? Would that not over fuse one of the three items?
Also what size fuse would I use to fuse the 90 amp fuse block? Just a guess but 80 amps?
Thanks,
Chip
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post #5 of 11 Old 11-09-2010
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Chip
You fuse for the wire gauge. If you are using 14 awg from the fuse block to the various loads the appropriate fuse is 15 amps. But the load is usually a lot less. Look at the specs for the various items and add up the usage. Assume all are on at the same time. You should not be anywhere near 90 amps. For example my Icom vhf uses 5.5 amps at 25 watt transmit and 1.5 amps on receive. A propane detector is very low amperage. The bilge alarm is probably less than 2 or 3 amps. Then size the wire that feeds the fuse block for max usage, not a total of the fuse sizes.

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post #6 of 11 Old 11-09-2010 Thread Starter
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finally get it!

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Chip
You fuse for the wire gauge. If you are using 14 awg from the fuse block to the various loads the appropriate fuse is 15 amps. But the load is usually a lot less. Look at the specs for the various items and add up the usage. Assume all are on at the same time. You should not be anywhere near 90 amps. For example my Icom vhf uses 5.5 amps at 25 watt transmit and 1.5 amps on receive. A propane detector is very low amperage. The bilge alarm is probably less than 2 or 3 amps. Then size the wire that feeds the fuse block for max usage, not a total of the fuse sizes.
OK I think I finally get it!
Thanks,
Chipo
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post #7 of 11 Old 11-09-2010
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Chip,

Do some research to find the ACTUAL power draw of the devices you plan to power. Your estimates, except for the SSB, seem to me to be very high.

Also, think about which devices will be in simultaneous use.

I'd be surprised if all-told your power draw exceeds 50A, including the 30A voice peaks on the SSB. (The SSB will only draw about 1-2 amps on receive and about 10-15amps on average when you're transmitting). The sensors draw very little current. The VHF will only draw about 5-6 amps when transmitting on high power (25 watt setting). The bilge alarm won't draw much either....almost nothing when just sensing and not alarming.

Bill
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post #8 of 11 Old 11-13-2010
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Fuse blocks are rated for their maximum amps, and it is fine to actually size the feed wire appropriate to the loads you intend to have. The feed wire gets protected with a fuse of the appropriate size. You really want the TOTAL voltage drop to be 3% or less at maximum load. That means you add the drop to the block, then the drop on the smaller wire to each load. In practice the voltage drop will be far less because you won't always have all loads on at the same time. Wire sizes should be chosen for voltage drop, but fuses should be chosen by actual load. This will always be safer and will better protect the load. Using fuses sized for the wire means you could deliver enough current into a bad connection to start a fire, without blowing the fuse.

Gary H. Lucas
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post #9 of 11 Old 11-13-2010
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Chip, as Bill says, the current a device draws is often less than the fuse it is connected to. Part of this has to do with how fast you want a fuse to blow, part of this is conservation, i.e. if my radio draws 5A, but the wiring coming out of it can support 10A, I'd probably out the radio on a 7.5A fuse. Using the logic that that fuse will blow faster than the wiring, but won't blow unless the radio has a dead short, even if there's something causing a slight extra load there, like a corroded contact or power problem.

I would suggest that using cables which support the maximum rating of the fuse block is good cheap insurance though. In time, you tend to add things, and the last thing you want to do is go back and replace primary cabling. So you make a personal decision about "this is all I could ever ever need" versus "I can't afford this!" and "How the hell does this cable fit on that bolt?!" Sometimes the answer to that is a pigtail, six inches or a foot of smaller cables that DOES fit. A piece that short won't cause a huge voltage drop, just make sure the primary fuse is sized to protect THAT rather than the larger cable.
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post #10 of 11 Old 11-14-2010
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A device generally always draws less current than the fuse it's connected to. If it drew more, the fuse would blow. A fuse doesn't blow instantly, so it's possible to draw more for a very short time and not have the fuse blow.

The 150A rating on the block is the limit on the sum total of all the fuses. To actually draw the 150A would require every single circuit to be drawing at the limit of its fuse at the same time. You actually had 150A worth of stuff to connect you'd want a bigger fuse block.

Size the wire for the load. The voltage drop calculator gives you a minimum wire size, you can always go bigger than that.

Then size the fuse for the load and the wire. The load gives you the minimum fuse size while the wire gives you the maximum fuse size.

Figure out that your device actually uses. It's usually in the manual. Then give say a 50% margin to that so the fuse doesn't blow if the current happens to be briefly a little high for some reason.

For the wire, look up the ampacity in a table for the wire you have chosen. This is the maximum amount of current the wire can carry before the insulation melts. It will be less if the wire in a hot engine compartment or stuffed in a conduit with other wires. This info should be in the ampacity table. Your maximum fuse size should be 80% of this.
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