Living Up to the 7" Rule .................

JohnShannon · Nov 28, 2012

Go read the data sheet for the fuses, typically they take some time to blow at their rating (like 20min)

Maine Sail · Nov 28, 2012

Ralph Johnstone said:
.................. is proving to be difficult to live up to (in more ways than one) .

I've finally decided to install two Blue Sea Systems terminal fuses #5191 (shown below). One on the houses battery bank and one on the single starter battery. This is a quick and easy way to ensure that any short circuit in a heavy cable within the engine compartment (the most likely place to happen) is not going to result in a melt down of the battery switch (250A rating) or worse.

A few details about the electrical system read as: The starter pulls about 135A on a cold wet day. It's supplied by #2 AWG cables with an ampacity of 200A. Perhaps a little on the light side but still does the job very well. From this, a 200A fuse on each battery #1 and #2 should serve the purpose.

However, this is only true when the battery selector is in the "1" or the "2" position but not on the "BOTH" position. The switch is normally on "BOTH" when motoring for charging. On "BOTH", we would see a flow of 400A before the 200A fuses would blow.

If the two fuses were to be reduced to 150A each, this would be too close to to the draw of the starter when only #1 battery was used.

I want to avoid installing a 200A fuse past the battery switch because this is greater than 7" and would also be difficult to access at the best of times.

Any thoughts on how to safely fuse this wiring without rewiring the entire setup would be greatly appreciated.

Thanks as always.

Does not work like that. The fuses will never blow at the same time so they don't get treated as one fuse..

If you can't do 7" then just get as close as you can. 7" is often impossible. Remember any fuse is better than none unless it nuisance trips. You are also allowed to go to 150% of the max ampacity under ABYC E-11....

MSter · Nov 28, 2012

Ralph,
Maybe I'm missing something, but what scenario would generate 400 amps ? If pos'n 1 or 2 could generate 200 amps max what would jump the demand up to 400 ?
I think you're still dealing with the draw on your starter whether from batt 1 or 2, It's still just 200.
Mster

Maine Sail · Nov 28, 2012

Ralph Johnstone said:
............. but not really clear on the comment that "The fuses will never blow at the same time". I can see that the first fuse will blow and then the second will blow immediately thereafter due to it's now handling the entire overload. What concerns me is the fact that the two fuses MAY just sit there at (a theoretical) 199A each and damage the cable and switch (250A max.) which will be passing (a theoretical) 398A, until damage is done. The fuses will not blow in this scenario.

However, as you say, any fuse is better than no fuse. In this case I'll just hope the short circuit amps are high enough to blow the first and second fuse quickly before damage is done.

A 250A switch is usually a continuous rating unless it is a real el-cheapo switch. A dead short is measured in milliseconds and a switch can easily handle that before the fuse blows.

I admit that I have eaten a number of ANL, MRBF and a few Class T fuses over the years due to "comfort with sparks" and not disconnecitng things I should have then shorting a wrench across it. It happens.. They blow and I've never once damaged a battery switch or melted a wire in a dead short.. I most recently blew an alt fuse, 150A, trying to get my Midtronics clamp on the B+ terminal to take a measurement. The clamp shorted to the case of the alt and popped the 150A fuse on 4GA wire. Some boats have NO ROOM.....

The batteries are also never going to be equal in the current they can throw into a dead short due to age, state of charge, bank size, type etc. etc.. The cables and connections are also never going to be 100% the same. I have actually tried to get fuses to blow at identical times under very controlled conditions and still not been able to do it. Fuses will also differ slightly from fuse to fuse so blow times are close but never identical. The long and short is there is but a gnats hairs chance that your 2GA wire would be pushed beyond its melt point with the switch in BOTH and a 200A fuse on each "bank"...

Your 2GA wire can safely handle 315A under ABYC E-11 max ampacity tables, using the 150% rule, if not bundled and not in an engine space and "hot"...

Beyond that as a point of reference I pull 450A - 500A through my carbon pile load tester for 15 seconds on 4GA leads and while they do get hot they don't melt. The fuse would go waaaaaay before you get to 15 seconds in a dead short scenario, which is what you are protecting against....

Again, any fuse is better than what you had......

anchorclanker · Nov 29, 2012

No one mentioned voltage drop at the starter, which would increase resistance/amperage. I think I would up the wire size just on principle.

Jalepeno · Nov 29, 2012

Hi Ralph,
Here is my DC battery setup:
Two battery banks with the typical four position battery switch, OFF- B1-BOTH-B2, with the common out going to the DC distribution panel (6 circuits with switches and 5amp fuses) and Yanmar instrument panel. The bilge pump / float switch is always connected to B1 with an inline fuse.
All of the lights including navigation lights are LED. The VHF, Tiller Pilot and Stereo draw about 2-3 amp maximum each. All other circuits draw fractional amperes only. LED lights are a very economical way to extend the usable capacity of the battery bank without increasing the expense and bulk of the batteries.
B1 is the main battery bank and consists of two, 6volt US2200XC batteries wired in series (2 x 6volt = 12volts), rated at 232AH (50% usage = 116AH available). They cost $119 + HST each (no deposit or other fees) at “Phil’s Batteries and More” in North Surrey. They also bought back my old batteries for $10 each to recover the lead and keep the acid out of the drug labs.
B2 is the back-up battery, a Canadian Tire sealed marine starting group 24 and is for backup motor starting only. I have never need to use B2 but do switch to it when motoring to keep it charged.
The BOTH switch position is used only to “make-before-break” when switching from B1 to B2 with the motor alternator running. Don’t be fooled into using the BOTH position to help start the motor if one of the batteries has a low charge. All you are doing is draining power from the battery with the higher voltage to try to charge the battery with the lower voltage.
B1 is monitored using a 500amp shunt and Xantrex LinkLITE for voltage, AH and % usage, etc. and B2 for voltage only. B1 is used for everything including normal house loads and Yanmar starting. B2 is a BACKUP BATTERY ONLY and is not normally used. My hand crank (with a squirt of WD40 and a bit of decompression) is a backup to B2.
The alternator is the original 35amp Yanmar/Hitachi and it has no problems recharging the batteries during normal motoring in/out of the marina. I sail solo and always motor or motor-sail until into the steady winds outside Passage Island (usually a minimum of one hour of Yanmar run time on the way out or in). Often I end up motor-sailing the 5 to 6 hours across to Vancouver Island if the winds are not favourable and I have a schedule. I rarely need to use the shore power battery charger at our marina or at other docks. The shore power is used for an electric heater when overnighting at a dock or to put a bit of heat in the cabin to keep the inside humidity around 50% or less during cold wet winter weather like now.
Each battery has a 175amp Blue Seas terminal fuse (carry spares). The battery fuse is to protect the wiring and is sized by the cable ampacity rating and length. Initially I blow a 150amp terminal fuse when trying to start the YSM12 with the piston at the start of the compression cycle. I now usually pull the decompression lever for the first second of starter operation to get everything moving. She normally always starts within two seconds.
p.s. I like to support our local merchants and will always give Penny at the Marina store an opportunity to price match the big boys (West Marine, Steveston Marine and Nika Industries).

Maine Sail · Nov 29, 2012

Ralph Johnstone said:
Thanks for the info Jalapeno. I am in the process of flipping coins right now to estimate the smallest fuses I can get away with. From your experience with the 150's blowing, I think I'll stick with the 200A fuses and avoid the nuisance blows.

As previously mentioned, the chances of having a partial short pulling 398A through both fuses combined (and NOT blowing the first fuse) is less than winning the Super Ball. Any short will be far greater amperage than the total 400A capacity of the two fuses. The two fuses will blow immediately with no damage to the wiring or switches.

And that's my logic so don't confuse me with facts .

You are best to go with the largest fuse that will protect the wire when starting loads are involved.. If no motors are on a circuit then you can size for smallest but with motors best to go larger... Remember these fuses are only to protect against catastrophic shorts. The last thing you want are nuisance trips. 200A should be fine though...

Living Up to the 7" Rule .................

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