Wire and Fuse Sizing

[FlyingScot]

I purchased a 2008 Leopard 40 Private Owner's Version. Pretty loaded from the factory plus a few things added afterwards.

How do you size the DC wiring and fuses? I'm familiar with the chart using bundles and or engine room runs, and both lengths added. But...

Problem is that my boat has a lot of potential things that COULD be running simultaneously. There is no possible way to allow for everything. So what's the rule of thumb?

Currently has twin 80 amp alternators. Soon to be adding solar. 250 amp fuses on the + and - DC.

Installing three 4V Firefly Oasis L15+ batteries for 450AH @ 12V.

I've purchased a bunch of Ancor 4/0 and plan to start by installing that for the battery + - and jumpers between them. Then go from there.

As best as I can find, this is complete list that could pull amps from the batteries. OVER 800 amps @ 12VDC!

Amps Max Item Description
180 Powered Winch Harken 46.2 ST EC
167 Inverter/Charger 2000 watt
140 Windlass Quick 1000watt 12v
88 Xantrex 1000w Inverter Main Cabin
43.1 Water Maker (uses Port AC Sea Water Pump) Eco Tec 260 DML 1 DC pump 39.58 Amps 12v, Boost pump 3.5 amps
30 Diesel Transfer Pumps Jabsco 23870 (15-20 amps @12v each)
20 Head Fwd Jabsco Quiet Flush
20 Head Aft
18 Auto Pilot Raymarine ST6002, Rotary Type 2 Drive,
10 Radio Clarion CMD8 18 watts RMS x 4
10 Wash Down Pump Marine Tough Blaster 2095-23
8.4 Bilge Pump Port Rule 2000 GPH
8.4 Bilge Pump Starboard Rule 2000 GPH
7.5 Shower Pump Discharge Port Jabsco Diaphram Pump
6.5 Freezer Sea Frost BD prior to 09 with one added 3 speed fan by coils (.1/.2/.3 amps) and one small fan in fridge (.18 apm)
6 VHF Raymarine Ray240
6 Electric Head Starboard Supply Pump Jabsco Par-Max4
6 Electric Head Port Supply Pump Jabsco Par-Max4
5 Fresh Water Pump Jabsco Par-Max3
4 Instruments ST60 Wind 2 amps likely, ST60 Tri Data 2 amps likely
3.68 Ventilation Fans Hella Turbo Fans .55 amps x 5, Carframo .31 amps x 3
3.3 Engine Room Bilge Pump Port Rule 1100 GPH
3.3 Engine Room Bilge Pump Starboard Rule 1100 GPH
3.3 Shower Pump Discharge Starboard Rule 1100
3.08 Cabin Overhead Lights Labcraft KLLED .44 amps x 7
1.8 Deck Light 20 watt 2 CREE bulbs
1.5 Wirie Pro Wifi and Cell Router
1.1 Gas Valve Propane solenoid valve
0.8 GPS / Chartplotter Raymarine RC435
0.7 Salon / Galley / Navigation Station Lights White G4 LED bulbs .075 amps each x 9
0.5 Cabin Reading Lights Flex LED reading lights .08 amps each x 6
0.5 Cockpit Lights Red/White G4 bulbs .15 amps each x 5
0.4 Navigation Lights Hella NaviPro 2NM Stern .27 amps , Hella NaviPro 2NM Bi Color .17 amps
0.2 Steaming Light Hella NaviLED 3NM
0.1 Panel Lights Small LED for each breaker label
0.1 Anchor Light Marine Beam LED w/ photocell
808.26

[john61ct]

Your call, to add up the largest load combination running concurrently over a given segment. Main trunks to distribution points are short.

Round way up to prevent nuisance tripping CP, which is sized to protect the wire.

Blue Seas Circuit Wizard app for calculating that, post back here for Qs.

[Stu Jackson]

www.bluesea.com also has good material on this and fusing, but it's a PDF download and doesn't link. Here's the link to the page, scroll down for the PDF download of the Fuse & Wire Sizing Tables: https://www.bluesea.com/support/reference
I've found tables & charts easier to use than computerized versions. The charts & tables allow me to see bigger and smaller and ranges, whereas the "calculators" just give you an answer, and if any of your input is off, GIGO. Like early spreadsheets before folks figured out they should cross check the answers.
Good luck.


Without a wiring diagram, no one can really answer your question. Have you tried a good boat electrical book, like Charlie Wing's? john's right: over a given segment. That's where the wiring diagram is ESSENTIAL.

[FlyingScot]

Being 46 I'm both young enough to use an app on my phone and old enough to use the charts.

The issue is I have a boat that could pull 800 amps under a exceedingly rare circumstance. Is it expected or customary to size wire for the absolute maximum?
Anyone know what ABYC says?

All I can find are tables and charts for length, amps, engine room and bundles.

[smac999]

The boat is never going to pull 800a.

Forget that. Run 4/0 to a pos and ground bus. From that bus run your main lines. House switch, windlass. Inverter switch. Alternator input. Each of those lines comming / going from the bus needs to be sized and fused.

[billknny]

Although it sounds odd, your fuse sizing has nothing to do with the amount of power you can draw. The fuses are sized based on the amount of power the wires can safety carry without overheating. The fuses protect the wire from accidental (or intentional!) power flow that will damage the wires, and potentially the boat.

The size of the WIRES is selected based on the maximum amount of power you will move through them on a normal basis. Usually (but not always), the voltage drop becomes too large to tolerate at currents below the maximum ampacity of the wire, so the fuse rating turns out to be higher than the current flow used in the design.

So...

The design process is:

1. Decide how much power you will draw under worst reasonable case. This sounds like the step that is giving you trouble?
2. Size the wire by app or table based on maximum amps needed and maximum voltage drop to be tolerated, length of the wire, rating of its insulation, where it will be installed, how many wires in the bundle, etc..
3. Size the fuse (by app or table) based on the maximum allowed ampacity of the size wires used.

If your design basis for the wire size is unrealistically low, you will have chronic voltage drop issues and/or nuisance fuse failures. If too high, you will install wire larger than needed. Everything will work fine, but your boat will be heavier than needed, and your wallet significantly lighter.

[rgleason]

I once was involved in design for the Nynex Computer center in Manchester, NH. We had power failure design issues in sizing the interim battery bank in the basement, the two Westinghouse massive generators on the roof, and the 40,000 gal insulated water storage tank in the ground out front, all because no Nynex Engineer was willing to commit to a % for load diversity. Not a single engineer would put his head on the line, basically because of the chilled water IBM Mainframes that had to be supported. Everything was sized for 100% load and a 10 minute transfer period to Generators when we had many disk drives and other acceessories in the building.


You can do this once you know your use patterns. Figure out minimum and maximum loads, being realistic. Add them up. Consider diversity. You can also get into details like when you do stuff, like try to keep big loads focused around when the engines are running. Don't be like those chicken.


Nynex engineers that, only knew what they didn't know and weren't willing to try to figure it out.

[john61ct]

Originally Posted by FlyingScot

Being 46 I'm both young enough to use an app on my phone and old enough to use the charts.

The issue is I have a boat that could pull 800 amps under a exceedingly rare circumstance. Is it expected or customary to size wire for the absolute maximum?
Anyone know what ABYC says?

All I can find are tables and charts for length, amps, engine room and bundles.

Use the app, directly based on ABYC specs.

Live update as you change inputs.

It's not amps alone, but for how long, momentary surges don't melt the wire insulation when continuous will.

[john61ct]

And the safe gauge will be **lots** smaller than trying to keep V from dropping more than 1-2%

CP only needs to be sized for the safety margin, not that fatter factor

[Stu Jackson]

Originally Posted by rgleason

>>>>>>>>>>>>>>>>>>>>> ... no Nynex Engineer was willing to commit to a % for load diversity. >>>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

You can do this once you know your use patterns. Figure out minimum and maximum loads, being realistic. Add them up. Consider diversity. >>>>>>>>>>>>
>>>>>>>>>>>>>>>>>>>>



Nynex engineers that, only knew what they didn't know and weren't willing to try to figure it out.

Diversity is NOT choosing a %. It DOES involve use patterns, which can be ENGINEERED.


I've done hundreds of computer centers with backup power and UPS.



The way to do it is to properly define design criteria and engineer to meet those goals.


It ain't a guess. Anyone who is requested to do it that way, in my book, is completely correct in refusing to answer.


Back to the OP: have you figured out you need to know more? Or tell us more than just everything you have on your boat that draws power?

[FlyingScot]

Originally Posted by billknny

... The fuses are sized based on the amount of power the wires can safety carry without overheating. The fuses protect the wire from accidental (or intentional!) power flow that will damage the wires, and potentially the boat....

Yes, I understand fuses protect the wiring and wiring is sized to handle the expected loads. My point in mentioning the 250 amp fuses, tells me that this boat has never drawn a sustained 250+ amps to trip the fuse.

Originally Posted by billknny

[*]Decide how much power you will draw under worst reasonable case. This sounds like the step that is giving you trouble?...

This is where I'm stuck. I can obviously make reasonable assumptions about peak demand.

Originally Posted by Stu Jackson

Back to the OP: have you figured out you need to know more? Or tell us more than just everything you have on your boat that draws power?

I thought there has to be some rule or guideline or something that has been codified and accepted somewhere.

[Suijin]

Originally Posted by FlyingScot

Yes, I understand fuses protect the wiring and wiring is sized to handle the expected loads. My point in mentioning the 250 amp fuses, tells me that this boat has never drawn a sustained 250+ amps to trip the fuse.







This is where I'm stuck. I can obviously make reasonable assumptions about peak demand.







I thought there has to be some rule or guideline or something that has been codified and accepted somewhere.

The use cases are pretty straight forward. Start with three: underway under power, underway under sail, and at anchor. What do you *reasonably* use in each scenario. Break it down further. Do you use the windlass while making water, or using the auto pilot? Of course not.

Wondering what will happen if you turn everything in the boat on at once is pointless. With 450 amp hours and all those draws you’d probably flatten your batteries in 10 minutes. Current draw would be irrelevant because either your fuses at the batteries would go or everything would brown out.

It’s a doomsday scenario. If it ever actually happened it would be the result of you loosing your mind.

[Suijin]

Originally Posted by Suijin

The use cases are pretty straight forward. Start with three: underway under power, underway under sail, and at anchor. What do you *reasonably* use in each scenario. Break it down further. Do you use the windlass while making water, or using the auto pilot? Of course not.

Wondering what will happen if you turn everything in the boat on at once is pointless. With 450 amp hours and all those draws you’d probably flatten your batteries in 10 minutes. Current draw would be irrelevant because either your fuses at the batteries would go or everything would brown out.

It’s a doomsday scenario. If it ever actually happened it would be the result of you loosing your mind.

If your Harken winch can pull 180 amps and is independently fused, I’d fuse your bank for 200 amps. Hard to imagine you’d be using another 20 amps concurrently when if ever the winch actually drew it’s full rated draw.

[FlyingScot]

Originally Posted by Suijin

If your Harken winch can pull 180 amps and is independently fused, I’d fuse your bank for 200 amps. Hard to imagine you’d be using another 20 amps concurrently when if ever the winch actually drew it’s full rated draw.

Winch at full pull while the other person flushes all the s**t would exceed 200 amps

I could have a lot of amps flowing through the batteries. I'm adding 4x360 watts solar with plans to make water during sunny days.

[Lake-Effect]

Originally Posted by FlyingScot

I thought there has to be some rule or guideline or something that has been codified and accepted somewhere.

Like what? Every owner will run their boat in a different way, and have different load combinations.


If your boat's electrics are currently working, why not run with that for a while and record your current draws at various points? You might even find that a total rewire isn't necessary.