Voltage drop/wire size from fuse block?

[Sailorman Ed]

I haven't seen this issue addressed before. Rather than the normal circuit breaker panel on most boats, I am going with distributed switch panels off a remote fuse block 16 feet from the battery. I plan on using #6 (fused at the battery) to the fuse block. And from the fuse block to 4 small switch panels. Other than the water pump, the loads are all quite small. How do I calculate the wire size/voltage drop % from the fuse block to the switch panels ( helm-7, Galley-12 and master-15 ft away)? Only the helm is 3% critical.


The windlass and fridge with be both on separate fused circuits.

[Nicholson58]

Originally Posted by Sailorman Ed

I haven't seen this issue addressed before. Rather than the normal circuit breaker panel on most boats, I am going with distributed switch panels off a remote fuse block 16 feet from the battery. I plan on using #6 (fused at the battery) to the fuse block. And from the fuse block to 4 small switch panels. Other than the water pump, the loads are all quite small. How do I calculate the wire size/voltage drop % from the fuse block to the switch panels ( helm-7, Galley-12 and master-15 ft away)? Only the helm is 3% critical.


The windlass and fridge with be both on separate fused circuits.

The recommended wire gage for DC voltage drop/foot, rated amperage is easily found on a Google search. ABYC no doubt has a chart. Whatever they say, I always nudge the size up if possible. All motors run better if they are not sucking electrons through a tiny straw. Voltage drop is lost power. Unless you plug in most of the time wire size is important. Be sure to use Marine grade tinned wire such as Ancor.

[SailingHarmonie]

What you want to know is what is the TOTAL voltage drop from the battery to the load. That is after all the only thing that matters.

It's a bit more complex in the way your system is designed, and it takes some assumptions, but it is not intrinsically difficult.

If I understand your system, you will have one wire from the battery to a central fuse block, and then wires running to several sub-panels, then wires from the panels out to the loads.

You need to calculate the total voltage drop from the battery to the loads. This is easy if you only have ONE load running at a time. You calculate the voltage drop for each run of wire, and add them together. But of course at least some of the time this simple situation will not be the case. Your wire from the battery to the fuse block will carry current for all your loads, the wires to the panels will carry all the currents out to those panels.

You need to assume a "highest reasonable load" for each of the distribution wires between the battery and the panels, and use that to calculate the voltage drop, first to the fuse block, then out to each panel, and then finally add the voltage drop on the dedicated wires to each individual load.

If you had a REALLY critical voltage sensitive device, you might want to use the "highest possible current" in the distribution wires to calculate the voltage drops, but in the real world that is likely to be overkill.

[Tupaia]

What SailingHarmonie says is exactly right. My system is similar in some respects wide cat and multiple banks.



I built a spreadsheet with my intended wire sizing and used the Cable Sizing "Formulas" (ABYC and European) rather than tables to prove what happens when various things are running.

[GordMay]

From "Ohm’s Law & Boats”

...
The industry standard formula for calculating minimum Wire Size for a given Voltage Drop is:
CM = (K x A x L) ÷ VD
Where: CM = The Circular Mil cross sectional area of the wire
(See Table 1, and ABYC Section E-8, Table III)
K = 10.75 Representing the Mil-Foot Resistance of Copper Wire @ 78o F.
L = The Total Length of the wire in Feet
(This is the ‘Round Trip’ length of both Positive + Negative wires)
VD = Permitted Drop in Volts
(Ie: 3% @ 12.5V = 0.375 ED or 3% @ 25V = 0.750 ED)
...

Much more ➥ https://www.cruisersforum.com/forums....html#post1256

[Bill O]

This is a good voltage drop calculator since you can change a few variables.
https://www.nooutage.com//vdrop.htm

[Lake-Effect]

Originally Posted by SailingHarmonie

What you want to know is what is the TOTAL voltage drop from the battery to the load. That is after all the only thing that matters.
...
You need to assume a "highest reasonable load" for each of the distribution wires between the battery and the panels, and use that to calculate the voltage drop, first to the fuse block, then out to each panel, and then finally add the voltage drop on the dedicated wires to each individual load.

It's simpler to come up with reasonable estimates for current required in each part of the system, estimate the lengths of runs, and look up the recommended wire gauge from the ABYC charts. (what nicholson58 said, basically)

Voltage drop on the runs is managed by using the right chart (10%/3%). Upsize the gauge for even less drop.

If you had a REALLY critical voltage sensitive device, you might want to use the "highest possible current" in the distribution wires to calculate the voltage drops, but in the real world that is likely to be overkill.

Maybe, but.... If you have a device that you want to provide full battery voltage to, then the whole path, including battery to fuseblock to switchpanel (as per OP) has to be sized to have minimal drop when each section is carrying its largest expected load. It's a possible downside to a broadly distributed approach.

On most boats with just a single DC fuse/switch panel, fed by a proper-sized heavy run from the battery, then getting full voltage to a given device is mainly a matter of correctly sizing just the run from the panel to that device.

[Sailorman Ed]

Nickolson - very familiar with the wire charts but they all assume a continuous gauge for the entire run.


SailingHarmonie - so take the highest load at each switch panel that I think will be on with the other highest loads since not everything is always on at the same time? You are correct about the total voltage drop, what I am asking is HOW to calculate this with 3 different wire gauges: large from the bat to fuse block, medium from the fuse block to the switch panel and small from the switch to the fixture?



Tupaia -

My system is similar in some respects wide cat

, this is one reason I thought to go to a distributive system. So maybe a spreadsheet is the way to go using the calculator that Bill O suggested. And total for each of the 3 conductors? I can see that dual monitors will be required while doing this.



Bill O - nice calculator. Where is asks for the operating temperature, safe to use the maximum the interior of the boat will see? 60C (140F) is much higher than I ever want to be inside the boat.



I know that fuses add a small resistance to an electrical circuit, is this even meaningful?

[wrwakefield]

I have a few 12V DC branch circuits similar to what you describe.

When designing, don't forget to account for/mitigate any voltage drop through the fuse you choose to use for each sub-feed.

See Rod's article on this point for more details.

Cheers, Bill

[Bill O]

Probably would go for ~40C for the temp unless in the engine room.


Maybe slightly off but if you did the independent calculations for each of your 3 "systems", then the V drop should be the sum of the 3 losses.

[wrwakefield]

As for establishing wire sizes for various DC circuits, I find the Wiresizer app extremely useful.

It does nothing more than the online tables, but it works offline and allows me quickly do what-if evaluations while reverse-engineering and/or planning/designing wire runs.

On Edit: It also allows you to create 'Project lists' from the outputs of your calculations. Very handy...

FWIW

Cheers, Bill