Cable sizing and voltage drop

[mermike]

I'm working through a redesign of my electrical system and would like confirmation of my understanding of voltage drop. The batteries are 4x Trojan T-125's. The boat was not designed for the weight of these batteries and she has a noticeable list. They're over 5 years old and I'm planning to swap them out for a pair of Firefly G31's. There is a separate Group 24 start battery, so no starting load on the house bank circuit except in emergency. The battery compartment (BC) is 25.92' round trip to/from the main bus (MB).

From MB, it's another 11.2 ft round trip to the DC panel (DCP) on 2 AWG cable. It's 10.0 ft round trip from MB to the 80 amp charger/inverter (CI) on 2/0 cable. The cables were sized according to the expected current load and to minimize voltage drop without sinking the boat or the bank account. I'd like to bring the batteries closer, but I'd have to switch to LiFePO due to space limitations.

Using the published Ancor standards for resistance I calculated voltage drop (VD = I × (2 × L × R / 1000)) below. I also added 0.00025Ω per termination per Maine Sail's article:

BC to MB @ 150 A = 2.61%
MB to DCP @ 135 A = .94%
MB to CI @ 100 A = .67%

If all that is correct that would make the total voltage drop from BC to CI=3.28% and BC to DCP=3.55%. Does the voltage drop and cable sizing look right, or do I need to go up a size?

Thanks,

Mike

 

[dlochner]

What size cable are you using from the Battery to the Main Bus?

What's the fusing at the battery? If there is the potential for 235 amp draw from the DCPanel and the Inverter, then the fuse at the battery has to be able to handle that load, so a minimum of a 250 amp fuse. That current draw would require 4/0 cable.

What's on your DC panel that would require 135 amps?

 

[Brian D]

Whenever I do anything regarding cabling, whether for the boat or my ham radio at home, I use this website.

American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength

AWG Wire size chart and ampacity table for design engineers including skin depth frequencies and tensile strength data; electrical cable size

www.powerstream.com

At the bottom of the AWG table is a calculator. You can use it just to confirm your findings. Just something to help you in your cabling.

 

[mermike]

Battery to Main Bus cable size was planned at 2/0.

DC Panel load is based on all items on the panel turned on simultaneously, but I inadvertently included my inverter loads in the total. The corrected total is a maximum of 59.71 amps. I could drop the MB-DCP cable to 4 AWG.

Good catch on adding the loads of the DC panel and and inverter, thank you. The charger is 80 amps, inverter is 2000 watts or 167 amps. Increasing cable size to 4/0 on BC to MB lowers the voltage drop on BC to DCP to 2.76%. For fusing at the batteries, I would use a 350 A Class T with 4/0.

 

[mermike]

Whenever I do anything regarding cabling, whether for the boat or my ham radio at home, I use this website.

American Wire Gauge Chart and AWG Electrical Current Load Limits table with ampacities, wire sizes, skin depth frequencies and wire breaking strength

AWG Wire size chart and ampacity table for design engineers including skin depth frequencies and tensile strength data; electrical cable size

www.powerstream.com

At the bottom of the AWG table is a calculator. You can use it just to confirm your findings. Just something to help you in your cabling.

I was just looking at the battery primer on that site - pretty comprehensive: Battery - 10 - Storage Battery Troubles

 

[Stu Jackson]

Some other resources:

Blue Sea Tables Reference - Blue Sea Systems Scroll down to find the PDF download: Protect Your Boat with the Correct Size Wire & Fuse Chart. Handy to keep around.

Battery Fuse Sizing with tables (from Maine Sail)

Battery Fuse Sizing - How?

Wire Size Calculators 101

Voltage Drop Calculator Genuinedealz.com

Help Center - Recommended Wire Gauge Calculator | Bulk Wire

 

[mermike]

Thanks, Stu. Charlie Wing's book is also a must-have for anyone contemplating this project. I also signed up for the free 5-day ABYC membership and downloaded the standards. I have researched this pretty carefully, and am now testing out my knowledge of the theory. I built a spreadsheet that records all the cabling and terminations and calculates the voltage drop in a pivot table. There are still a few bugs of course. When the cables are all identified and measured, I'll complete the fusing.

 

[sea_casper]

1)Your DC load max seems really high, I hope you are not using fuse/circuit breaker sizes for load.
2)When do you run the inverter, and for how long?
What is actual ac load?
3)Can you relocate charger and inverter closer to batteries, if you can't relocate batteries closer to DC panel?

 

[mermike]

1)Your DC load max seems really high, I hope you are not using fuse/circuit breaker sizes for load.
2)When do you run the inverter, and for how long?
What is actual ac load?
3)Can you relocate charger and inverter closer to batteries, if you can't relocate batteries closer to DC panel?

Right now the AC load is about 65 amps but with some investment it could be a lot lower. Switching from a plug in heater to diesel heat would help a lot. I've got a propane heater but still using an AC fan to circulate heat.

Faced with another $500 in 4/0 cable I compromised on the battery location and brought it closer to the panel and charger. Too high, and to far in the stern but all boats are compromises. At least the list will be gone. The picture shows where the batteries will live. I'll have to fab some support and a box. The charger will mount on the bulkhead to the left of the batteries. The round trip run from the batteries to the main distribution drops from 25' to around 9'.

 

[twalker H260]

It may help the forum if you were to list the current draw of all your devices and break out what is on Ac and what is on dc. 4/0 cable can handle 200+ amps. If this is primarily to supply the 12v side of the inverter I would expect to run no more than a few feet and have the inverter physically located near the batteries. The AC side of the inverter is only about 16 amps and can run through 12guage wire. Heavier draw items may be connected to a buss bar then direct to the load (engine starter) and not run through your dc panel.

I’m assuming that your AC powered heat is only run On shore power or do you have a generator to replenish your batts to run when away from shore power? As mentioned previously your 12v load number seems high. does everything have to be capable of running simultaneously? Ie. if you have a anchor windlass it would be unlikely to run at the same time as an autopilot at full draw(what is its draw on standby?). It may also make sense to run a separate battery for this purpose near the windlass using heavy cables then controlling it through a solenoid wired with lighter gauge wire.

Additional information from your end will give better information from the forum.

What AC heat are you running that takes 65 amps? Is that at 12v or 120? 65x12=780. 600w Ac space heater? Maybe look at alternative source? 65x120=7800w. Do you run two 50amp shore inlets?

 

[mermike]

Plug in heater, shore power only. No generator. I'll post my energy budget a little later. The 65 amp figure has got to be wrong, I'll go back and see what I did wrong. The heater draws 7 amps on full, as measured with a clamp meter.

 

[twalker H260]

7amps is a shade under 900watts, which would be a medium heat setting on most adjustable output space heaters. 1500watts is fairly standard for 120v.

if you have a standard twist lock 30amp shore power inlet I would try to ensure that you current draw on the Ac side is less than 20amps. - or change to smart plug connector for the inlet/cable.

Items on this side are normally water heater, air conditioning, lighting, charger and plug in circuits. Charger has 80amps 12v out but likely uses 10amps Ac input.

on Ac side wire is fairly simple. From 30amp shore inlet would normally be 10guage to initial breaker then on to circuit panel. Most circuits (15amp) from there will be 12 or 14g depending on country and circuit load (I would likely just go with 12 gauge. )

On dc side it is more complex. From what you’ve indicated previously I would think of moving the charger/inverter (Ac side in/out likely only requires 12g wire) closer to batts ( is a remote control available for it?) and also would likely consider moving the main bus and a few of the high draw loads As well to reduce round trip distance (Starter, windlass etc).

running through my dc panel are:
dometic fridge/freezer max draw 5amps
auto pilot 2.5amps
radio max 5a (transmitting) - not often
nav lights (max draw sailing) 4amps (non led)
interior lighting incandescent - .25-1amps each
usb plug in chargers (1amp each?)

max through panel normally less than 20 amps. = 10-12g wire. (From main buss to my panel is wired with 8guage)