best large to small wire connection?

[ksanders]

Here’s how 10% voltage drop works in a 12 volt system. These are the rules of thumb that I go by.

I generally do not have to worry about wire run length because my boat has a centralized main breaker panel, and sub panels in reasonable places.

Breaker size wire size max run
20A. 12 AWG. 37’
30A. 10 AWG. 40’
40A. 8 AWG. 47’ (I AMOST NEVER RUN 8 AWG)
50A. 6 AWG. 60’

The maximum run above is the round trip 10% voltage drop wire length. If you look at almost every piece of DC gear on a boat it will be manufacturer rated for 10.5 volts to 14.5 volts.

Thesae are the exact same rules of thumb that we use in land based systems. The only difference being the max run is in practical terms never an issue in a land based system.

BTW, the OP figured out his original problem. He had a 10 amp breaker installed for a motor load of 16 amps.

Even though I never worry about the exact voltage drop calculation it’s easy math. In his case 16 amps is his current. 10% voltage drop is 1.2 volts. 1.2/16 =.075 ohms maximum resistance. 12 AWG wire is .0015888 ohms per foot. .075/.0015888= 47’ round trip wire run.

My money says his actual wire run is not longer than 47’ making 12AWG wire appropriate for his application.

 

[psneeld]

Guess it depends on what one "needs" (10% drop) , or what one "wants" (3% drop) or somewheres between no drop and a safe wire ampacity.

Once again blowing the "only one way" or "only correct way" words of wisdom out the door, let alone doing the job with different possibilities of a remote battery, remote sub panel, different connctor types, etc...etc....

 

[caltexflanc]

The "power source" in this case is the first circuit breaker preceding the pump. That is the critical "round trip" distance here. The breaker is mounted to a panel that in turn has larger cables running either to a master panel or the batteries. The breaker is sized to protect the cable downstream from it. I too recommend getting Calder's book which is quite well written for the layman rather than depending on a cacophony of anonymous posters who are not there to see your situation and are amateurs themselves (including me).

Also check the body of the pump or the head manual and see what the operating voltage range is for the pump.

I am going to (virtually) guarantee that when you get the correct information, 6ga is going to be WAY WAY too big regardless of what voltage loss you pick.

 

[angus99]

I have been calculating the distance from the pump to the power source (battery) X 2.

Does your negative go all the way back to the source battery or does it go to a common ground bus closer to the head? If the latter, this could (depending on how the bus is wired back to the battery) reduce your total wire run rather than pegging it on X 2 of your positive. (Electricians, check me on this.)

I’m finishing the install of two Marine Elegance heads (18 amp draw) and am using number 8 per Raritan’s instructions and Blue Seas calculator for the runs I have—including the engine room and bundling discounts. I have a negative bus near the house bank with 4/0 connecting them that shaves 20 feet off my total run.

 

[ksanders]

Any wire run calculations can be from your main or sub panel to the load.

This is because you will have a negligible voltage drop from the battery to the main or sub panel in a well designerd electrical system.

 

[PMF1984]

Folks,

Just to give a final update...

Used a terminal block at both ends to insert the #6 wire between the panel board and the head. Also reread the manual and inserted a 25 amp breaker as instructed.

Also rewired the water pump with a inline 7.5 amp fuse at the pump end as instructed and ran new #10 wire, again as instructed by directions in the manual.

 

[Comodave]

Sounds like a good fix.

 

[Steve DAntonio]

There's no reason to guess at any of the wire size requirements. Use the Blue Seas Circuit Wizard web page or app. While a head is not a "critical" item as far as ABYC is concerned, voltage drop at motors is problematic, they run slower and hotter as a result, and have shorter lives when chronically supplied low voltage. For most motor loads I use the 3% voltage drop calculation.

Agreed, a terminal block is the best means of connecting disparate wire sizes. Another common area this occurs is auto pilot pumps.

 

[rgano]

WAIT A MINUTE! You are planning to hook large wire for umpteen feet to carry a heavy load and run a much smaller wire into the apparatus at the last foot or two? Seems to me the smaller wire is still carrying a load larger than designed for.

 

[PMF1984]

My "smaller wire". is #10. Interesting to note that the wire coming out of the motor is #14.

 

[Rough Magic]

One other note on 12 gauge house wire, it is solid, whereas boat wire is not. The copper area of 10 gauge stranded wire is closer to 12 gauge solid. Solid wire not recommended as it won't stand up to the shaking and moving in a boat.



Sounds like a well designed repair.

 

[Xsbank]

Running the “proper” wire, #6, to the head is like moving the power source there, #10 to the pump is a perfect solution, imho. Happy flushing!

 

[C lectric]

WAIT A MINUTE! You are planning to hook large wire for umpteen feet to carry a heavy load and run a much smaller wire into the apparatus at the last foot or two? Seems to me the smaller wire is still carrying a load larger than designed for.

You misunderstand.


The smaller wire size is that provided by the manufacturer and is more than adequate for the current for the very short distance it has to finish travelling to the motor. The Vdrop in that very short section will be absolutely negligible.

The discussion is for the distance BETWEEN that short final section and the supply point which is many feet away and for the proper means to connect the supply and the load wires. It is that long run that may create excessive Vdrop and it is that which is the concern here.


As for that very short final section that actually enters the load, the motor, that is very common since no mfgr. knows how far it will be from the supply point to the actual load. They supply a short section which is adequate for that load and that short lead length. It is fully adequate for the load.

It is up to us, the installer, to ensure that the supply wiring between the load and the panel is adequate since that can be very different from one installation to another. The wire must be sized to keep Vdrop in line with the motor needs.

THe load HAS NOT CHANGED. THe 16A load is easily handled by that short section.

The problem has been solved by reading the instructions properly, replacing a too small fuse with the proper unit and installing an adequate size wire, a #10, which is more than adequate for the load and still allow for minimal Vdrop.

 

[rgano]

MY ERROR! Carry on, McDuff, and I think they say in Hamlet.

 

[PMF1984]

Great explanation C lectric