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Wire sizes

bgary

Advanced Beginner
Blogs Author
Here's today's contribution to the pantheon of dumb questions.

I'm working out my plan to install a new pump. Water-witch switch in the bilge, water-witch 3-way switch panel somewhere close by, and the pump itself under the transom... about 15-ish feet of wire-run between the switch/panel and the pump.

Jabsco manual says that for a 12v run of 1-25 feet I should use 12ga wire. okay.

….but… it appears to me that the wires on the pump itself are 14ga, just eyeballing them, and I'd guess that the wires from the breaker panel to the existing bilge pump are the same (14ga). From the photos, it also looks like the wires on the water-witch itself are pretty small.

If 12ga is required.... should I plan to rewire the whole run so that it is 12ga end-to-end? It seems to me that when the pump is running the "load" will be carried end-to-end (from the breaker, through the switch panel, through the water-witch, to the pump and then to ground)… and if the wires on the actual devices are 14ga or smaller, will I have actually accomplished anything useful when I am done?

Mr. Metzger, can I have another martini's worth of insight?

Bruce
 

gadangit

Member III
Each wire size has a different resistance (ohms) per foot. You can calculate the total resistance of your total wiring run by multiplying the ohms/foot times each length of different size cable. Once you have the total resistance of your wiring run you can calculate voltage drop by multiplying full load amperage of the motor times your total wiring resistance.

I'll let Mr. Metzger expand from there.
 

Slick470

Member III
This is a relatively common condition in the electrical distribution world. You are trying to keep the % voltage drop under a certain limit for the entire run from the source to the load. As long as the entire run is under the required % voltage drop limit you are ok. I have seen cases with feeders serving site lighting where you needed #8 wires for voltage drop, but you could only actually make the final connection to the fixture with #12.

You could do the math for the run to see if you can get away with #14 for the entire run, but if it is easy to run the #12, I would probably do that but leave wires for the pump and water witch alone.
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
I have always believed that you look at the length of each size of wire, since a very short piece of final connection will be a fraction of the total distance to breaker panel and back. Of course that scheme is only valid until any portion of the circuit wire is too small and literally overheats.
I may be agreeing with Andy and Chris, but am not sure enough to declare it!
 
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bgary

Advanced Beginner
Blogs Author
Very useful, but (as usual) spawns some stupid follow-up questions.

Does that imply that using "big wire" to reduce voltage drop over the long run is good, but that a small wire on the device at the working end is okay? If I'm interpolating correctly, the big wire would deliver sufficient power over the long part of the run, and the smaller wire at the connection doesn't have much voltage drop over a much shorter distance so it's okay....?

Sort of like a main-line for water.... big pipe under the street, to make sure that there's enough (volume and pressure / voltage and amps) to feed all the little pipes that connect to it....?

(hey, I'm a software guy...)
 

Slick470

Member III
In a nutshell, yes. As long as the small wire is sized properly for the amperage of the load, and it's additive voltage drop doesn't doesn't bring the total voltage drop of the run above the required %VD.
 

Tom Metzger

Sustaining Partner
Mr. Metzger, can I have another martini's worth of insight?

Too late for tonight, but I'll try. Out in the world there are different wiring standards for "equipment" and for "wiring". The difference, besides the obvious - cost, ours vs their's, is that the equipment's wire length is small compared to the wiring run.

The voltage drop is equal to the sum of the resistance of each length's wire times the current flowing. The current is the same in each segment. The resistance in the equipment's segments is generally small compared to the resistance allowed in the wiring.
 
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