voltage drop question

diat150 · May 23, 2009

I have a quick question, I want to verify the data that this online calculator provides.

a company installed some equipment for me and used 14-3 to power my panel. the 14-3 run was around 80 feet. the system voltage is usually between 24-26 volts DC. The panel current is usually around 1-2 amps, but can get as high as 7-8 amps depending on a dc motor that is powered out of this panel. we have had some issues with this panel because of this and before I present this to the installer I want to make sure I have all of my ducks in a row.

I used this online claculator I found to verify what i already knew, that 14 gauage wire is too small to run 6-7 amps with a 80 foot run.

I am sure it is correct because my voltage is 26-ish and we see it get down to the 22-23 volt range when we run the dc motor.

it looks like we should go with at least a 8 gauge cable for this run of 80 feet. what do you think?

http://www.nooutage.com/vdrop.htm

milldrone · May 23, 2009

diat150 said:
I am sure it is correct because my voltage is 26-ish and we see it get down to the 22-23 volt range when we run the dc motor.

Diat150,

I got similar results when I use a 160 foot (coming and going) wire length on this wire resistance calculator.
http://www.stealth316.com/2-wire-resistance.htm

leitmotif · May 23, 2009

In this case you have two things to consider
1. Voltage droop of power supply
2. Voltage drop of your conductor.

I see no mention of voltage droop on power supply. I would bench test motor and power supply to eliminate as much conductor voltage drop as I could and get a good reading of droop.

Given limited informatin and assuming the 22 or 23 volt (measured at motor) is a steady state run voltage and not voltage during start I do not think I would necessarily be alarmed over this.

Dan Bentler

diat150 · May 23, 2009

leitmotif said:
In this case you have two things to consider
1. Voltage droop of power supply
2. Voltage drop of your conductor.

I see no mention of voltage droop on power supply. I would bench test motor and power supply to eliminate as much conductor voltage drop as I could and get a good reading of droop.

Given limited informatin and assuming the 22 or 23 volt (measured at motor) is a steady state run voltage and not voltage during start I do not think I would necessarily be alarmed over this.

Dan Bentler

let me add more information. this is a solar and win powered system with a battery bank. the panel has a plc, quickpanel, radio, and other components. If it were just a motor it might not be an issue, but this is causing issues with my other equipment.

leitmotif · May 23, 2009

diat150 said:
let me add more information. this is a solar and win powered system with a battery bank. the panel has a plc, quickpanel, radio, and other components. If it were just a motor it might not be an issue, but this is causing issues with my other equipment.

OK now we are getting places. You still have a voltage droop problem. Basically
not enough battery
OR not enough generation to fully charge the battery
OR you are not charging enough (charging voltage too low)

Let me state why I am somewhat qualified - am former nuc submarine electrician - generation and battery experience.
I also have a sail boat with 3 battery banks Engine start, Vital and Hotel.
Engine Alternator charges Starting battery first.
THEN Vital is paralled to charge.
Last Hotel is paralleled to charge.

BATTERY Power out = power in X 1.3 (approx)
I would split the batteries when not charging and parallel them when you have charge capacity (ie solar and or wind)
Put your most vital and most droop sensitive on one bank.
Put less sensitive on one or two other banks.

Dan Bentler

diat150 · May 23, 2009

leitmotif said:
OK now we are getting places. You still have a voltage droop problem. Basically
not enough battery
OR not enough generation to fully charge the battery
OR you are not charging enough (charging voltage too low)

Let me state why I am somewhat qualified - am former nuc submarine electrician - generation and battery experience.
I also have a sail boat with 3 battery banks Engine start, Vital and Hotel.
Engine Alternator charges Starting battery first.
THEN Vital is paralled to charge.
Last Hotel is paralleled to charge.

BATTERY Power out = power in X 1.3 (approx)
I would split the batteries when not charging and parallel them when you have charge capacity (ie solar and or wind)
Put your most vital and most droop sensitive on one bank.
Put less sensitive on one or two other banks.

Dan Bentler

I think you re misunderstanding. the batteries can handle the load, the problem is the 14 gauge 80 foot piece of wire cant. as current goes up the voltage goes down.

leitmotif · May 23, 2009

This 80 foot conductor serves only the motor right??
Check voltage across your DC line at battery terminals when unloaded and loaded.

Dan Bentler

controlled · May 23, 2009

leitmotif said:
This 80 foot conductor serves only the motor right??
Check voltage across your DC line at battery terminals when unloaded and loaded.

Dan Bentler

I think the OP stated that the 14/3 feeds the entire control panel

leitmotif · May 23, 2009

controlled said:
I think the OP stated that the 14/3 feeds the entire control panel

OOOPS missed that.

Dan

surferb · May 24, 2009

So for the edification of the casual lurker...he needs to go with lower gauge wire?

leitmotif · May 24, 2009

surferb said:
So for the edification of the casual lurker...he needs to go with lower gauge wire?

Nate he has three questions to answer
1. What is voltage droop of batteries under load?
2. What is voltage drop of the conductor.
3. He may not have large enough batteries or they are undercharged.

It is hard to do this with only a crystal ball and no factual information on his setup but
He is not going to overcome battery droop with larger conductor - he will reduce voltage drop of course.
If mine I would run another circuit off a different battery that is isolated from the battery feeding the motor. That should
A reduce battery droop from current drawn by motor.
B reduce the voltage drop problem.

When his generation is back on line then parallel the batteries.

Dan Bentler

diat150 · May 24, 2009

leitmotif said:
Nate he has three questions to answer
1. What is voltage droop of batteries under load?
2. What is voltage drop of the conductor.
3. He may not have large enough batteries or they are undercharged.

It is hard to do this with only a crystal ball and no factual information on his setup but
He is not going to overcome battery droop with larger conductor - he will reduce voltage drop of course.
If mine I would run another circuit off a different battery that is isolated from the battery feeding the motor. That should
A reduce battery droop from current drawn by motor.
B reduce the voltage drop problem.

When his generation is back on line then parallel the batteries.

Dan Bentler

the batteries are more than adequate, 8 100 amp hour batteries in a 24 volt configuration. the small conductor cannot handle the amount of current when the motor is running.

when the motor isnt running, the voltage drop is around 3.6%.

when the motor is running the voltage drop goes to 12-14%, which is causing the issue.

I know the issue is the wire. I have similar setups in numerous location, the only difference is the inadequate feed wire to that panel. I just wanted to double check these numbers.

Lancie1 · May 24, 2009

So for the edification of the casual lurker...he needs to go with lower gauge wire?

Yes, the smaller the number of the American Wire Gauge, the larger is the diameter of the wire.

Just by the seat-of-my-pants, #14 AWG is too small for an 80' run of 6 Amps at 24 VDC.

The general formula is:

VD% = {[(2 X L X A)/1000] X Ohms per 1000' X 100} / V

Where VD% = Voltage Drop Percent
L = Length of circuit, one-way
A = Amps load in circuit
Ohms/1000' = wire resistance per 1000 feet
V = Voltage of circuit

The resistance of #14 is about 3.26 Ohms, therefore with a 6 Amp load,
VD = {[(2 X 80 X 6)/1000] X 3.26 X 100} / 24 = 13.04 %
3% is considered to be the maximum allowed voltage drop.

Using a #8 wire will get you close enough. Alternatively, you can raise the voltage to 105 volts and that gets you a 3% drop at 6 amps, but that might be hard on the equipment!

Your contractor is going to say that #8 is impossible and #10 is a maybe. So you might try raising your power supply voltage to 30 VDC and using #10 wire. That will get you down to a 4% drop, which you might could live with. Hey, this world is about making the best of what is possible to do.

diat150 · May 25, 2009

Lancie1 said:
Yes, the smaller the number of the American Wire Gauge, the larger is the diameter of the wire.

Just by the seat-of-my-pants, #14 AWG is too small for an 80' run of 6 Amps at 24 VDC.

The general formula is:

VD% = {[(2 X L X A)/1000] X Ohms per 1000' X 100} / V

Where VD% = Voltage Drop Percent
L = Length of circuit, one-way
A = Amps load in circuit
Ohms/1000' = wire resistance per 1000 feet
V = Voltage of circuit

The resistance of #14 is about 3.26 Ohms, therefore with a 6 Amp load,
VD = {[(2 X 80 X 6)/1000] X 3.26 X 100} / 24 = 13.04 %
3% is considered to be the maximum allowed voltage drop.

Using a #8 wire will get you close enough. Alternatively, you can raise the voltage to 105 volts and that gets you a 3% drop at 6 amps, but that might be hard on the equipment!

Your contractor is going to say that #8 is impossible and #10 is a maybe. So you might try raising your power supply voltage to 30 VDC and using #10 wire. That will get you down to a 4% drop, which you might could live with. Hey, this world is about making the best of what is possible to do.

thanks for the response.

the contractor is going to have to run 8 or he wont get payed for the 2 trips that he made after the installation to troubleshoot. 30vdc isnt going to happen as this is solely powered by solar and windpower.

voltage drop question

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