Voltage Drop on long Cable

[UFO]

I have noticed that the approx 15m 10AWG wire to my water maker boost pump has a large voltage drop on it down to around 11.75V. The pump still appears to work and is pulling its rated 6 Amps.


I have not tested it to make water as I am currently stopped in a marina (to fix another issue) and do not want to run marina water through the membranes.


My question is: Is this a normal drop over such a distance and as the pump is pulling its rated 6 AMPS will the pump work as it should at this lower voltage? or should I be hunting cause of the drop?

[barnab]

Without knowing the voltage at the batteries it's impossible to assess the drop. Drop is the difference between to voltages and you give only one.



If the 15m are one way and I got the math right the voltage drop should be around 0.7V with your cable. Assuming your batteries are around 12.5V this would be the expected drop. I would say 0.7V drop is too much and you need to go to 6 or 8 AWG.If your batteries are higher than 12.5V, you have a bad connection on top of the cable being too small.

[mvweebles]

See attached wire size chart from Blue Sea. Note, the distances are "round-trip" distances across both + and - conductors. If you have 15-meters of 2-conductor cable to your pump, distance is 30-meters. Also of note is connections and crimps are not accounted for and sometimes affect the overall circuit resistance.

Looks like loss for 30-meters of 10AWG at 6-amps will approach 10%. Personally, I'd go with 6AWG and be done with it. I know proper tinned wire is expensive but its a one time cost.

[GordMay]

YES, the pump should work fine, as described.


Assuming a nominal 12.0VDC at the battery [source]; 11.75V, at the water maker boost pump [load] represents only about a 2.1% Voltage Drop - which is better than the [max] 3% VD, often specified*.

* ABYC E11, for instance, specifies a maximum Voltage Drop of 3%, for critical/essential items; and max 10% VD, for non-critical items.
To some extent, ABYC leaves it up to the user, to determine what is critical, or non-critical.
I always sized ALL wiring to accommodate a maximum VD of 3%.

Originally Posted by ABYC 2009 Edition

“... E11.14.2.6 Voltage Drop
Conductors used for panelboard or switchboard main feeders, bilge blowers, electronic equipment, navigation lights, and other circuits where voltage drop must be kept to a minimum, shall be sized for a voltage drop not to exceed three percent [3%].
Conductors used for lighting, other than navigation lights, and other circuits where voltage drop is not critical, shall be sized for a voltage drop not to
exceed 10 percent [10%].”

The conductor size, necessary to keep voltage drop below the maximum permitted level, may be calculated by means of the following formula:

CM = [K x I x L] ÷ E

Where:
CM = Circular mil area of conductor [consult wire specs]
K = 10.75 (constant representing the resistivity of copper)
I = Load current in amperes
L = Length of conductor from the positive power source connection to the electrical device, and back to the negative power source connection [round trip], measured in feet.
E = Maximum allowable voltage drop at load, in volts [e.g., for a three percent voltage drop at nominal 12V, E= 0.03 x 12 = 0.36; for a 10 percent voltage drop at nominal 12V, E = 1.2]

Your specific case:
CM = [10.75 x 6A x 50 Ft] ÷ 0.36VD
CM = 3225 ÷ 0.36 = 8958 CM
#10 AWG is 9343 CM; which exceeds the minimum requirement [8958 CM] for 3% voltage drop.


See also "Ohm's Law & Boats" ➥ https://www.cruisersforum.com/forums....html#post1256

[mvweebles]

Originally Posted by GordMay

YES, the pump should work fine, as described.


Assuming a nominal 12.0VDC at the battery [source]; 11.75V, at the water maker boost pump [load] represents only about a 2.1% Voltage Drop - which is better than the [max] 3% VD, often specified*.

* ABYC E11, for instance, specifies a maximum Voltage Drop of 3%, for critical/essential items; and max 10% VD, for non-critical items.
To some extent, ABYC leaves it up to the user, to determine what is critical, or non-critical.
I always sized ALL wiring to accommodate a maximum VD of 3%.

12V at-rest voltage is about 50% SoC. At-rest voltage of a healthy AGM battery at 90% SoC is 12.75V so that should be the numerator, not nominal (12.00V). Assuming the OP has something similar, he's seeing almost 8% voltage drop across the 10AWG conductor.

I would think the boost pump would be fine with a 10% loss, though start-up is likely double the 6-amp draw. Unless there was some malady rearing it's head, swapping for 6-AWG wouldn't be the first priority on my project list, but it would be on the list.

QUESTION for the OP: What made you check the voltage? Was something not operating correctly?

[GordMay]

Originally Posted by mvweebles

12V at-rest voltage is about 50% SoC. At-rest voltage of a healthy AGM battery at 90% SoC is 12.75V so that should be the numerator, not nominal (12.00V). Assuming the OP has something similar, he's seeing almost 8% voltage drop across the 10AWG conductor.[1]

I would think the boost pump would be fine with a 10% loss, though start-up is likely double the 6-amp draw. Unless there was some malady rearing it's head, swapping for 6-AWG wouldn't be the first priority on my project list, but it would be on the list...

[1] TRUE.
But, I used the nominal 12V, for consistency, because that's what ABYC, (the specifier of the 3% & 10% values) uses.

FWIW: Both, the NEC & CEC recommend a maximum combined voltage drop of 5% at the farthest outlet, across feeders and branch circuits; and 3% at the farthest outlet, across the branch circuit alone.

[s/v Jedi]

No, voltage drop is the actual drop of voltage over the length of the wire, not anything to do with opinions nor institutions.

The best way to measure it is with long test leads: one probe at the pump positive and one at the breaker/fuse positive. Note voltage reading, ignore plus/minus sign. Now repeat for the negative conductor.

Add absolute values of both readings for total voltage drop. If they are different, there is a problem with contact resistance.

You want a 3% drop maximum for this circuit so I guess that’s right at the step from 8AWG TO 6AWG.

Current should go down with a bigger wire or more water gets pumped. Either case is better.

[Wolfe10]

Originally Posted by UFO

I have noticed that the approx 15m 10AWG wire to my water maker boost pump has a large voltage drop on it down to around 11.75V.

Before you can determine the minimum gauge for max of 3% drop in voltage, please let us know if the 15m run is one way (positive) or round trip (positive plus negative).


Quite a difference between 15m and 30m.

[sailorboy1]

Originally Posted by UFO

I have noticed that the approx 15m 10AWG wire to my water maker boost pump has a large voltage drop on it down to around 11.75V. The pump still appears to work and is pulling its rated 6 Amps.


I have not tested it to make water as I am currently stopped in a marina (to fix another issue) and do not want to run marina water through the membranes.


My question is: Is this a normal drop over such a distance and as the pump is pulling its rated 6 AMPS will the pump work as it should at this lower voltage? or should I be hunting cause of the drop?

something worth knowing exists

https://www.calculator.net/voltage-drop-calculator.html

[Jenta]

Several good replies here about voltage drop so no need to address that any differently. However, I did want to add something to think about when sizing your wiring for any job.

Critical or not, I prefer my voltage drop to be as low as reasonably possible given the cost and weight for sizing up.

That means as short of a run as needed to get the job done which is a major concern during the design or redesign phase of any electrical project.

It also means sizing the wire to handle the load with a good margin.

Why? Because that voltage drop means that you're losing power to the cable. Your essentially using extra power to heat the wire instead of the intended load.
4 percent may not be a lot for a small LED bulb with very little current draw, but if you were running a water maker that took 750 watts (60 amps at 12.5v) to operate and had a .5 volt drop then 30 watts are being lost on the cable.
Run that water maker for 5 hours and that's at least 150 Wh or 12 Ah in this scenario that went to heating the wire instead of doing the work.

I understand that isn't a lot by itself, but it does add up. Especially if you're on any kind of power budget.

(Note - this was on the fly so my math could be off - but I think it's right!)

[UFO]

Voltage at Breaker Board and batteries was 13.3V, so a pretty big drop above what the wire should account for. Also it is both + & - doing the 15M

[sailorboy1]

Originally Posted by UFO

Voltage at Breaker Board and batteries was 13.3V, so a pretty big drop above what the wire should account for. Also it is both + & - doing the 15M

so a 6A load should have a 0.7V drop on a 10awg wire. So you have a bad connections etc somewhere

at same time unless this is a Spectra type of WM it probably doesn't matter.

[UFO]

Originally Posted by sailorboy1

so a 6A load should have a 0.7V drop on a 10awg wire. So you have a bad connections etc somewhere

at same time unless this is a Spectra type of WM it probably doesn't matter.

No not a spectra, but I think ill find that issue anyway - I think its at the main control panel and will investigate further.

[s/v Jedi]

Originally Posted by UFO

No not a spectra, but I think ill find that issue anyway - I think its at the main control panel and will investigate further.

Use a multimeter across suspect points like the terminals on a breaker. Zero Volt means it is good, any reading is the voltage drop between those two points.

I often use an IR temperature gun and look for hot spots.

[GordMay]

Originally Posted by Jenta

Several good replies here about voltage drop so no need to address that any differently. However, I did want to add something to think about when sizing your wiring for any job.

Critical or not, I prefer my voltage drop to be as low as reasonably possible given the cost and weight for sizing up.

That means as short of a run as needed to get the job done which is a major concern during the design or redesign phase of any electrical project.

It also means sizing the wire to handle the load with a good margin. ...

It, also, means eliminating as many unnecessary connections, as possible; and insuring the all necessary connections are of the highest quality. [both items reducing voltage drop].