Why is string voltage drop over 3% bad?

[socratic gadfly]

I am using a system design tool. When one designs a string with low voltage (eg 100v), moderate current (eg 10a), long string (eg about 300 feet), with 10 ga wire, the voltage loss is about 6%. The tool complains that the voltage drop is too high and the "string doesn't work."

I can understand that the string is not efficient, but I still have 94v at the end of the string. This is still enough volts to run the inverter. This may be undesirable, but is it dangerous or harmful to equipment?

I don't know why the inverter should care. All that it sees is 94v. As for wires heating up due to more resistance, a longer wire may generate more heat, but it has a longer distance to dissipate it so that seems to even out.

Please help me with this.
Thanks.

 

[sunshine_eggo]

3% is code.

6% drop also means 6% power loss. That 6% power goes into heating the wires. The wire rating factors in the rate of dissipated, length, etc.

Personally, I would make a judgment call for my own needs based on cost and convenience.

 

[zanydroid]

The voltage drop may be enough to hit a boundary condition where the MPPT cannot operate. I think if the string is riding the edge of a voltage limit voltage drop might be more important.

Could have happened at 3% too.

 

[OffGridInTheCity]

The main reason is loss of power.

I don't believe heat 'evens out' across the distance. I have 6awg carrying 45a over 150ft from the PV array combiner boxes to the Midnite Classic 150 charge controllers at full power. My wires (in conduit) are warmer as they near the Midnites per measurements of temp on the conduit. At 120ft from the combiner boxes they are noticeably warmer (50C/122F) than 50ft from the combiner boxes.

I could be wrong as to the cause but my temp measurements are correct - which is why I comment that heat does not seem to 'even out'. Based on my observations and if you're doing conduit, you could wind up with some significant temps (60C?) at 300ft and 6% drop.

 

[zanydroid]

Physics wise the same waste heat / power loss should be generated per unit length of conductor, modulo the positive thermal coefficient of resistance.

However heat is not temperature. Temperature is related to how the heat is dissipated/not dissipated, and this definitely will not be symmetric down the length of a circuit.

Anyway the details of the distribution doesn’t matter if you follow code. It’s supposed to have enough margin so that you stay out of trouble.

 

[socratic gadfly]

The main reason is loss of power.

I don't believe heat 'evens out' across the distance. I have 6awg carrying 45a over 150ft from the PV array combiner boxes to the Midnite Classic 150 charge controllers at full power. My wires (in conduit) are warmer as they near the Midnites per measurements of temp on the conduit. At 120ft from the combiner boxes they are noticeably warmer (50C/122F) than 50ft from the combiner boxes.

I could be wrong as to the cause but my temp measurements are correct - which is why I comment that heat does not seem to 'even out'. Based on my observations and if you're doing conduit, you could wind up with some significant temps (60C?) at 300ft and 6% drop.

It seems that heat produced by current in wires would be a function of the number of watts are lost due to the resistance. In my example, I would lose 60w (6v * 10a) to heat.

If in your example you also lost 60w, your equation would be 60w = ?v * 45a. That means that the drop in v in your case would be 1.33v. That's due to your higher current.

It seems that if heat were the problem, simply considering % voltage drop would be a poor criterion.

 

[timselectric]

Voltage drop is a calculation of conductor size, length, amps, and volts.
Choosing the correct conductor size. For the voltage and amperage required at the end of a length.
If you can stay under 3% , you have minimal losses due to heating of the conductors.

 

[zanydroid]

If you dissipate 60W in a 1” length of wire you are approaching a soldering iron. If you dissipate it across 50’, it’s probably fine.

 

[socratic gadfly]

If you dissipate 60W in a 1” length of wire you are approaching a soldering iron. If you dissipate it across 50’, it’s probably fine.

My reason for doing this is that I have existing wires in the ground and want to use a 3000w growatt whose max volts is 115.

The length of my wires is 280 feet and they are direct bury. That seems like a good radiator to me.

 

[zanydroid]

My reason for doing this is that I have existing wires in the ground and want to use a 3000w growatt whose max volts is 115.

The length of my wires is 280 feet and they are direct bury. That seems like a good radiator to me.

Do the NEC limits not work for you? Its based on wire type, environmental conditions, etc. It’s probably safe to assume 60C for the calculations. And probably not that high of ambient temperatures.

Does the design tool let you use a shorter distance? And allow it?

There is a formula for calculating ampacity for a DC string, and then checking whether the cable is good for it.

 

[sunshine_eggo]

If I already had direct bury in place over 280 ft, and it gave me a 6% voltage drop, I would go with it.

 

[timselectric]

My reason for doing this is that I have existing wires in the ground and want to use a 3000w growatt whose max volts is 115.

The length of my wires is 280 feet and they are direct bury. That seems like a good radiator to me.

1. 280 ft
2. What type of wire is it?
3. What will the voltage be?
4. What will the amperage be?

 

[zanydroid]

10A is barely an inconvenience for #10 UF, assuming you applied the derating calculations when quoting that

Did you apply the derating calculations? I think it’s 1.25 or 1.56

15.6A is still barely an inconvenience

You don’t apply the derating for voltage drop calculation

 

[Hedges]

"Why is string voltage drop over 3% bad?"

It isn't.

Within MPPT voltage range (temperature adjusted) and wire ampacity (derated), "if it feels good, do it"

 

[MisterSandals]

(eg 100v), moderate current (eg 10a),

(100V x .06) x 10A = 60W of heat
60W / 300ft = .2W per foot does not sound like a lot, especially if there is reasonable heat dissipation.

Not saying yea or nay, just messing with numbers.

 

[socratic gadfly]

Do the NEC limits not work for you? Its based on wire type, environmental conditions, etc. It’s probably safe to assume 60C for the calculations. And probably not that high of ambient temperatures.

Does the design tool let you use a shorter distance? And allow it?

There is a formula for calculating ampacity for a DC string, and then checking whether the cable is good for it.

The tool allows for any distance but I already have wires in the ground at 280 feet.

It seems like NEC is designed for general purpose circuits that may have motors,computers, etc that would be ruined by under voltage.

It is my understanding that if you have enough volts to turn on the mppt, you're good to go. I want to know if there is anything other than volts and amps at the mppt connection that could mess things up

 

[socratic gadfly]

1. 280 ft
2. What type of wire is it?
3. What will the voltage be?
4. What will the amperage be?

I have 3 buried uf 10-3 cables (a total of 9 wires) . I will start low with 10a and 110v. I plan to combine wires to start. But if I decide to grow i want to know how far I can push it.

 

[SparkyJJO]

I would just hate to waste the watts.

 

[socratic gadfly]

10A is barely an inconvenience for #10 UF, assuming you applied the derating calculations when quoting that

Did you apply the derating calculations? I think it’s 1.25 or 1.56

15.6A is still barely an inconvenience

You don’t apply the derating for voltage drop calculation

My tool does not consider derating. It only has a places for gauge, length and conductor count. I don't know what derating is, but it sounds interesting.

 

[zanydroid]

It seems like NEC is designed for general purpose circuits that may have motors,computers, etc that would be ruined by under voltage.

That’s the voltage drop recommendation. But the ampacity calculation methods is fine for this application. In fact it’s actually the law.

On #10 UF you can go up to 30A after PV derating rules from a fire safety angle. Assuming you don’t stuff the UF in a shared conduit with other conductors at some point.