Thick Feeder Cable for PV

[sjkted]

I live in the forest and have some solar shading from trees. I am adding an array with (3) used 36V 250 watt panels. They will be east facing in order to capture sunlight in the morning. The feeder cable to connect them to my solar combiner box would be about 80 feet. From the voltage drop calculator, I estimate I would need a 4AWG cable with 2.6% voltage drop or maybe I could get by with 6 AWG. I'm not sure the cost difference is enough to try to skimp on it. Anyways, from my research the minimum MC4 connector appears to be something like 8AWG. Is there a "correct" way to do this? I'm sure I could do a butt connector or splice them with shrinkwrap tubing, but I would like to do something more standard if possible. I don't have code checks or anything so I can do whatever I want, I just want it to last.

sjkted

 

[Mattb4]

Just to clear things a bit. Is this a grid tie?
If not, an Array would connect to its own SCC and not to another array at a combiner box. If this is intended to be a string in parallel with other strings (combiner box) it would require the other strings to have the same voltage. Not knowing your entire panels specs, nor you proposed setup for either Parallel or Series connected panels, it is hard to say what kind of voltage drop or wire size you need.

 

[seneysolar]

I live in the forest and have some solar shading from trees. I am adding an array with (3) used 36V 250 watt panels. They will be east facing in order to capture sunlight in the morning. The feeder cable to connect them to my solar combiner box would be about 80 feet. From the voltage drop calculator, I estimate I would need a 4AWG cable with 2.6% voltage drop or maybe I could get by with 6 AWG. I'm not sure the cost difference is enough to try to skimp on it. Anyways, from my research the minimum MC4 connector appears to be something like 8AWG. Is there a "correct" way to do this? I'm sure I could do a butt connector or splice them with shrinkwrap tubing, but I would like to do something more standard if possible. I don't have code checks or anything so I can do whatever I want, I just want it to last.

sjkted

3 panels in series shouldnt need anything that heavy. Likely 10-12 awg should suffice. Voltage drop is not critical, look at ampacity rating.

 

[sjkted]

The 3 panels would be in parallel at 36V. I have heard you need to be within 3% - 5% for voltage drop. Is that wrong?

This is for an off-grid system. It connects to the solar combiner box and then solar charge controller.

sjkted

 

[zanydroid]

The 3 panels would be in parallel at 36V. I have heard you need to be within 3% - 5% for voltage drop. Is that wrong?

Voltage drop is always dependent on the thing being fed. Somethings are more tolerant than others. General rule of thumb is that MPPTs are tolerant of voltage drop b/c they have to be tolerant of use with different solar panels.

For solar the voltage drop concern is whether the voltage drop at maximum power point (calculated by the current at max power point + Ohm's law) will be below the operating range of the MPPT. Which is published in the specs, and determined by the voltage of your batteries (unless you have a boost MPPT rather than the standard buck MPPT)

 

[zanydroid]

FWIW if you googled for "voltage drop calculator", on average it is highly unlikely to be customized for solar. A solar customized one would have to intake the MPPT, battery specs, and solar panel specs. I've never seen one, though it probably ought to exist to help designers.

 

[sjkted]

The voltage drop calculator is not for solar in particular, but you can feed in volts, amps, and length of the cable run for DC current and it will spit out a % of voltage drop. This is what the NEC uses. I'm not sure why 36V solar would be any different than any other low voltage system.

sjkted

 

[Ampster]

I have heard you need to be within 3% - 5% for voltage drop. Is that wrong?

Not wrong but as others have said not as important for solar voltages because of the ability of the MPPT controller to handle varying voltage. The only thing to make sure is that the voltage drop does not put you below minimum startup voltage for the MPPT as mentioned earlier.

 

[sjkted]

I think I'm going to do something like this:

Polaris 4-14 AWG Bagged Insulated Tap Connector, Black IT-4B - The Home Depot

IT series insul-tap connectors are suitable for connecting 2-wires. Which are provided with open wire entry ports on both sides of the connector, which allows for access from either side. This style is

www.homedepot.com

The run is going to be buried in conduit, so I really don't want to touch it again after I set it up.

sjkted

 

[zanydroid]

The voltage drop calculator is not for solar in particular, but you can feed in volts, amps, and length of the cable run for DC current and it will spit out a % of voltage drop. This is what the NEC uses. I'm not sure why 36V solar would be any different than any other low voltage system.

Didn't I walk through in detail why?

Happy to go point by point clarification if you start it.

 

[zanydroid]

I think I'm going to do something like this:

Polaris 4-14 AWG Bagged Insulated Tap Connector, Black IT-4B - The Home Depot

IT series insul-tap connectors are suitable for connecting 2-wires. Which are provided with open wire entry ports on both sides of the connector, which allows for access from either side. This style is

www.homedepot.com

The run is going to be buried in conduit, so I really don't want to touch it again after I set it up.

sjkted

Polaris connectors are an option. DIN blocks / combiner boxes are potentially more common on here

With 3P you already need fusing (2P and 1P do not need fusing). In that case fuses in a DIN box, + a combiner bus / terminal blocks as appropriate, would be common.

 

[zanydroid]

From your posting history, looks like you are using 12V battery, you have huge space to accept voltage drop. With 36V panels you are definitely using a buck MPPT

It is an efficiency question. Is the extra Wh you can gain from bigger wire, worth the cost and effort?

 

[Mattb4]

The 3 panels would be in parallel at 36V. I have heard you need to be within 3% - 5% for voltage drop. Is that wrong?

This is for an off-grid system. It connects to the solar combiner box and then solar charge controller.

sjkted

You did not answer the questions. I doubt that you are running wire from each panel 80ft to your combiner box. That would take 6 wires and if so they would only need to carry the amperage of 1 panel each.

The purpose of voltage drop calculations is to maintain a sufficient voltage depending on amperage at the end of a conductor run. Your SCC selects the best voltage and current necessary to output at the maximum possible rate to the battery. So it is less dependent on standard voltage drop considerations.

 

[sjkted]

If this is intended to be a string in parallel with other strings (combiner box) it would require the other strings to have the same voltage.

Yes it is a string in parallel with other strings. All of them will be 36V. So, if we are not dependent on standard voltage drop considerations, what is the best way to size wire? From the answers I am getting it sounds like it is somewhat guess work.

sjkted

 

[zanydroid]

I don't think I can help further without an itemized pointer at which parts of the explanation need more explanation.

You got two non-guesswork, semi-detailed technical answers.

There is a pretty closed form (like fully modeled and calculated), explicit answer possible here. For instance, the meme calculation would be, we can find the minimum start voltage of your MPPT for the 12V battery, and compute Impp * (Vmpp - Vmin) to see what the power loss would be if we target the exact voltage drop level at which your system would fail to operate.

You may need #8 to be strictly code compliant, maybe #10 will barely fit. It depends on the Solar panel specs, which you haven't shared. This will most likely give the 3-5% drop level. 5% * 36V (I don't know your Vmpp, because you haven't shared it) gives an operating voltage at MPPT of 34.2V. This is clearly enough to start the MPPT. But you will lose some amount of power (which I can't calculate, because you haven't shared your specs / expected insolation level)

I think that's enough from me.

 

[Q-Dog]

Size the wire based on how many amps you need to carry.

Are these 3 panels in series? Then parallel to another string of 3 panels.

 

[sjkted]

Well, if the SCC is less dependent on standard voltage drop calculations, how do you size wire for it? There has to be some percentage of voltage drop that is unacceptable.

sjkted

 

[sjkted]

Size the wire based on how many amps you need to carry.

Yes, this is what I am planning to do.

sjkted

 

[Rocketman]

The best thing would be to raise the voltage- by putting the the panels in series and using a mppt like a Victron 150/35 (or whatever size your system needs) you solve both issues. Because the amps are much lower you can use much smaller wire and because the voltage is higher it doesn’t matter if you lose a volt or two on voltage drop.

80’ is not a long distance to go…

 

[zanydroid]

Well, if the SCC is less dependent on standard voltage drop calculations, how do you size wire for it? There has to be some percentage of voltage drop that is unacceptable.

sjkted

Assuming your minimum MPPT voltage is 20V. Then you can drop all the way down to that point, which is 45% assuming 36V is Vmpp.

 

[sjkted]

Wow, that's crazy. I had no idea there was such a range. I'll have to look at it.

sjkted

 

[robbob2112]

three thumbs up - higher voltage, smaller wire, leave 20% or so voltage overhead if your location gets cold - there should be a thermal figure on the back you can use to calculate exactly what the voltage rise is for your lowest ever temp

 

[Mattb4]

Before you buy anything I suggest you know all the specifications. Panels are not just watts and volts. SCC have requirements for minimum and maximum voltages. All of it must match from panels to feeder wires, combiner boxes, fuses, breakers, SCC to battery and on to inverter if you need AC.

It does no good to buy things that are incompatible.