Learning how to fish, or, how to correctly calculate wire size and other misc. Qs

[kybishop]

Hey all,

I'm trying to teach myself how to fish and was hoping for some sanity checks on my research thus far.

I'm looking to manually calculate the required wire size for a given build. The formula I've come across a couple times is the one listed here: http://www.paigewire.com/pumpWireCalc.aspx

Wire Circular Mils = (Conductor Resistivity)(Amps)(2)(One Way Distance in Feet) / Allowable Voltage Drop
Allowable Voltage Drop = volts * percent voltage drop as integer
// allowable voltage drop for a panel with a 12 volt VOC and a 3% voltage drop would = 12*3

Let me know if I messed up the interpretation of allowable voltage drop!

Sanity checks regarding wire size:

1. Am I right in thinking you then need to use a table to convert from circular mils (CM) to American wire gauge?
2. When calculating allowable voltage drop for the panels, use cumulative VOC from panels if in series, or singular VOC if in parallel
3. Are the voltages for the other components (charge controller to batteries, batteries to inverter) simply the system voltage (12, 24, etc.), or is there another number one should use, similar to VOC for the panels?

Misc Sanity checks:

1. When calculating max amperage, multiply by 1.3 to account for edge of cloud effect? Does anyone think you should go higher or lower? Is there an industry-standard for this? Does this multiplier change depending on the size of your system? (aka 1.3 for small systems, 1.4 for house sized systems, etc.)
2. When calculating total panel voltage, should you multiply by anything to account for things like edge of cloud effect? I swear I read something that recommended multiplying by 1.1

Charge controller questions:

1. Does one have to take max VDC, max Amps, AND max wattage into account when selecting the appropriate charge controller? It just seems redundant that a charge controller also lists max wattage. Can a combination of high VDC and high amperage somehow overload the charge controller even if these stats individually fall below the charge controller's max specifications?
2. Assuming one never wanted to upgrade with more panels, how much headroom should one allow for max VDC, max amps, and max wattage? AKA if I'm running at 99% of max specs, is my charge controller going to die an early death?

Thanks so much for any help!
- Kyle

[Sunking]

Kyle you are asking way too many questions in one thread, Si l will tackel as many as I can in 5 minutes.

Sanity checks regarding wire size:

1. Am I right in thinking you then need to use a table to convert from circular mils (CM) to American wire gauge?
2. When calculating allowable voltage drop for the panels, use cumulative VOC from panels if in series, or singular VOC if in parallel
3. Are the voltages for the other components (charge controller to batteries, batteries to inverter) simply the system voltage (12, 24, etc.), or is there another number one should use, similar to VOC for the panels?

A1 Yes, any wire manufacture website has tables. Just Google AWG Sizes.
A2. You use the operating voltage. If you use MPPT controller then you use the total series Vmp. Voc is too high and can lead to smaller wire gauges. If you use PWM you use battery voltage + 1 volt. Having said that you are looking at 2% maximum voltage drop
A3 operating voltage always, not open circuit voltages.

Misc Sanity checks:

1. When calculating max amperage, multiply by 1.3 to account for edge of cloud effect? Does anyone think you should go higher or lower? Is there an industry-standard for this? Does this multiplier change depending on the size of your system? (aka 1.3 for small systems, 1.4 for house sized systems, etc.)
2. When calculating total panel voltage, should you multiply by anything to account for things like edge of cloud effect? I swear I read something that recommended multiplying by 1.1.

A1 & 2 If if use 2% voltage drop, the wires are over sized to meet any code requirements unless the runs are really short.

Charge controller questions:

1. Does one have to take max VDC, max Amps, AND max wattage into account when selecting the appropriate charge controller? It just seems redundant that a charge controller also lists max wattage. Can a combination of high VDC and high amperage somehow overload the charge controller even if these stats individually fall below the charge controller's max specifications?
2. Assuming one never wanted to upgrade with more panels, how much headroom should one allow for max VDC, max amps, and max wattage? AKA if I'm running at 99% of max specs, is my charge controller going to die an early death?

Assuming you are talking about MPPT charge controllers, the maximum voltage input will be listed as Voc. Most is 150 Voc. So if using NEC guidance if your maximum panel array voltage would be 150 volts / 1.25 = 120 volts. The exception woul dbe if you live in a cold climate requiring even lower voltage.

MPPT controllers output current = Panel Wattage / Battery Voltage. Generically a 80 amp MPPT controller has the maximum wattage input:

1000 watts @ 12 volt battery
2000 watts @ 24 volt
4000 watts @ 48 volt

[kybishop]

Kyle you are asking way too many questions in one thread, Si l will tackel as many as I can in 5 minutes.

My apologies, and a huge thanks for taking the time! I'll separate my questions into their own threads in the future, just didn't want to swamp the forum with so many threads all at once.

A1 Yes, any wire manufacture website has tables. Just Google AWG Sizes.
A2. You use the operating voltage. If you use MPPT controller then you use the total series Vmp. Voc is too high and can lead to smaller wire gauges. If you use PWM you use battery voltage + 1 volt. Having said that you are looking at 2% maximum voltage drop
A3 operating voltage always, not open circuit voltages.

I just read up a bit more on Vmp and Voc. This was very helpful http://www.altenergy.org/renewables/...ing-solar.html Anyways, is it safe to use Vmp alone when determining wire size? It just seems like there is potential variability (in a worst-case scenario, up to the Voc) in voltage. I'm guessing that the distance that the wire needs to travel determines whether or not you need to be more careful with this kind of thing (shorter == need to be more careful)?

A1 & 2 If if use 2% voltage drop, the wires are over sized to meet any code requirements unless the runs are really short.

How short is "really short"? The lengths I'll be dealing with will often range between 1 and 10 feet. I'd love to understand the breaking point where one has to start looking at max voltage and amperage more carefully.

Assuming you are talking about MPPT charge controllers, the maximum voltage input will be listed as Voc. Most is 150 Voc. So if using NEC guidance if your maximum panel array voltage would be 150 volts / 1.25 = 120 volts. The exception woul dbe if you live in a cold climate requiring even lower voltage.

MPPT controllers output current = Panel Wattage / Battery Voltage. Generically a 80 amp MPPT controller has the maximum wattage input:

1000 watts @ 12 volt battery
2000 watts @ 24 volt
4000 watts @ 48 volt

Just making sure I understand correctly; do you mean that the NEC recommends operating a charge controller at its max input voltage / 1.25? (unless in a colder climate). Also, does "maximum panel array voltage" == total series Vmp?

All the MPPT charge controllers I saw on Amazon seem to only show max input Vdc, with no mention of Voc. Are these terms interchangeable, or might you have suggestions on how I could find resources that explain their differences?

Once again, thank you so much!

[sensij]

Just a suggestion... instead of trying to learn it all in general, and then apply it to your system, you might want to approach it the other way. Let's talk about your system, and from that, you will have a better perspective on the general understanding you are missing right now.

Just to pick on a few points in particular:

To save you the trouble of looking up wire resistance and cross sectional area for voltage drop, there are a number of calculators that do it for you.
This one requires that you enter the round trip distance, and generates a table showing the voltage drop at different wire sizes.

With regard to wire ampacity, NEC would require that you use the Isc (short circuit current) of the PV array, multiplied by (1.25*1.25) = 1.56.

With regard to the charge controller, the amp rating is often on the output. The max voltage and max array wattage are usually input ratings. When all three ratings are on the input or output together, the wattage rating is saying that it can't handle the max voltage and the max amperage at the same time.

I wouldn't worry about leaving headroom in the rating to try to extend the life. You'll be paying more for a higher rated component and not using it, which, in the long run, will probably be more expensive than what life shortening would occur by operating a lower rated component at its rating. Some components lose efficiency when you get further away from their rating, as well.

The temp correction Sunking suggested is take whatever Voc you calculate for the array and multiply it by 1.25. The temp corrected Voc should be less than the Vdc input rating of the charge controller. In cold climates, the correction factor may be higher, and in warm climates, you might be able to get away with less.

[foo1bar]

Assuming you are talking about MPPT charge controllers, the maximum voltage input will be listed as Voc.

I think you're assuming a battery system - but this is posted in the "systems for the grid" section.

SO - Kyle - what kind of system are you looking to have?

[Amy@altE]

I did a 20 minute video explaining NEC compliant wire sizing. I think it would help you. In YouTube, search for "Wiring Your Solar Electric System". It should be one of the first ones that comes up. There's a bunch other I did that will help with some of your other question, and I'm trying to finish up how to size a battery based system, hopefully that will be up soon.

[kybishop]

Just a suggestion... instead of trying to learn it all in general, and then apply it to your system, you might want to approach it the other way. Let's talk about your system, and from that, you will have a better perspective on the general understanding you are missing right now.

Hey sensij. Although I do have a certain system in mind for myself, I'm actually more interested in building some calculators of my own! My system will be ~500 watts on top of a van, and the calculator(s) I hope to build have similar sized systems in mind.

Just to pick on a few points in particular:

To save you the trouble of looking up wire resistance and cross sectional area for voltage drop, there are a number of calculators that do it for you.
This one requires that you enter the round trip distance, and generates a table showing the voltage drop at different wire sizes.

Thanks! I'm happy to have other people's calculators to sanity check the ones I'm building! All the research I've been doing has me really psyched about building some calculators of my own (I'm a programmer by nature)

The temp correction Sunking suggested is take whatever Voc you calculate for the array and multiply it by 1.25. The temp corrected Voc should be less than the Vdc input rating of the charge controller. In cold climates, the correction factor may be higher, and in warm climates, you might be able to get away with less.

Hmm, so sunking mentioned using Vmp to determine panel voltage, but here you mentioned using Voc. Perhaps I'm mixing up terms, but do you perhaps mean "take the calculated Vdc (which is the cumulative of the series Vmps)"? I'll have to look into temp corrections for voltage calculations to see how much things can vary.

I think you're assuming a battery system - but this is posted in the "systems for the grid" section.

SO - Kyle - what kind of system are you looking to have?

You are very right! This was my mistake; I actually have off-grid battery systems in mind, but mistakenly posted here. I'm still learning my way around the forum. Please move to the appropriate sub-forum if it doesn't belong here.

I did a 20 minute video explaining NEC compliant wire sizing. I think it would help you. In YouTube, search for "Wiring Your Solar Electric System". It should be one of the first ones that comes up. There's a bunch other I did that will help with some of your other question, and I'm trying to finish up how to size a battery based system, hopefully that will be up soon.

Thank you! I skimmed the video and it has some very useful information. I'll be taking a deeper dive once I'm out of work.

[Sunking]

I just read up a bit more on Vmp and Voc. Anyways, is it safe to use Vmp alone when determining wire size? It just seems like there is potential variability (in a worst-case scenario, up to the Voc) in voltage. I'm guessing that the distance that the wire needs to travel determines whether or not you need to be more careful with this kind of thing (shorter == need to be more careful)?

It is Vmp that you work with. Panels are current sources, not voltage. Current is what heats up wire. Current is at maximum at Vmp and ZERO amps at Voc. As current flows through resistance generates heat, and voltage along the length of the wire. For the wiring between the panels and charge controller you want to keep the voltage loss as low as possible of 2% or less. So lets say we have a 2000 watt panel operating with a Vmp of 50 volts and Imp of 40 amps. Voc = 70 volts, and Isc = 50 amps. Distance between panels is 50 feet 1-Way or 100 feet loop distance.

2% of 50 volts = 1 volt. That means with 40 amps the wire resistance must be 1 volts / 40 amps = .025 Ohm's or less. If we use a Calculator like this one we see it takes a #4 AWG copper conductor to meet the Voltage Drop Requirement. Next we determine if the conductor meets the NEC requirement. For NEC we take the ISC current (50 amps) and multiply by 1.56 and we get 78 amps. We now look at NEC table in table 310.15 and see the minimum size is #8 AWG. Our number #4 is way larger than the minimum safety requirement.

Now from the controller to the battery we still want 2% or less but the loop distance is only 6 feet. On the output of the controller we have a 24 volt battery so the current is 80 amps. If we use the calculator inputting 24 volts, 80 amps, and 6 feet at 2% only requires a #10 AWG wire. But when we check NEC table 310.15 the minimum size wire is #8 AWG so we must use the larger 8 AWG wire.

Motto to this story is we do it both ways and always use the larger of the two. Doing so achieves both performance and safety objectives so we can sleep at night.

[kybishop]

It is Vmp that you work with. Panels are current sources, not voltage. Current is what heats up wire. Current is at maximum at Vmp and ZERO amps at Voc. As current flows through resistance generates heat, and voltage along the length of the wire. For the wiring between the panels and charge controller you want to keep the voltage loss as low as possible of 2% or less. So lets say we have a 2000 watt panel operating with a Vmp of 50 volts and Imp of 40 amps. Voc = 70 volts, and Isc = 50 amps. Distance between panels is 50 feet 1-Way or 100 feet loop distance.

2% of 50 volts = 1 volt. That means with 40 amps the wire resistance must be 1 volts / 40 amps = .025 Ohm's or less. If we use a Calculator like this one we see it takes a #4 AWG copper conductor to meet the Voltage Drop Requirement. Next we determine if the conductor meets the NEC requirement. For NEC we take the ISC current (50 amps) and multiply by 1.56 and we get 78 amps. We now look at NEC table in table 310.15 and see the minimum size is #8 AWG. Our number #4 is way larger than the minimum safety requirement.

Now from the controller to the battery we still want 2% or less but the loop distance is only 6 feet. On the output of the controller we have a 24 volt battery so the current is 80 amps. If we use the calculator inputting 24 volts, 80 amps, and 6 feet at 2% only requires a #10 AWG wire. But when we check NEC table 310.15 the minimum size wire is #8 AWG so we must use the larger 8 AWG wire.

Motto to this story is we do it both ways and always use the larger of the two. Doing so achieves both performance and safety objectives so we can sleep at night.

This makes perfect sense and clears up most of my confusion. Thank you!

[Sunking]

This makes perfect sense and clears up most of my confusion. Thank you!

You are welcome. Learn this from the drill. NEC only objective is MINIMUM SAFETY REQUIRMENTS. Those minimum safety requirements may or may not meet performance requirement. Thus you must do both. NEC is NOT A DESIGN GUIDE or HOW TO book.

You first do the design to meet the performance objective. Once that is done you check to see if it meets the minimum NEC requirement. Then use the one that cost the most money.

[sensij]

Hmm, so sunking mentioned using Vmp to determine panel voltage, but here you mentioned using Voc. Perhaps I'm mixing up terms, but do you perhaps mean "take the calculated Vdc (which is the cumulative of the series Vmps)"? I'll have to look into temp corrections for voltage calculations to see how much things can vary.

You need to look at the voltage in two ways. The temp corrected Voc cannot exceed the input voltage rating of the charge controller. The operating voltage (Vmp) is what you use when looking at voltage drop for wire sizing. The things Sunking and I have posted here do not conflict.

[kybishop]

You are welcome. Learn this from the drill. NEC only objective is MINIMUM SAFETY REQUIRMENTS. Those minimum safety requirements may or may not meet performance requirement. Thus you must do both. NEC is NOT A DESIGN GUIDE or HOW TO book.

You first do the design to meet the performance objective. Once that is done you check to see if it meets the minimum NEC requirement. Then use the one that cost the most money.

http://memecrunch.com/meme/2NXBN/my-wallet-is-ready/image.png?w=440&c=1

You need to look at the voltage in two ways. The temp corrected Voc cannot exceed the input voltage rating of the charge controller. The operating voltage (Vmp) is what you use when looking at voltage drop for wire sizing. The things Sunking and I have posted here do not conflict.

Yep yep. Sunking's post alleviated my confusion; I now see how voltage and current are correlated.

[Amy@altE]

You first do the design to meet the performance objective. Once that is done you check to see if it meets the minimum NEC requirement. Then use the one that cost the most money.

Nice, I may steal that line!