Clarifying Questions on AC Voltage Drop

[Anode]

A question was asked about ac voltage drop today in the design room with a microinverter system that is single phase 120/240 and traveling 300 feet one way with a 37.5 nominal amperage (not with 125%), the intention is to bring back a single combined circuit back in a single conduit, buried.

So I am not really looking for wire type/gauge/vdrop answers here, as I think I have them, but rather some clarifying questions to better understand. Although your answers on the aforementioned are appreciated as well.

1. So first question I have is, are we basing the voltage in the equation on 120 or 240? These are YC-500 microinverters
2. If parallel feeders are used to not have oversized wire, mainly because we stock lots of standard smaller wire (like #6), does each set of parallel feeder just cut the resistance on the group of wires, or does it cut the amperage down as well? I presume the former. So if 37.5 A was for single set, would parallel feeders mean each wire could be looked at as 18.75A ; again I advised this was not the case.
3. Lastly, if parallel feeders are used, I presume they are then all treated as they should, as current carrying conductors, and then they also have to be able to carry the load after derate considerations for multiple conductors in a raceway.

Thanks in advance - We love learning

 

[jaggedben]

1) You should use the nominal output voltage of the inverters. I'm not familiar with those inverters so can't say. Single phase in the US would normally be 240V but I've heard of some 120V microinverters.
2) I believe you're over thinking it. Simplest way to look at it is that the current will divide in half and the resistance of each conductor will be the same as one conductor, so V drop is indeed divided in half. Try searching the forum for more info.
3) Yes, for each conductor that is otherwise considered a CCC you would count one CCC for each actual conductor.

 

[Zee]

...and in the future consider a DC string inverter AFTER the conduit run.
This will bump your voltage to approx. double (good)
AND
your amperage down to approx. half (good).....
and a win-win all around in terms of wire size, and hence conduit size, and labor blood sweat and tears...:rant:and $$.

I just did this on a 250' conduit run from a ground-racked array up a hill....I re-wrote the whole contract eliminating the micro's once I saw what microinverters at 240 VAC mean in terms of wire size....hell no!

 

[Smart $]

1. ...
2. If parallel feeders are used to not have oversized wire, mainly because we stock lots of standard smaller wire (like #6), does each set of parallel feeder just cut the resistance on the group of wires, or does it cut the amperage down as well? I presume the former. So if 37.5 A was for single set, would parallel feeders mean each wire could be looked at as 18.75A ; again I advised this was not the case.
3. ...

If running parallel sets to achieve the exact same voltage drop as compared to one large set, the resistance of the group will be the same. Each conductor's resistance will be that of the large times the number of sets. There may be some variance in the resistance though, depending on how well the voltage drop matches, because you are limited to specific sizes rather than making an exact voltage-drop-matching size.

The current will divide equally on each member by the number of sets.

 

[Carultch]

A question was asked about ac voltage drop today in the design room with a microinverter system that is single phase 120/240 and traveling 300 feet one way with a 37.5 nominal amperage (not with 125%), the intention is to bring back a single combined circuit back in a single conduit, buried.

So I am not really looking for wire type/gauge/vdrop answers here, as I think I have them, but rather some clarifying questions to better understand. Although your answers on the aforementioned are appreciated as well.

1. So first question I have is, are we basing the voltage in the equation on 120 or 240? These are YC-500 microinverters
2. If parallel feeders are used to not have oversized wire, mainly because we stock lots of standard smaller wire (like #6), does each set of parallel feeder just cut the resistance on the group of wires, or does it cut the amperage down as well? I presume the former. So if 37.5 A was for single set, would parallel feeders mean each wire could be looked at as 18.75A ; again I advised this was not the case.
3. Lastly, if parallel feeders are used, I presume they are then all treated as they should, as current carrying conductors, and then they also have to be able to carry the load after derate considerations for multiple conductors in a raceway.

Thanks in advance - We love learning

1. If the inverters connect to both Line 1 and Line 2, then your nominal voltage is 240V. Likely they also will connect to the neutral as well, but is the Line-to-Line voltage that matters when both line connections are present.

2. Read NEC310.4. To parallel wires for a common feeder, the wires to be paralleled need to be #1/0 or larger. If you need to run separate wires that are smaller, then each set needs to be treated as a separate feeder.

For example. A 250A circuit parallels two sets of #1/0 Cu in the same conduit. It is common practice to parallel for anything 400A and larger. And you will notice that parallel lugs start appearing on equipment 250A and larger.

However, you aren't permitted to make a 120A feeder by parallelling two sets of #6 Cu. Or to parallel in this manner this for voltage drop curtailment. If you need a 120A feeder, you need #1 Cu. If you need the equivalent of two sets of #6 Cu for voltage drop curtailment, you need a single set of #3 Cu.

To run two parallel sets of #6, they each need to be treated as separate feeders. Originating at separate devices (inverters), and terminating at their own overcurrent devices at the AC combiner.


3. YES. Any conductor that intentionally carries current should be counted as CCC's for derate purposes. Neutral may or may not count, depending on the circumstances. Spare conductors shall count as active.

If you make a 250A single phase feeder by paralleling two sets of #1/0 in the same conduit, then there are 4 current-carrying conductors in the conduit, assuming neutral doesn't count.

If you carry two separate 60A single phase feeders in the same conduit, then that is a total of 4 CCC's in the conduit. Also assuming neutral doesn't count.

 

[Anode]

If running parallel sets to achieve the exact same voltage drop as compared to one large set, the resistance of the group will be the same. Each conductor's resistance will be that of the large times the number of sets. There may be some variance in the resistance though, depending on how well the voltage drop matches, because you are limited to specific sizes rather than making an exact voltage-drop-matching size.

The current will divide equally on each member by the number of sets.

Okay this does make sense. That resistance and amperage would divide by each parallel. Thank you.

...and in the future consider a DC string inverter AFTER the conduit run.
This will bump your voltage to approx. double (good)
AND
your amperage down to approx. half (good).....
and a win-win all around in terms of wire size, and hence conduit size, and labor blood sweat and tears...:rant:and $$.

I just did this on a 250' conduit run from a ground-racked array up a hill....I re-wrote the whole contract eliminating the micro's once I saw what microinverters at 240 VAC mean in terms of wire size....hell no!

Yes, a good suggestion.

This was not my project, but we do consider this, and I think after hundreds of systems always sending DC back for the HV (as this is normally our philosophy as well), a long ac run finally slipped through. Im not sure what the cost difference will be on wire, but I doubt we would rewrite the contract. But maybe the pm on this has not crossed that bridge costing it yet.

Thanks for your input

1. If the inverters connect to both Line 1 and Line 2, then your nominal voltage is 240V. Likely they also will connect to the neutral as well, but is the Line-to-Line voltage that matters when both line connections are present.

2. Read NEC310.4. To parallel wires for a common feeder, the wires to be paralleled need to be #1/0 or larger. If you need to run separate wires that are smaller, then each set needs to be treated as a separate feeder.

For example. A 250A circuit parallels two sets of #1/0 Cu in the same conduit. It is common practice to parallel for anything 400A and larger. And you will notice that parallel lugs start appearing on equipment 250A and larger.

However, you aren't permitted to make a 120A feeder by parallelling two sets of #6 Cu. Or to parallel in this manner this for voltage drop curtailment. If you need a 120A feeder, you need #1 Cu. If you need the equivalent of two sets of #6 Cu for voltage drop curtailment, you need a single set of #3 Cu.

To run two parallel sets of #6, they each need to be treated as separate feeders. Originating at separate devices (inverters), and terminating at their own overcurrent devices at the AC combiner.

I wasn't aware of this, thank you for pointing it out.

Thank you everyone for clarifying this for me.