# Subcombiner (4) to Final Combiner Voltage Drop Variations

#### AWinston

##### Member
I have designed a large scale PV system (close to 1MW) that has 38 inverters going to 4 different sub combiners. The sub combiners then feed into a final combiner that is fed into a 4160V-480V transformer. There are a lot of long cable runs from the sub combiners. Each sub combiner puts out 450A. See below.

SC-1 (2,385FT - Voltage drop (5.42% using 3 sets of #500 CU THHN))
SC-2 (204FT - Voltage drop (1.42% using #600 CU THHN))
SC-3 (603FT - Voltage drop (2.10% using 2 sets of #500 CU THHN))
SC-4 (978FT - Voltage drop (3.41% using 2 sets of #500 CU THHN))

I know I am violating voltage drop values (2% for feeders) so I plan to take care of that. My question is....does each sub combiner have to have the same voltage drop? Does the bus at the final combiner have to receive the exact same voltage from each sub combiner?

#### jaggedben

##### Senior Member
You're combining AC here?

#### AWinston

##### Member
You're combining AC or DC?
Sorry, I should have specified. All the combiners are 480V AC.

#### Carultch

##### Senior Member
I have designed a large scale PV system (close to 1MW) that has 38 inverters going to 4 different sub combiners. The sub combiners then feed into a final combiner that is fed into a 4160V-480V transformer. There are a lot of long cable runs from the sub combiners. Each sub combiner puts out 450A. See below.

SC-1 (2,385FT - Voltage drop (5.42% using 3 sets of #500 CU THHN))
SC-2 (204FT - Voltage drop (1.42% using #600 CU THHN))
SC-3 (603FT - Voltage drop (2.10% using 2 sets of #500 CU THHN))
SC-4 (978FT - Voltage drop (3.41% using 2 sets of #500 CU THHN))

I know I am violating voltage drop values (2% for feeders) so I plan to take care of that. My question is....does each sub combiner have to have the same voltage drop? Does the bus at the final combiner have to receive the exact same voltage from each sub combiner?
On the AC side, voltage drop is really a voltage rise. You start out with the voltage of the service, and each inverter increases its output voltage to be as large as necessary (within the inverter's range), such that the current can travel through the entire AC system of circuits and satisfy Ohm's & Kirchhoff's laws.

#### AWinston

##### Member
On the AC side, voltage drop is really a voltage rise. You start out with the voltage of the service, and each inverter increases its output voltage to be as large as necessary (within the inverter's range), such that the current can travel through the entire AC system of circuits and satisfy Ohm's & Kirchhoff's laws.
The output of the inverters are constant at 480V. My issue is the voltage drop at the combiners.

#### jaggedben

##### Senior Member
...does each sub combiner have to have the same voltage drop? Does the bus at the final combiner have to receive the exact same voltage from each sub combiner?
I prefer to think of it as voltage rise from the final combiner to the inverters.

It's basically the same calculation, except that your variables must be based on the assumption that the voltage at the transformer is the given. Don't use the nominal voltage of the inverter output and calculate a drop from that. Calculate the drop to the transformer voltage from the inverter. The inverter will output at whatever voltage is necessary to push the current to the utility; the voltage at the inverters will rise too high if they voltage drop from the inverters to the transformer is too high. The busbars of the final combiner of course cannot be at more than one voltage to each other; that voltage will be determined by the interplay of all components.

Your engineering problem, which you need to avoid here, is that if voltage at the inverter terminals is too high they will shut off. If the transformer is higher than nominal, the voltage at the inverter terminals will be that voltage plus the voltage drop on the conductors in between. You are probably good with keeping all of your voltage drop/rise under 2%, but you may want to consult the utility on the engineering on their side. I have only dealt with this on much, much smaller systems where really I didn't have to worry about the effect of my system on the local grid.

I don't believe it's a problem to have different voltage drops for the different sub combiners but your concern is the maximum you can afford for any one of them.

#### jaggedben

##### Senior Member
The output of the inverters are constant at 480V.
No, it's not. Your inverters will output at any voltage within a UL proscribed window. That's what you need to understand. Assuming, that is, that this all a grid tied system (which I can't imagine it wouldn't be).

#### AWinston

##### Member
I prefer to think of it as voltage rise from the final combiner to the inverters.

It's basically the same calculation, except that your variables must be based on the assumption that the voltage at the transformer is the given. Don't use the nominal voltage of the inverter output and calculate a drop from that. Calculate the drop to the transformer voltage from the inverter. The inverter will output at whatever voltage is necessary to push the current to the utility; the voltage at the inverters will rise too high if they voltage drop from the inverters to the transformer is too high. The busbars of the final combiner of course cannot be at more than one voltage to each other; that voltage will be determined by the interplay of all components.

Your engineering problem, which you need to avoid here, is that if voltage at the inverter terminals is too high they will shut off. If the transformer is higher than nominal, the voltage at the inverter terminals will be that voltage plus the voltage drop on the conductors in between. You are probably good with keeping all of your voltage drop/rise under 2%, but you may want to consult the utility on the engineering on their side. I have only dealt with this on much, much smaller systems where really I didn't have to worry about the effect of my system on the local grid.

I don't believe it's a problem to have different voltage drops for the different sub combiners but your concern is the maximum you can afford for any one of them.
I checked the specs on the inverter and maximum voltage it will output is 528V. So if the inverter only has to compensate for less than 10% of voltage drop losses...then I should be fine correct?

#### Carultch

##### Senior Member
I checked the specs on the inverter and maximum voltage it will output is 528V. So if the inverter only has to compensate for less than 10% of voltage drop losses...then I should be fine correct?
The inverters might state that they have a 10% high setting on the voltage, but the unspoken rule is that the as-built setting may be tighter as a safety margin, to avoid ever exceeding the IEEE prescribed voltage range. Also, it is difficult to know just how much different the voltage at the service is, from its nominal value. It is best if you can stay within the NEC recommendations, and avoid exceeding 3% on either half of the system, while also limiting yourself to no more than 5% total. Job specifications commonly call for tighter limits than this.

#### AWinston

##### Member
The inverters might state that they have a 10% high setting on the voltage, but the unspoken rule is that the as-built setting may be tighter as a safety margin, to avoid ever exceeding the IEEE prescribed voltage range. Also, it is difficult to know just how much different the voltage at the service is, from its nominal value. It is best if you can stay within the NEC recommendations, and avoid exceeding 3% on either half of the system, while also limiting yourself to no more than 5% total. Job specifications commonly call for tighter limits than this.
Great thanks. This is for a farm and the owner is very loose in the way things are done. I may need to have them add additional transformers to reduce the 480V runs and run 4160V as long as possible.

#### BillK-AZ

##### Senior Member
Your customer may be "very loose in the way things are done" right now, but voltage rise can cause loss of production and be expensive to correct later.

#### jaggedben

##### Senior Member
I checked the specs on the inverter and maximum voltage it will output is 528V. So if the inverter only has to compensate for less than 10% of voltage drop losses...then I should be fine correct?
If the utility voltage runs high (which I see all the time) the you have less than 10% to work with.

I agree with Carultch's suggestions.

#### ggunn

##### PE (Electrical), NABCEP certified
If the utility voltage runs high (which I see all the time) the you have less than 10% to work with.

I agree with Carultch's suggestions.
But to answer the original question: No, you do not have to ensure that the voltage drop is the same for every inverter - within reason. Each inverter will adjust its AC voltage as needed within its range. Of course, voltage drop is power loss so the less the better, balanced with the cost of the wire.