Voltage Drop Issue

System

I have a friend who is installing a grid tied PV system on their barn roof. They just got a quote from a "reputable firm" who is telling them there is no problem with voltage drop from the barn (sub panel) to the house (Main Ac service panel). That is exactly opposite what I told them. Using SMA's voltage drop calculator I come up with a 6% drop; way more than the recommended 1 to 2% drop.
I'm only an amateur at this stuff so how can they figure it's okay with the existing underground cable? If anyone wants to do the math themselves they have aluminum 2 awg cable run 600' they are putting in a 10 KW system that I believe will generate at least 40 amps at 240V.

Thanks in advance for all your insights.

BB.

Re: Voltage Drop Issue

Excluding code issues for the moment... There can be issues with the 6% voltage drop...

1. You are talking about a 6% loss in power (heating of the cable). Not a huge number--but does make the 10kW array really a 9.4kW array just because of the losses. Changes the economics a bit--but it may simply not be worth the costs (trenching, wire, etc.) to install a new cable to recover 3% of the losses.
2. Another issue is that GT Inverters have to meet fairly tight specifications for Grid Voltage ranges. For example, my grid will be as high as 245 VAC midday. 245*1.06=260VAC. The inverter should shutdown around 263 VAC or even a bit less when measurement accuracy is taken into account (over-volt fault; under voltage fault is around 212 VAC). That is not a lot of margin if the local line "runs" high in your neighborhood. Many times, the Grid Voltage is set to be somewhat towards the High End of the acceptable voltage range--and this can cause nuisance faults with a GT inverter (which causes voltages to rise, unlike loads, which cause voltages to fall).

In the end, it is almost always less issues if the "long" cable run is between the solar panels and the PV array/GT inverter vs between the GT Inverter/Utility Drop.

It would seem that (if money was available) the best solution would be to dig a new line just to run the PV array cable back to the inverter installed next to the utility drop. If your area requires a separate meter for the GT Inverter--may also reduce issues for meters reader access.

Obviously, I do not know the installation details and there may be other reasons things will workout OK as designed.

-Bill

Cariboocoot

Re: Voltage Drop Issue

I have to comment: aluminium wire? Did anyone else just go "yuch!"?

niel

Re: Voltage Drop Issue

it's probably some service entrance wire as my service entrance wire is #4 aluminum. some electric companies don't want to spend $ on copper. would i use it for solar? that depends, but i do prefer copper.

tallgirl

Re: Voltage Drop Issue

Milmil wrote: »

I have a friend who is installing a grid tied PV system on their barn roof. They just got a quote from a "reputable firm" who is telling them there is no problem with voltage drop from the barn (sub panel) to the house (Main Ac service panel). That is exactly opposite what I told them. Using SMA's voltage drop calculator I come up with a 6% drop; way more than the recommended 1 to 2% drop.
I'm only an amateur at this stuff so how can they figure it's okay with the existing underground cable? If anyone wants to do the math themselves they have aluminum 2 awg cable run 600' they are putting in a 10 KW system that I believe will generate at least 40 amps at 240V.

Thanks in advance for all your insights.

They need to dig up that #2 and put in something more reasonable, like 2/0. The voltage drop is just plain WRONG.

dwh

Re: Voltage Drop Issue

Cariboocoot wrote: »

I have to comment: aluminium wire? Did anyone else just go "yuch!"?

I would have, had it been for small branch circuits, but for utility drops or panel-panel I don't mind aluminum wire. Basically, if both ends are terminated in lugs and gooped and torqued properly I'm okay with it.

And for long runs of fat wire...copper is bloody heavy.

However, I definitely twitch when I see aluminum wire with twisted connections and wirenuts - that's just bad mojo there.

Windsun

Re: Voltage Drop Issue

If the run is that far, I have to wonder why they did not run the high voltage DC and put the inverter closer to the house.

One reason perhaps is that DC in Aluminum wiring is even more tricky than AC, but if they are OK with a 6% voltage drop, not sure they would know that anyway.

dwh

Re: Voltage Drop Issue

Windsun wrote: »

If the run is that far, I have to wonder why they did not run the high voltage DC and put the inverter closer to the house.

I assumed from the sound of it that they were going to tie the PV into the barn sub, then backfeed to the main house via the existing underground barn feed.

n3qik

Re: Voltage Drop Issue

We are all assuming the the barn will use zero electric. For all we know, the barn may use all 10KWs the array will produce. If so, then the wire size is non debatable.

System2

Re: Voltage Drop Issue

It was/is an existing sub panel. With the cable all ready in place. The barn will use some but very little of the power produced. Again my concern is, if you go to inverter manufacturers' web-sites they recommend (in the case of SMA max of 1.5%) no more than a 1-2% drop. This is going to be 400% higher. Are the inverter manufactures just being overly conservative or is this going to effect inverter operations?

Thanks in advance.

BB.

Re: Voltage Drop Issue

Usually, 3% or less voltage drop is recommended around here. With solar, because power is realatively expensive to generate--you try and design a system not to waste power (in this case, using relatively small diameter wire with a very long run).

The big issue (in my humble opinion) is the potential problem with the large voltage drop potentially causing the inverter to "fault" with high line voltage. And this is a real issue--not just being overly conservative. Remember, there is even more voltage drop between the meter and the pole transformer--and we still do not know the normal operating range of the voltage at the site (if the voltage goes above 245 VAC--the known voltage drop between the home and the barn will just about be enough to cause the inverter to fault in normal operation). Add another couple percent for voltage drop between the home and the pole transformer, it will be come a big problem.

Each time there is a voltage fault, the inverter will shut down for 5 minutes before attempting to reconnect.

-Bill

crewzer

Re: Voltage Drop Issue

I've started taking what I consider to be a "balanced" perspective on this issue.

The wire from the inverter to the service panel should be spec'd per the NEC for inverter specs, wire type, conditions of use, etc.

However, the barn-mounted array will rarely ever deliver full rated power. If/when it does, the delivery period(s) will be short, so little marginal energy (voltage drop x Amps x time) will be lost.

Accordingly, for calculating operational voltage drop vs. wire size, I suggest you consider using current derived from the modules' NOCT specs -- perhaps 80% of STC -- to determine typical power loss.

IOW, what would be an acceptable voltage drop and wire size (but not smaller than the NEC minimum) for, say, 32 A instead of 40 A?

HTH,
Jim / crewzer

BB.

Re: Voltage Drop Issue

Another issue is that we do not know the details... A 10 kW system could be 10kW of solar panels (more or less marketing rating as Jim/Crewzer says)--Or it could be 11.7+ kWatts of solar panels on a "10kWatt inverter" installation...

The problem is not that anything will "burn up" or be unsafe--it is the issue of power stability at the GT inverter on a long wire run back to the pole--and the peak normal voltage that the site will experience (peak utility voltage + wire drop > 263 VAC is not good).

Could also have the problem running the other way--Say the utility voltage is low, and there is a large arc welder and/or compressor in the barn (I can dream--can't I )... Large surge loads could drop the local power below 212 VAC and knock the GT inverter off-line too for five minutes. Won't damage anything--but can be frustrating for the owner.

The "6%" power loss can just be a cost of the installation (new, heavier wire, is just too expensive for the 6% power loss) and not a major issue in and of itself.

-Bill

System2

Re: Voltage Drop Issue

The problem is not that anything will "burn up" or be unsafe--it is the issue of power stability at the GT inverter on a long wire run back to the pole--and the peak normal voltage that the site will experience (peak utility voltage + wire drop > 263 VAC is not good).

I am trying to wrap myself around the concept that the voltage drop will cause to high a voltage at the inverter. Could you explain why?

boB

Re: Voltage Drop Issue

Milmil wrote: »

I am trying to wrap myself around the concept that the voltage drop will cause to high a voltage at the inverter. Could you explain why?

Because a grid tie inverter produces current, it inherently tries to raise the grid voltage.

If the grid impedance, as seen at the inverter, is high, because of long wire and its associated resistance,
the terminals of the inverter will have a high voltage across them.

i.e. the inverter (normally) is a current source.

Put into terms of Ohms Law:
Voltage drop across wire = Current through the wire TIMES the resistance of the wire.

boB

BB.

Re: Voltage Drop Issue

The "voltage drop" is from the source of the power to the load...

In "the normal home"... The pole on the street is the source of the power (say 240 VAC), and the load is the home (or in your friend's case--through the home and out to the barn). So, a 6% drop (assuming heavy load in the barn) would mean that 94% of the voltage from the pole (240 VAC) would be at the barn (the load). Or 240v*0.94=225.6VAC (and probably even more drop for the line from the pole to the home panel).

In your friends case--the load is the 240 VAC load on the pole (the transformer)... And the source of the power is the 10kW GT inverter in the barn. And the voltage drop is now the other direction... The voltage at the GT inverter (Barn) is 6% higher than the pole (ignoring all other issues for the moment)... So, the voltage is "set" by the pole transformer (say 240 VAC), and the "drop" to the source (GT inverter) is 240v*1.06=254.4VAC... That leaves about 6-8 volts of "head room" between the inverter's protective circuits kick in at around 210-212 VAC.

The problem is that you have no control over what voltage the utility sets at the pole transformer... Say they assume that they want to provide 240 VAC power at the home (fewer complaints from their customers about low voltage during surges). And they assume the 3% NEC voltage drop standard... That says they should set the pole transformer to 240v*1.03=247.2volts

Now, your friend has the 6% drop from his GT inverter to the pole (using the number you supplied) and we get 247.2v*1.06=262volts at the GT inverter. right in the range of the 263 volt or so inverter shutdown voltage (plus any devices in the barn are running at near maximum voltage--which could be an issue too).

So, now the inverter occasionally shuts down. The installer says--well everything is working OK on their side--call your utility and complain about the "high voltage".

Call the utility and tell them that 247 volts is too high--and they say no it is not, and we are not going to change anything on our end.

Now--the customer is left with a less than idea installation and the only choice is to redo the install (install heavier cable between house and barn, or install GT inverter near meter box and trench a new cable from the array to the inverter) to get a fully functional system.

Again, I am not saying the above will happen--but it is certainly within the realm of possibilities (perhaps, even likely). Distribution systems are designed to distribute power to loads--the utility is probably under no compulsion to adjust their network for distributed generators such as GT solar (which they probably don't want anyway and are only doing it because of the state PUC).

Without knowing the voltage drop from the home to the pole, and the voltage extremes of the pole transformer (utilities have load related issues too)--It is impossible to give you a 100% yes or no answer.

Your installer should be able to gather the data and give your friend an answer to the question though (what is the local voltage, and what is the resistance from the meter to the pole and what will be the worst case voltage at the GT inverter).

People at the end of the utility distribution system (rural areas) frequently have issues with high or low voltage at their meters (and unknown causes voltage fluctuations).

I hope this helps.

-Bill

Or, you can read boB's answer in 100 words or less.

System2

Re: Voltage Drop Issue

Got it.

You guys/gals have all been very helpful. Thank you.

t00ls

Re: Voltage Drop Issue

I have to concur with Jim's conclusion and B.B.'s last post....I wouldnt worry with it

BB.

Re: Voltage Drop Issue

t00ls,

Actually, if that is what is appears I was saying--not to worry about it... Then I was not very clear (at 800 words--how can it be clear :roll: ).

I would not install my own system like that (6% voltage drop from inverter to meter) unless:

1. It is too expensive to trench and install the proper size cable to bring power from the solar panels on the barn to the GT inverter(s) on the home.
2. If #1 is true--then I would make the installer prove that the average high line + voltage drop of barn feed + voltage drop of meter to pole gives at least 5-10 volts of headroom.

And #2 is actually pretty hard to garrantee... It would take a year's worth of readings (when the home is drawing low amounts of power) and to trust that the utility is not going to change the feed voltage in the future.

If a person has an accurate voltage meter (calibrated/known good DVM or equivalent)--it would not be difficult to estimate the scope of the utility problem.

Measure the voltage at the home (ideally with no major loads in the home) for a few days and see how much it varies over the day and the week. Technically, you only care what the voltage is during ~9am-3pm or so (when the inverter is operating at high power levels).

The other test is to measure the line voltage, then turn on a known load (1.5 kWatt heater or a 4.8 kWatt water heater, etc.) and measure the voltage again and see how much the voltage in the home dropped.

If, for example you can see a 0.5 volt drop turning on and off a 1 kWatt heater, then the increase of a 10kW solar GT inverter would be (home to pole line drop):

0.5 Volt * 10kW inverter / 1 kWatt heater = 5 volt change at 10kWatts

Take all of the data and see what you get:

Max Line Voltage Mid-day + Max line increase 10 kW + 6% line increase home to barn

Example:

242 VAC + 5 Volts + 0.06*240VAC = 261.4 volts

261.4 volts is awfully close to the 263 volt cutout of the GT Inverter assume the inverter is accurate to 1%:

261.4 volts * 1.01 (assume GT reads high) = 264.1 volts apparent to GT CPU

So--with my made up numbers above--it is possible that even a very nominal 242 VAC pole voltage could put the GT inverter in near fault condition assuming some very average numbers (based on what was supplied and some guess work).

Again, make the installer prove to you (and themselves) that the installation will function correctly over time.

Heavy loads in the home and barn will reduce the problem--but than that assumes that those loads are always present--not usually a good assumption (examples: vacation, A/C in summer, no A/C in winter, etc.).

-Bill