DC trunkage

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Open Neutral

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A friend/client is planning to deploy a 20-30 thousand ft^2 array on a roof.
The rub is the array is to be ~250 ft from the house.
There's an existing, unused, feed of #2 copper.

I was wondering about bringing DC back to the house vs locating the inverters by the arrays.
If the wire is rated at [say] 300V AC, what's the Code say about DC, given the AC is in RMS?
 
Perhaps some more details would be in order. A 25,000 sq ft area would be a square about 160 feet on a side. That would be about 400kw array. You say this would run to /supply/interconnect at a house? That would be nearly 400 amps even at three phase 480......Even with 1500V DC strings, you would be looking at over 300 amps at 1250 VDC.
 
Open Neutral said:
You are right. I've botched the numbers somehow. (Can I blame COVID??? More accurately, posting while answering the phone, etc.)
It's ~5000 ft^2 of array.
Click to expand...
Ok so thats around 300 amps 240 AC. Two sets of 500 AL may do it (depending on what you want for voltage "rise"). I just ordered some triplexed 500-500-/1/0 and it was$3.60 a foot. Fronius inverters do 1kv strings so you could try the math on that. Remember PV wire is available in AL too. I recently did two systems that were long runs but not as big (600 feet 20kw, 800 feet 10 kw) and did not find it worthwhile to run the DC back, even with 1kv strings.
 
General rule of thumb is it's better from a voltage drop perspective to run high voltage DC rather than 240Vac.

5,000 sq. ft is still a massive array for residential purposes, somewhere in the 80-100 kWdc range depending on module selection and racking choice. Is his service capable of handling that much current?

You/he will have to price out the cost of running multiple #10 or #8 strings back to the house versus running some massive sets of aluminum. Pulling the string wiring will likely be easier than 500 kcmil aluminum.

If you do pull the strings back to the house, make sure you have a way to disconnect them before they enter the building. Either with a dedicated disconnect or by putting the inverters outside.
 
BandGap1.1eV said:
General rule of thumb is it's better from a voltage drop perspective to run high voltage DC rather than 240Vac.
Click to expand...
Hence my thinking of HVDC.
5,000 sq. ft is still a massive array for residential purposes, somewhere in the 80-100 kWdc range depending on module selection and racking choice. Is his service capable of handling that much current?
Click to expand...
That is the available roof area; the actual panel area will be less but how much less is TBD.

As for his service.... it's like this. We needed to upgrade from a shared 37.5KVA transformer giving him 200A service.
PSE decided he needed a dedicated 100KVA transformer. I fell over but that was their position.
That spawned issues with AIC, and he ended up with a 600A fused main disconnect to get a series rating.

You/he will have to price out the cost of running multiple #10 or #8 strings back to the house versus running some massive sets of aluminum. Pulling the string wiring will likely be easier than 500 kcmil aluminum.
Click to expand...

Bigger issue is the existing cabling is buried; replacing it would be very disruptive and have SWMBO issues. It's likely that house capacity will be limited to what will feed several Tesla Powerwalls. I've been told by an installer I know that unlike most inverters, they have an "island" mode where, when the PoCo takes a holiday, the inverters+PowerWalls can accept solar input. The rest of the output would grid-tie in the barn, it remains on the 37.5 XFMR,

If you do pull the strings back to the house, make sure you have a way to disconnect them before they enter the building. Either with a dedicated disconnect or by putting the inverters outside.
Click to expand...

The 'house' end emerges in a separate garage, connected by a breezeway. We can put a disconnect there, but would rather have it at the other end where the meter, panelboards, and disconnect are located.
 
Open Neutral said:
As for his service.... it's like this. We needed to upgrade from a shared 37.5KVA transformer giving him 200A service.
PSE decided he needed a dedicated 100KVA transformer. I fell over but that was their position.
That spawned issues with AIC, and he ended up with a 600A fused main disconnect to get a series rating.
Click to expand...

A bit off topic, but that doesnt make any sense.
 
electrofelon said:
A bit off topic, but that doesnt make any sense.
Click to expand...

Well, I'm not an experienced power engineer, but the electrical contractor seems to know what they are doing. (Unlike the first two on the job...)
The AIC from the 100KVA transformer, and short secondaries, exceeded the rating of the Square-D panelboards. The solution someone here suggested was a OCP main, with a "series rated" document from Square-D attesting that the main fuses shall open before the breakers self-destruct. That was in fact what the EC installed & inspector OK'ed.
 
Open Neutral said:
Well, I'm not an experienced power engineer, but the electrical contractor seems to know what they are doing. (Unlike the first two on the job...)
The AIC from the 100KVA transformer, and short secondaries, exceeded the rating of the Square-D panelboards. The solution someone here suggested was a OCP main, with a "series rated" document from Square-D attesting that the main fuses shall open before the breakers self-destruct. That was in fact what the EC installed & inspector OK'ed.
Click to expand...
What doesn't make sense is the 600 amp size. In fact the larger the upstream device, the harder it is to get a series rating ( not as much with fuses, but definitely with circuit breakers).
 
It may be difficult to utilize the existing #2 for DC given typical residential inverter equipment nowadays. That decision will likely be determined by what you can find to work with it rather than what is theoretically most efficient.
 
I was wondering about bringing DC back to the house vs locating the inverters by the arrays.
If the wire is rated at [say] 300V AC, what's the Code say about DC, given the AC is in RMS?
………..
Equivalent.
…………
Ok so thats around 300 amps 240 AC. Two sets of 500 AL may do it (depending on what you want for voltage "rise"). I just ordered some triplexed 500-500-/1/0 and it was$3.60 a foot. Fronius inverters do 1kv strings so you could try the math on that. Remember PV wire is available in AL too. I recently did two systems that were long runs but not as big (600 feet 20kw, 800 feet 10 kw) and did not find it worthwhile to run the DC back, even with 1kv strings.
…………….
You/he will have to price out the cost of running multiple #10 or #8 strings back to the house versus running some massive sets of aluminum. Pulling the string wiring will likely be easier than 500 kcmil aluminum.
…...
It may be difficult to utilize the existing #2 for DC given typical residential inverter equipment nowadays. That decision will likely be determined by what you can find to work with it rather than what is theoretically most efficient.
……….
In other words, you might be kind of forced to use it for AC and put inverters at the building with the arrays.
………..

My response to above comments:

Most comments are not accurate.
There is a definite difference between current capacity of a given size of conductor when passing energy on DC versus AC.

DC doesn’t suffer from reactance which ultimately cause reduced current flow—that is passing through a conductor. This is mostly affected by SKIN EFFECT and to a lesser extent . . . PROXIMITY EFFECT. The latter is induced by induction caused by AC current.
Both of these phenomena don’t happen in DC circuit.

The formula for impedance provides another way to show these relationships:

impedance= square root of resistance in ohms squared + (inductive reactance in ohms minus capacitive reactance in ohms squared)

Other factors that are present in AC circuits--that are not present in DC circuits also include frequency and inductance.

The higher the frequency, the more pronounced effect the above equation has on the overall current capacity of a conductor.
Check these links for starter:
https://waveguide.blog/impedance-skin-effect-implications-high-frequency-circuits/
https://electricalbaba.comh/cause-of-skin-effect-in-ac-conductors/
https://electricalbaba.com/comparison-of-ac-and-dc-transmission/

Yes, as alluded to by another poster-- multiple runs of smaller conductors could mitigate the adverse anomaly that SKIN EFFECT or PROXIMITY EFFECT has on current flow. But could be cost- prohibitive. This approach is often called the Litz Effect but is more effective on electronics, inductors and switching transformer circuits.

There is a lot to say about skin effect but that would be better covered from a different platform.

In summary:
Running DC from the array to the house has benefit than running AC--after the inverter that is located in the vicinity of the DC source.
 
myspark said:
I was wondering about bringing DC back to the house vs locating the inverters by the arrays.
If the wire is rated at [say] 300V AC, what's the Code say about DC, given the AC is in RMS?
………..
Equivalent.
…………
Ok so thats around 300 amps 240 AC. Two sets of 500 AL may do it (depending on what you want for voltage "rise"). I just ordered some triplexed 500-500-/1/0 and it was$3.60 a foot. Fronius inverters do 1kv strings so you could try the math on that. Remember PV wire is available in AL too. I recently did two systems that were long runs but not as big (600 feet 20kw, 800 feet 10 kw) and did not find it worthwhile to run the DC back, even with 1kv strings.
…………….
You/he will have to price out the cost of running multiple #10 or #8 strings back to the house versus running some massive sets of aluminum. Pulling the string wiring will likely be easier than 500 kcmil aluminum.
…...
It may be difficult to utilize the existing #2 for DC given typical residential inverter equipment nowadays. That decision will likely be determined by what you can find to work with it rather than what is theoretically most efficient.
……….
In other words, you might be kind of forced to use it for AC and put inverters at the building with the arrays.
………..

My response to above comments:

Most comments are not accurate.
There is a definite difference between current capacity of a given size of conductor when passing energy on DC versus AC.

DC doesn’t suffer from reactance which ultimately cause reduced current flow—that is passing through a conductor. This is mostly affected by SKIN EFFECT and to a lesser extent . . . PROXIMITY EFFECT. The latter is induced by induction caused by AC current.
Both of these phenomena don’t happen in DC circuit.

The formula for impedance provides another way to show these relationships:

impedance= square root of resistance in ohms squared + (inductive reactance in ohms minus capacitive reactance in ohms squared)

Other factors that are present in AC circuits--that are not present in DC circuits also include frequency and inductance.

The higher the frequency, the more pronounced effect the above equation has on the overall current capacity of a conductor.
Check these links for starter:
https://waveguide.blog/impedance-skin-effect-implications-high-frequency-circuits/
https://electricalbaba.comh/cause-of-skin-effect-in-ac-conductors/
https://electricalbaba.com/comparison-of-ac-and-dc-transmission/

Yes, as alluded to by another poster-- multiple runs of smaller conductors could mitigate the adverse anomaly that SKIN EFFECT or PROXIMITY EFFECT has on current flow. But could be cost- prohibitive. This approach is often called the Litz Effect but is more effective on electronics, inductors and switching transformer circuits.

There is a lot to say about skin effect but that would be better covered from a different platform.

In summary:
Running DC from the array to the house has benefit than running AC--after the inverter that is located in the vicinity of the DC source.
Click to expand...
I think you are stuck on the theoretical side of things and what might be better for say a massive transmission line. The effects you speak of are not very relevant to the OP.
 
electrofelon said:
I think you are stuck on the theoretical side of things and what might be better for say a massive transmission line. The effects you speak of are not very relevant to the OP.
Click to expand...

This is a follow-up on the post that I think didn’t go through.
This is in response to the felon’s post about the claim regarding the irrelevance of my post.
If this is a deliberate deletion on the part of the powers-that-be . . . then I want to know.
I wanted to correct bogus information that undermines the credibility of this board.

If this is the goal of this board . . . then it’s not for me say that it needs to improve.

I understand that this board is for entertainment purposes...and no can be held accountable for what is said: bogus or non-bogus.
However, whether statements are mostly unverifiable or just made-up. . . it will help if they (comments ) are partly based on science not just plain pile of balderdash.
 
myspark said:
This is a follow-up on the post that I think didn’t go through.
This is in response to the felon’s post about the claim regarding the irrelevance of my post.
If this is a deliberate deletion on the part of the powers-that-be . . . then I want to know.
I wanted to correct bogus information that undermines the credibility of this board.

If this is the goal of this board . . . then it’s not for me say that it needs to improve.

I understand that this board is for entertainment purposes...and no can be held accountable for what is said: bogus or non-bogus.
However, whether statements are mostly unverifiable or just made-up. . . it will help if they (comments ) are partly based on science not just plain pile of balderdash.
Click to expand...
I am not sure what information you thought was "bogus". I looked back and did not see anything posted that was incorrect. Proximity and skin effect are negligible for the wire sizes and lengths being discussed. The higher voltage of the DC is what provides the significant advantage, but even this is often negated by other factors such as utilizing available MPPTS, logistics of pulling many conductors vs few, cost of CU vs AL, having to provide for cold open circuit voltages, cost of PV wire over XHHW or USE, cost of inverters that can take 1kv strings vs 600v strings...so no its often not worth running DC instead of AC. I have lots of time in the field installing stuff every which way and pricing out different PV system topologies and components, do you?
 
On the solar side, the GC looked today at the disconnected Barn<->House run. He found two 1/0, and two #2.

That makes little sense to me, did someone use an undersize neutral or ??
That's a vote for using DC.
 
Open Neutral said:
On the solar side, the GC looked today at the disconnected Barn<->House run. He found two 1/0, and two #2.

That makes little sense to me, did someone use an undersize neutral or ??
That's a vote for using DC.
Click to expand...

It's not a vote for using DC. It doesn't matter. Unless perhaps you forgot to mention that you wanted the AC to be 3-phase.

Those wire sizes are a little odd but keep in mind that 225.61 allows neutrals to be sized smaller than hots when the unbalanced load calculates accordingly. Further, with most solar inverters there is essentially zero unbalanced load. See 705.95(B). (2017 NEC reference.)

So you'd be fine using the 1/0 for hots and the #2 for neutral and EGC. All else being equal.

In my opinion, it's really going to come down to whether you can find a competitively priced inverter for your service voltage that will accept that much DC current on a single circuit. Ten years ago you probably could, now I'm not so sure. But admittedly the array you're talking about is a few kW above what I typically work with.
 
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