mbrooke
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- United States
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- Technician
Running power 4000 feet
- Thread starter mbrooke
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
That is what the drawing cites, yes.
mbrooke
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- United States
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But considering the length, this is technically an MGN? I need ground rods across this cable.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
The difference is that the outbuilding us NOT being fed from the service. The building is being fed by an SDS and the loads inside the building are fed from a second SDS.
Whether the second transformer is in/on the building or separate from it (pad or pole mount), the feeder to the building is not electrically continuous with the original service.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
Yes, that is the key. That is why the exception is called out on the drawing (250.30(A)(1) Exception 2), as most think at first glance this is not a compliant installation. PVC conduit (no metallic path). This way you only need 2 conductors for the long 600 volt conductors. Makes for a very efficient use of conductor. Safe and code compliant.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
I designed a similar job for use in South Africa. A little bit longer but almost the same specification. It was over hard terrain which added to the cost.
The bespoke transformers came in at ZAR 20,000/£1000/$1500 each.
You have to work out the savings in cable CSA and obviously the labour costs involved in laying.
I like jobs like this, the customer doesn’t want poles spoiling his view so will pay the required amount. What the customer wants, the customer gets (for a price).
The South African job was laid UG to remove the possibility of the copper being stolen from OH lines. It’s a problem here, I don’t know about your side of the pond but in SA it’s a foregone conclusion it will go missing.
The bespoke transformers came in at ZAR 20,000/£1000/$1500 each.
You have to work out the savings in cable CSA and obviously the labour costs involved in laying.
I like jobs like this, the customer doesn’t want poles spoiling his view so will pay the required amount. What the customer wants, the customer gets (for a price).
The South African job was laid UG to remove the possibility of the copper being stolen from OH lines. It’s a problem here, I don’t know about your side of the pond but in SA it’s a foregone conclusion it will go missing.
mbrooke
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Correct, however there will be at least a 10 to 15 volt drop across the grounded conductor which will transfer into the structure. 15 volts while standing outside on earth can become a problem.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
A good point to consider!
It would not transfer into the structure if the *second* transformer used only the local GES and did not have a connection to the long neutral.
Not quite as in the diagram, but acceptable I think.
Worst case make the long link ungrounded with a ground sensor.
Maybe....
mbrooke
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- United States
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But would that be to code? Dont all incoming neutrals need to be bonded to the system?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Perhaps, but the long feed does not have a neutral and need not have a grounded conductor either.
But I do not think that there is any requirement that the grounded conductors of two buildings, one of which is part of an SDS, be bonded, just that be separately earthed.
And the GES systems of two buildings without a common service need not be bonded to each other.
JMO.
I would give the most credibility to a working electrician who has gotten this accepted by an AHJ subject to the NEC.wwhitney
Senior Member
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- Berkeley, CA
- Occupation
- Retired
Question about the diagram in Post 49:
The diagram shows one conductor on the long 600V feeder being grounded, so the other conductor is 600V to ground. Could one instead use a transformer whose 600V coil is center tapped, ground the center tap, and then run only the two ungrounded conductors in the long feeder? Are such transformers readily available? Maybe this is what GoldDigger just proposed in the previous post. Would this arrangement have any advantages over the Post 49 diagram?
Just wondering, thanks.
Cheers, Wayne
The diagram shows one conductor on the long 600V feeder being grounded, so the other conductor is 600V to ground. Could one instead use a transformer whose 600V coil is center tapped, ground the center tap, and then run only the two ungrounded conductors in the long feeder? Are such transformers readily available? Maybe this is what GoldDigger just proposed in the previous post. Would this arrangement have any advantages over the Post 49 diagram?
Just wondering, thanks.
Cheers, Wayne
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Texie, that is ingenious! I have debated these transformered long wire runs a number of times and the EGC was always an annoying thing that there seemed to be no way around.
Regarding the 1000V option, note that 1KV PV wire is also rated 2KV so that is super tempting. I am not sure what your transformer options would be and what would dictate terminations. 1 KV terminations are just like <600V ones.
Fulthrotl
~Autocorrect is My Worst Enema.~
- Occupation
- E
when you go to buy that 750 copper, you'll need 12,000' of it, to save those transformers.....
so you know when you have enough, you'll need four times this amount:
we need a bigger boat......
Fulthrotl
~Autocorrect is My Worst Enema.~
- Occupation
- E
+1 on all of that....
and the nod goes to the voice of reason from san jose.....
575 volts is off the shelf on most of this.... you'll have
to get someone to make up the transformers, as they
are not really used much in the US, but i'd bet it adds
maybe 20% to the cost of the transformers, tapping them
at 575 instead of 480.... everything else is just toss it together,
and you are done.
and 1,000 volts makes it into a bastard thing, and what
you are going to save in reduced wire size will disappear
like the morning mist when you upgrade the wire insulation, and
get specially wound transformers that don't exist anywhere.
stuff made out of unobtanium is more expensive, generally speaking.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Maybe. Going to 2 KV saves dramatically on wire. Say we are talking single phase, 20kva, 3% VD. At 2KV, #8 copper will do it. 1KV/2KV PV wire is about 50% more than THHN, so two 4000' runs would be $4440. At 600V you would need 500 AL which, extrapolating from the last time I priced 250, would be $2/ft so $16000 for the 600V option. That is a lot and it doesnt figure increased labor and conduit costs.
curt swartz
Electrical Contractor - San Jose, CA
- Location
- San Jose, CA
- Occupation
- Electrical Contractor
I just punched the info into one of the online calculators and I get 2.7% drop using 350 kcmil AL single phase. 3/0 AL would give you a 2.8% drop if using 3 phase.
In Canada 600Y/347 is a common utilization voltage so getting transformers should be easy.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
If you want the long feed to be just two conductors, with no EGC or bonding jumper, then I think it needs to have a grounded conductor. 250.30(A)(2) requires a supply-side bonding jumper be installed with the circuit conductors, with the following exception:
Exception: A supply-side bonding jumper shall not be required between enclosures for installations made in compliance with 250.30(A)(1), Exception No. 2.
So ignore my previous post, I see now the point of having one circuit conductor grounded is so that you can use 250.30(A)(1), Exception No. 2, to earth the grounded conductor at both ends of the long run, and use the above exception to avoid any EGC or bonding jumper.
Cheers, Wayne
mbrooke
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- United States
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One option I heard and researched today is buying two 10kva pad-mounts and interconnect them via URD concentric neutral cable. Only issue is I have no idea how to make that NEC complaint.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
As shown in the image in post 49. That post makes me have to take back some of what I said in post 33 on running medium voltage but needing a third conductor for equipment grounding - but you can not have other conductive paths between the two structures either, shield on a telephone cable could get you if you have this, or a metal piping system between them.
Still don't know if cost will be less then larger low voltage conductors/cables. Don't know how much a 2.4 or 7.2 padmount would cost, guessing both are common enough they would be in same price range as POCO's use a lot of them. Next item that a POCO would not typically need is a disconnect (on the meduim voltage line) at the second structure, my guess is they cost just a little more then a typical 240 volt 100 amp safety switch.
mbrooke
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- United States
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If the Pad is outside, I can skip the switch and run the secondary directly into a disconnect?
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