Service transformers

[hhsting]

I have never seen something like this but here goes its on the plan:

The site is residential apartments with few shops commercial first floor not high rise.

I have two POCO xfmrs 208/120V three phase feeding one 4000A main service swbd. Their are 12 sets of 4#600kcmil AL from each xfmr to the switchboard.

The Switchboard has one 4000A section and another 2000A section. 4000A section powers meter stacks. 2000A section powers fire pump and house panels.

I am confused why there are two poco xfmr or is it even allowed by NEC? Does POCO not provide 1442 kva xfmr? Why have two? How can switchboard handle such currents from2 xfmr while main bus of swbd is 4000A?

 

[don_resqcapt19]

The NEC has ZERO authority to restrict the size and number of transformers that a power company chooses to use to supply a building.

 

[hhsting]

The NEC has ZERO authority to restrict the size and number of transformers that a power company chooses to use to supply a building.

So fault currents on poco swbd be addition of two poco xfmr? Also main bonding jumper be sized from 24 sets of 600kcmil AL?

 

[hhsting]

So fault currents on swbd be addition of two poco xfmr? Also main bonding jumper be sized from 24 sets of 600kcmil AL?

Time ran out first paragraph edit as above:

 

[MyCleveland]

Cannot say I have seen this arrangement ever. You really stumble into some strange projects. Such is the life of a plans examiner.
I guess you have to assume author coordinated with utility, specific issues involved but they know what they are. Others will have to comment on whether SCCs are a simple addition or not. Curious what that number is.
Have never seen a mlo section that accepted 24 sets.
Must be quite a few apartments….with three 1200A meter stacks.
I assume the fourth 1200A main is feeding the house 2000A ct cabinet.
4800A’s of OCP…4000A bus, what is the total load calc ?
Fire pump tap downstream of house main…? Old fuzzy mind, but not sure it is ok.

 

[Hv&Lv]

Larger transformers are expensive both to energize and purchase.
They don’t make a 1442 transformer…

Cheaper to put in two 500 kVA pad mounts and parallel them, but parallel transformers like that sometimes don’t play well together.
You don’t need 4000 amps worth of transformers..

Or better yet have one for apartments and one for fire pumps.
What size are the transformers anyway?

 

[hhsting]

Larger transformers are expensive both to energize and purchase.
They don’t make a 1442 transformer…

Cheaper to put in two 500 kVA pad mounts and parallel them.
You don’t need 4000 amps worth of transformers..

Or better yet have one for apartments and one for fire pumps.
What size are the transformers anyway?

no size given


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[don_resqcapt19]

So fault currents on poco swbd be addition of two poco xfmr? Also main bonding jumper be sized from 24 sets of 600kcmil AL?

Yes and yes, but typically the main bonding jumper on switchgear is a bus bar provided by the manufacturer.

 

[DrSparks]

The NEC has ZERO authority to restrict the size and number of transformers that a power company chooses to use to supply a building.

That would be the NESC.

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[DrSparks]

I have never seen something like this but here goes its on the plan:

The site is residential apartments with few shops commercial first floor not high rise.

I have two POCO xfmrs 208/120V three phase feeding one 4000A main service swbd. Their are 12 sets of 4#600kcmil AL from each xfmr to the switchboard.

The Switchboard has one 4000A section and another 2000A section. 4000A section powers meter stacks. 2000A section powers fire pump and house panels.

I am confused why there are two poco xfmr or is it even allowed by NEC? Does POCO not provide 1442 kva xfmr? Why have two? How can switchboard handle such currents from2 xfmr while main bus of swbd is 4000A?

They're connected in a network. They do that quite frequently, especially in dense urban areas. Fault currents can be quite high.

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[don_resqcapt19]

That would be the NESC.

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But that is not typically adopted by a unit of government and so not enforceable by the inspection authority.

 

[hhsting]

But that is not typically adopted by a unit of government and so not enforceable by the inspection authority.

*cough*. Public Service Commission

 

[Hv&Lv]

But that is not typically adopted by a unit of government and so not enforceable by the inspection authority.

??
Was that a guess?
the NESC is adopted by most governments.
Those that don’t, never fear. For USDA borrowers the NESC is mandatory.
Cali writes their own (of course) called general rule 95
Even in Cali USDA borrowers are still mandated to the NESC or general rule 95 whichever is stricter.

only a handful of states don’t.
I would hate to sit in a witness stand against a jury and explain why my company didn’t follow these rules…

https://standards.ieee.org/content/dam/ieee-standards/standards/web/documents/other/2017-nesc-state-adoption-reference-survey.pdf

 

[DrSparks]

??
Was that a guess?
the NESC is adopted by most governments.
Those that don’t, never fear. For USDA borrowers the NESC is mandatory.
Cali writes their own (of course) called general rule 95
Even in Cali USDA borrowers are still mandated to the NESC or general rule 95 whichever is stricter.

only a handful of states don’t.
I would hate to sit in a witness stand against a jury and explain why my company didn’t follow these rules…

https://standards.ieee.org/content/dam/ieee-standards/standards/web/documents/other/2017-nesc-state-adoption-reference-survey.pdf

I can't help but suspect that CAs code was partially, of not entirely written by Enron.

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[don_resqcapt19]

??
Was that a guess?
the NESC is adopted by most governments.
Those that don’t, never fear. For USDA borrowers the NESC is mandatory.
Cali writes their own (of course) called general rule 95
Even in Cali USDA borrowers are still mandated to the NESC or general rule 95 whichever is stricter.

only a handful of states don’t.
I would hate to sit in a witness stand against a jury and explain why my company didn’t follow these rules…

https://standards.ieee.org/content/dam/ieee-standards/standards/web/documents/other/2017-nesc-state-adoption-reference-survey.pdf

I guess I said that because, the local units of government around here have not adopted it, but the State of Illinois has....however there is no state enforcement of the NESC or the 2008 NEC that our state has adopted.

 

[kwired]

??
Was that a guess?
the NESC is adopted by most governments.
Those that don’t, never fear. For USDA borrowers the NESC is mandatory.
Cali writes their own (of course) called general rule 95
Even in Cali USDA borrowers are still mandated to the NESC or general rule 95 whichever is stricter.

only a handful of states don’t.
I would hate to sit in a witness stand against a jury and explain why my company didn’t follow these rules…

https://standards.ieee.org/content/dam/ieee-standards/standards/web/documents/other/2017-nesc-state-adoption-reference-survey.pdf

May be more widely into law than many might think, but is probably little enforcement other than self enforcement by individual POCO's. And yes if it is in the law books and litigation comes up you better hope you followed it if you were supposed to or your case gets harder to defend.