Parallel Utility Transformers for Service

[bwat]

I see no reason not to treat the utility as a black box, what they do should be of no concern to you. The NEC sizing of conductors, and their enforcement, only come into play after the demarcation point. Where is the utility metering being performed, before or after your main device?

Also, if the paralleled conductors do not have the same impedance the current will not divide across them equally

In my experience, utilities rarely worry about overloading their transformers, based on NEC sizing methodologies. Their protection engineers are usually involved when paralleling transformers becomes a concern.

I’m not concerned about their transformers overloading or being protected. I don’t think I’ve said anything here that should indicate that (or at least I hope not). If their transformers burn down, I don’t want to say I don’t care, but that’s not my responsibility and they can do what they want.

What I’m concerned about are the secondary conductors, which are my responsibility. The current demarcation point is the secondary terminals of their transformers. So the secondary conductors I need to care about. If one of their transformers were to fault and clear it’s primary fuse, my secondary conductors on the other transformer are only protected from overload by the primary fuse.

Agreed and understood on the different impedances causing different currents in parallel conductors. With what’s being proposed, my secondary conductors are not actually paralleled. They’re not all connected together on the source end.

Metering is before me. I believe they’re planning on doing primary metering since there are the two transformers.

 

[bwat]

I think the confusion here is the OP seems to be implying that the POCO wants him to run from each POCO xformer to the line side of his service disconnect. That in my view has all kinds of NESC and NEC issues. As I said in post #13 and others seem to be saying and they run to a common bus that they own and control and the customer attaches at that point then I see no issues. Done all the time in large large city downtown networks.

Yes, you are properly understanding the situation.

 

[synchro]

Metering is before me. I believe they’re planning on doing primary metering since there are the two transformers.

And so they want the customer to pay for any excitation currents and losses in the transformers.

 

[texie]

Metering is before me. I believe they’re planning on doing primary metering since there are the two transformers.

This does not sound correct to me. I can't imagine why they would do this and wonder if their tariff would even allow metering on the primary side where they own the transformer. Again, another reason you can't do what they are proposing by not having a common bus.

 

[Hv&Lv]

I don't follow this. Unless the fuses you're referring to in the last sentence are different than the ones you are referring to in 2nd and 3rd sentence. Almost sounds like you're talking about transferring load from phase A to phase B when A-fuse clears.

Sorry for the confusion.
I was talking about the fuse on the primary dip feeding the transformers.

Not under NEC..

 

[Hv&Lv]

Primary metering can be done and the transformer usage can be deducted.
Compensated metering,
Or totalizers on the secondary of both transformers

 

[bwat]

I agree with Jim (Post 17). POCOs address their distribution methods is various ways and our concern should be from the demarcation point.
Parallel conductors from that point are the norm and, if you are concerned about protecting individual conductors, as I've mentioned in earlier posts, install cable limiters to protect those individual conductors.

I agree with a lot of what you're saying here as well. But again, I think you might be misunderstanding my descriptions of the situation. The transformer secondary terminals are the demarc point. So the secondary conductors are mine, and if they are not connected together on the source side, they aren't parallel and therefor aren't meeting an important part of your post (my bold). I would be having conductors on my side of the demarc line that are not parallel.

I should not care, and do not care, what the poco does on their side of the demarcation point.

 

[electrofelon]

I agree with a lot of what you're saying here as well. But again, I think you might be misunderstanding my descriptions of the situation. The transformer secondary terminals are the demarc point. So the secondary conductors are mine, and if they are not connected together on the source side, they aren't parallel and therefor aren't meeting an important part of your post (my bold). I would be having conductors on my side of the demarc line that are not parallel.

I should not care, and do not care, what the poco does on their side of the demarcation point.

Could you express you concerns to the POCO and ask if they could to tie the two secondaries together to give you a common point to connect to? If they are unwilling I suppose you could just do it, although i understand it is not an insignificant cost in connectors and what not. Perhapas something lay in like this and you could just continue thru one transformer to the next with your 8 conductors:

Transformer Spade Connector ABK 8750 | eBay

We will take care of you! We are not an authorized dealer of this product. This product has no manufacturers warranty.

www.ebay.com

 

[Hv&Lv]

Would 450.6 come into play here?

 

[don_resqcapt19]

Would 450.6 come into play here?

Not with the service point being the secondary terminals of the transformer...the transformer itself is outside of the scope of the NEC.

 

[bwat]

So it seems like we're mostly all on the same page here: we have to do something different than what is being proposed since it seems like what is being proposed would be a violation. I think that if I could clearly show the poco what exactly is a violation, they would go along with the plan and be willing to change something on their side with the transformer connections.


So my question is, what is the specific code section that is being violated?

The ones that I listed earlier as potential:
310.10(H),
240.4(F),
240.21(C) and 240.21(C)(4),
230.90:

 

[bwat]

So I'm thinking these two seem to be the most clear that it would be non-compliant, if they apply. So I suppose this is my question, does 240.4(F) and 240.21(C) apply to my secondary conductors in this instance?

240.4(F):

(F) Transformer Secondary Conductors. Single-phase (other
than 2-wire) and multiphase (other than delta-delta, 3-wire)
transformer secondary conductors shall not be considered to
be protected by the primary overcurrent protective device.
Conductors supplied by the secondary side of a single-phase
transformer having a 2-wire (single-voltage) secondary, or a
three-phase, delta-delta connected transformer having a 3-wire
(single-voltage) secondary, shall be permitted to be protected
by overcurrent protection provided on the primary (supply)
side of the transformer, provided this protection is in accordance
with 450.3 and does not exceed the value determined by
multiplying the secondary conductor ampacity by the
secondary-to-primary transformer voltage ratio.

240.21(C) and 240.21(C)(4):

(C) Transformer Secondary Conductors. A set of conductors
feeding a single load, or each set of conductors feeding separate
loads, shall be permitted to be connected to a transformer
secondary, without overcurrent protection at the secondary, as
specified in 240.21(C)(1) through (C)(6). The provisions of
240.4(B) shall not be permitted for transformer secondary
conductors.

(4) Outside Secondary Conductors. Where the conductors
are located outside of a building or structure, except at the
point of load termination, and comply with all of the following
conditions:
(1) The conductors are protected from physical damage in
an approved manner.
(2) The conductors terminate at a single circuit breaker or a
single set of fuses that limit the load to the ampacity of
the conductors. This single overcurrent device shall be
permitted to supply any number of additional overcurrent
devices on its load side.
(3) The overcurrent device for the conductors is an integral
part of a disconnecting means or shall be located immediately
adjacent thereto.
(4) The disconnecting means for the conductors is installed
at a readily accessible location complying with one of the
following:
a. Outside of a building or structure
b. Inside, nearest the point of entrance of the conductors
c. Where installed in accordance with 230.6, nearest the
point of entrance of the conductors

 

[augie47]

So I'm thinking these two seem to be the most clear that it would be non-compliant, if they apply. So I suppose this is my question, does 240.4(F) and 240.21(C) apply to my secondary conductors in this instance?

240.4(F):


240.21(C) and 240.21(C)(4):

IMO, they apply. Good luck

 

[synchro]

In 240.21(C) I would emphasize the phrase "a transformer secondary" because its allowances pertain when there is only one transformer secondary. In your case there are not only two secondaries but they are even on two different transformers.

 

[electrofelon]

So I'm thinking these two seem to be the most clear that it would be non-compliant, if they apply. So I suppose this is my question, does 240.4(F) and 240.21(C) apply to my secondary conductors in this instance?

240.4(F):


240.21(C) and 240.21(C)(4):

IMO, they apply. Good luck

So guys do tell how 240 applies to these conductors? These sections are for customer owned transformers. We have utility transformers which don't even exist as far as the NEC is concerned.

 

[xformer]

I have some concerns about code permissibility and questions on how this should actually be executed.

Need a 3000A @480V service. Utility has (2) 1250kVA pad mount transformers, which would be 1500A each, that they said they are willing to parallel them in order to get our 3000A service capacity. The secondaries of both would terminate on a single 3000A CB that would service as the service disconnect.

Trying to navigate 230.2 and other sections for what applies. I don't think this counts as two services. But also, if utility isn't connecting their secondaries together at source side, this isn't a truly parallel feed. I'd be concerned about not knowing if one of sets of conductors to one of the transformers is overloaded. The main CB wouldn't see this as overload scenario.

I suspect the secondaries will have to get connected together at source side, but is that 100% true for requirement? Anything else I'm missing?

The Secondary Conductors are part of a SDS. What happens on the Primary side should not be a concern since that falls under POCO purview.

 

[bwat]

The Secondary Conductors are part of a SDS. What happens on the Primary side should not be a concern since that falls under POCO purview.

I appreciate you joining the discussion, but please read the rest of this thread. I'm not concerned about the primary side, or the transformers themselves. Also, I would not agree that this is an SDS since I don't own the transformers.

 

[electrofelon]

I appreciate you joining the discussion, but please read the rest of this thread. I'm not concerned about the primary side, or the transformers themselves. Also, I would not agree that this is an SDS since I don't own the transformers.

These are not transformer secondary conductors, they are service conductors.

 

[xformer]

I appreciate you joining the discussion, but please read the rest of this thread. I'm not concerned about the primary side, or the transformers themselves. Also, I would not agree that this is an SDS since I don't own the transformers.

I agree with you that this still qualifies as a service and not an actual SDS. But I also like to think of this as just a big house. All Secondary conductors should terminate in a single OCPD, that the Owner has access to and can disconnect the facility from if need be.

 

[zbang]

should terminate in a single OCPD

That's exactly where this started, see the original post.