Parallel Utility Transformers for Service

[bwat]

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?

 

[ron]

The utility can do what they like. In NYC they often provide service from a 4-bagger (four paralleled transformers under the sidewalk).

You get your single 3000A service and terminate in the disconnect / meter and go on your way.

https://www.coned.com/-/media/files/coned/documents/small-medium-large-businesses/transformervaultplacement.pdf?la=en

 

[bwat]

Thanks Ron. But for our service conductors, do we just treat it the same as if there was just one large transformer and ignore that the conductors aren't connected at the source end? (4) 500MCM to each transformer, for example. That would be 1500A ampacity on each circuit more or less, terminating on a 3000A CB. They'd be tied together at the CB, but going to two different transformers on the other side, so it would seem to me that these aren't parallel conductors and would be undersized.

 

[ron]

The utility will have a parallel bus that they will extend to you for connection. It will be a a parallel service feeder in the true sense of the term.

 

[augie47]

Agree with Ron. IF POCO is installing the conductors to your main, smile and walk away.
(They may protect the individual conductors with cable limiters which addresses your concern)

 

[bwat]

It sounds like I'd agree if the assumptions that you're making were the case in reality, but I don't think that is so. How do these items change things with your opinion?

• We (customer) are responsible for installing the conductors from our main to their transformer secondaries, not the poco
• They have explicitly stated that they won't have any kind of protection on the secondary of their transformers. Just bayonets on the primary, and the config won't be delta-delta for 240.4(F) purposes

The utility will have a parallel bus that they will extend to you for connection. It will be a a parallel service feeder in the true sense of the term.

The utility having a "parallel bus" that they extend for me sounds like what I would have expected; that they're tying their secondaries together on the transformer side of those conductors. But that's not the case. They're not doing that. All secondary conductors are tied to together on load side, but not source side.

It's simply something like this:

[Transformer 1 Secondary Terminals] -- (4 sets of 500mcm) -- [3000A Main]

[Transformer 2 Secondary Terminals] -- (4 sets of 500mcm) -- [same 3000A Main]


Assume (4) parallel 500s are ~1500A

 

[ron]

Definitely strange.

As a formality, if the (2) 1500A feeders are dictated to you by the utility, are you just installing them under their jurisdiction and the line of demarcation is the line side of your 3000A service disconnect? Can you document it like that? Other than that, it is not ok by the NEC.

 

[winnie]

What ron is pointing out is that the utility follows a _different_ set of safety rules than what electricians follow in building wiring.

Inside the building you generally have to follow the NEC. Of course, the NEC is adopted by some local government, and until this is done it is just a 'model code'.

The utility follows some version of the NESC.

There is a dividing point where the rules change from one to the other.

The installation would violate the NEC, but is probably acceptable under the utility's rules. So you need to find out what the dividing point is.

-Jon

 

[synchro]

... 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.
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.

• We (customer) are responsible for installing the conductors from our main to their transformer secondaries, not the poco
• They have explicitly stated that they won't have any kind of protection on the secondary of their transformers. Just bayonets on the primary, and the config won't be delta-delta for 240.4(F) purposes

The utility having a "parallel bus" that they extend for me sounds like what I would have expected; that they're tying their secondaries together on the transformer side of those conductors. But that's not the case. They're not doing that. All secondary conductors are tied to together on load side, but not source side.

It's simply something like this:

[Transformer 1 Secondary Terminals] -- (4 sets of 500mcm) -- [3000A Main]

[Transformer 2 Secondary Terminals] -- (4 sets of 500mcm) -- [same 3000A Main]

Assume (4) parallel 500s are ~1500A

I think if any of the conductors get disconnected, then the others on that same phase could get overloaded whether the transformer outputs are directly connected in parallel or not. The main breaker is not going to prevent this in either situation.

Now if a winding in one of the transformers or a primary fuse for it opens, then that could shift the load to the other transformer and then overload it (because its primary fuses may not open since it's not a delta-delta as bwat mentioned). But in that case having the transformer outputs paralleled would still allow the feeder conductors to continue share the load on each phase evenly between its respective conductors. That would not be the case if the transformer outputs were not directly connected together.
And protecting the transformers from overloading is really the POCO's responsibility.

 

[augie47]

I think if any of the conductors get disconnected, then the others on that same phase could get overloaded whether the transformer outputs are directly connected in parallel or not. The main breaker is not going to prevent this in either situation.

Now if a winding in one of the transformers or a primary fuse for it opens, then that could shift the load to the other transformer and then overload it (because its primary fuses may not open since it's not a delta-delta as bwat mentioned). But in that case having the transformer outputs paralleled would still allow the feeder conductors to continue share the load on each phase evenly between its respective conductors. That would not be the case if the transformer outputs were not directly connected together.
And protecting the transformers from overloading is really the POCO's responsibility.

would that not promote the use of cable limiters ?

 

[synchro]

would that not promote the use of cable limiters ?

I think cable limiters are not really good for responding to overloads. They are more for disconnecting a faulted conductor of a parallel set quickly so that the others can stay online and keep supplying the loads.

 

[Hv&Lv]

I think if any of the conductors get disconnected, then the others on that same phase could get overloaded whether the transformer outputs are directly connected in parallel or not. The main breaker is not going to prevent this in either situation.

Now if a winding in one of the transformers or a primary fuse for it opens, then that could shift the load to the other transformer and then overload it (because its primary fuses may not open since it's not a delta-delta as bwat mentioned). But in that case having the transformer outputs paralleled would still allow the feeder conductors to continue share the load on each phase evenly between its respective conductors. That would not be the case if the transformer outputs were not directly connected together.
And protecting the transformers from overloading is really the POCO's responsibility.

We have one installation like this.
we serve “A” phase on both transformers under the same fuse. ”B” phase under one fuse, etc.
reason is exactly as you stated. One fuse blows, load transfers, blows the other fuse.

 

[texie]

It would seem to me that if the POCO wanted to do this they would use a tap box (owned and controlled by them) and the customer would would connect at that point. How they handle their stuff from the tap box upstream to the xformers would be under their ownership and control and be governed by the NESC. Down stream from the tap box to the service disconnect would be the customers responsibility and fall under the NEC. If this was in my jurisdiction that is what we would likely require in a review.

 

[bwat]

Thanks for the responses thus far. It seems like we're on the same page now that this is a potentially odd and/or NEC violation depending on specifics and where the demarcation point is between us and them.

I think if any of the conductors get disconnected, then the others on that same phase could get overloaded whether the transformer outputs are directly connected in parallel or not. The main breaker is not going to prevent this in either situation.

Agreed. True, but that's pretty much the case for any parallel conductor installation, so that's not necessarily my concern.

Now if a winding in one of the transformers or a primary fuse for it opens, then that could shift the load to the other transformer and then overload it (because its primary fuses may not open since it's not a delta-delta as bwat mentioned). But in that case having the transformer outputs paralleled would still allow the feeder conductors to continue share the load on each phase evenly between its respective conductors. That would not be the case if the transformer outputs were not directly connected together.
And protecting the transformers from overloading is really the POCO's responsibility.

This is more along the lines of my concern. The bayonet opens on one of the transformers, but the load isn't high enough to overload the bayonet on the other. The site won't be consuming near the whole capacity all the time. I could very well see 2000A being somewhat of a normal site load, and I wouldn't want that on (4) 500s with the only hope of protecting it being the bayonets on the other side of the transformer. Who knows what they bayonet primary fusing will be in comparison to secondary conductor ampacity. I don't usually see those fuses being that tight on the curve.

Also, let's not forget some of the reasoning behind 240.4(F) (requirements to say that we're protecting secondary conductors via primary fusing). It's quite possible that the transformer is delta-wye, so there won't be an equal relationship between primary conductor current and secondary conductor current modified only by the turns ratio.

It would seem to me that if the POCO wanted to do this they would use a tap box (owned and controlled by them) and the customer would would connect at that point. How they handle their stuff from the tap box upstream to the xformers would be under their ownership and control and be governed by the NESC. Down stream from the tap box to the service disconnect would be the customers responsibility and fall under the NEC. If this was in my jurisdiction that is what we would likely require in a review.

This is essentially my thought as well. Sure, they can certainly do whatever they want on their side of the line. But if the secondary conductors are mine, then we're talking about something being a violation.

(I have more thoughts on other replies, but I'll break that into another post as this is already getting too long)

 

[bwat]

Definitely strange.

As a formality, if the (2) 1500A feeders are dictated to you by the utility, are you just installing them under their jurisdiction and the line of demarcation is the line side of your 3000A service disconnect? Can you document it like that? Other than that, it is not ok by the NEC.

What ron is pointing out is that the utility follows a _different_ set of safety rules than what electricians follow in building wiring.

Inside the building you generally have to follow the NEC. Of course, the NEC is adopted by some local government, and until this is done it is just a 'model code'.

The utility follows some version of the NESC.

There is a dividing point where the rules change from one to the other.

The installation would violate the NEC, but is probably acceptable under the utility's rules. So you need to find out what the dividing point is.

-Jon

Glad to hear I'm not crazy and that you agree this would be a violation. I had 'softly' thrown out the idea to them of the secondary conductors being their responsibility, and they didn't seem to like it. But I didn't propose the idea of us still supplying and installing the conductors, but they're on their side of the demarc and our service point is then the line side of the main CB. Might be a good idea there. The secondary terminals of their transformer is the currently proposed service point.

Thankfully, this utility is very nice to work with. I'm fairly sure they thought I was being a bit overly cautious on this topic when I brought it up, but I think if I could clearly demonstrate to them that this is an NEC violation, they would agree we need to do something different here. What code section would be my "smoking gun" so to speak? Here are the ones that come to mind for me.



310.10(H):

Conductors in Parallel.
(1) General. Aluminum, copper-clad aluminum, or copper
conductors, for each phase, polarity, neutral, or grounded
circuit shall be permitted to be connected in parallel (electrically
joined at both ends) only in sizes 1/0 AWG and larger
where installed in accordance with 310.10(H)(2) through
(H)(6).

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

And 230.90:

Part VII. Service Equipment — Overcurrent Protection
230.90 Where Required. Each ungrounded service conductor
shall have overload protection.
(A) Ungrounded Conductor. Such protection shall be provided
by an overcurrent device in series with each ungrounded
service conductor that has a rating or setting not higher than
the allowable ampacity of the conductor. A set of fuses shall be
considered all the fuses required to protect all the ungrounded
conductors of a circuit. Single-pole circuit breakers, grouped in
accordance with 230.71(B), shall be considered as one protective
device.

One other way around this, as terrible as it would be, is to run (8) 500s from the main CB to each transformer. Although I'm not sure this is even possible based on available lugs/terminals at both equipment ends.

 

[bwat]

We have one installation like this.
we serve “A” phase on both transformers under the same fuse. ”B” phase under one fuse, etc.
reason is exactly as you stated. One fuse blows, load transfers, blows the other fuse.

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.

 

[jim dungar]

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.

 

[augie47]

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.

 

[texie]

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.

 

[synchro]

One other way around this, as terrible as it would be, is to run (8) 500s from the main CB to each transformer. Although I'm not sure this is even possible based on available lugs/terminals at both equipment ends.

If the POCO is OK with connecting (8) 500s to each transformer, then just loop half of those conductors between the two transformers somewhere and save a lot of wire (half joking when I say this).