# 1500kVA transformer



## don_resqcapt19 (Jul 18, 2010)

What size is the primary OCPD?


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## Gatspy (Sep 14, 2011)

This is the layout sir.


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## cabletie (Feb 12, 2011)

That seems to be fine.
Table 310-15(b) (16) of the 2011 code shows 500 kcmil at 75 degrees is good for 380 amps. For not more than three current carrying conductors in a conduit
380*6=2280 amps
Looking at it the other way around 
2000/6=333.3 amps
 What else are your concerns? Ambient temp? Are the service entrance conductors in 12 conduits, if so than derating is not an issue? What is the wiring method? As long as a major portion of the load is not non-linear (harmonic currents) than the neutral conductor would not count for derating.


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## chicken steve (Mar 22, 2011)

conductors aside, wouldn't the 230 _rule of six_ apply to two MDP's ?

~CS~


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## Gatspy (Sep 14, 2011)

Ambient temp is somewhere around 95degF. Those cables were direct burial type and Yes the derating is not an issue sir. I don't know if this comply with the 230 rule. What I know close to this is the 240.21(C). What do you think Sir's?..


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## cabletie (Feb 12, 2011)

We would have to know if the conductors are service laterals covered under article 230 (services) or feeder conductors supplied by a transformer with the tap rules you stated in 240.21(c). Even if the customer owns the outside distribution system and transformer it would still be considered a service if it meets the definition of a service

*Article 100 (definitions)*

*Service. *The conductors and equipment for delivering electric
energy from the serving utility to the wiring system of
the premises served.

*Service Lateral. *The underground service conductors between
the street main, including any risers at a pole or other
structure or from transformers, and the first point of connection
to the service-entrance conductors in a terminal box
or meter or other enclosure, inside or outside the building
wall. Where there is no terminal box, meter, or other enclosure,
the point of connection is considered to be the point of
entrance of the service conductors into the building.

*Feeder. *All circuit conductors between the service equipment,
the source of a separately derived system, or other
power supply source and the final branch-circuit overcurrent
device.

I was assuming you were talking about a service to a building. It is interesting to note that two services are allowed to the same building where the capacity requirements are in excess of 2000A see 230.2(c)(1) 
You would be allowed up to six service disconnects in each MDP before you would need a main breaker in each MDP (service)

*230.71 Maximum Number of Disconnects*
*(A) General. *The service disconnecting means for each
service permitted by 230.2, or for each set of serviceentrance
conductors permitted by 230.40, Exception No. 1,
3, 4, or 5, shall consist of not more than six switches or sets
of circuit breakers, or a combination of not more than six
switches and sets of circuit breakers, mounted in a single
enclosure, in a group of separate enclosures, or in or on a
switchboard. There shall be not more than six sets of disconnects
per service grouped in any one location.
For the purpose of this section, disconnecting means installed
as part of listed equipment and used solely for the
following shall not be considered a service disconnecting
means:
(1) Power monitoring equipment
(2) Surge-protective device(s)
(3) Control circuit of the ground-fault protection system
(4) Power-operable service disconnecting means

Here is the commentary

Section 230.71(A) specifies the maximum number of disconnects
permitted as the disconnecting means for the service
conductors that supply the building or structure. One set
of service-entrance conductors, either overhead or underground,
is permitted to supply two to six service disconnecting
means in lieu of a single main disconnect. A
single-occupancy building can have up to six disconnects for
each set of service-entrance conductors. Multiple-occupancy
buildings (residential or other than residential) can be provided
with one main service disconnect or up to six main
disconnects for each set of service-entrance conductors.
Multiple-occupancy buildings may have serviceentrance
conductors run to each occupancy, and each such
set of service-entrance conductors may have from one to six
disconnects (see 230.40, Exception No. 1).
Exhibit 230.25 shows a single enclosure for grouping
service equipment that consists of six circuit breakers or six
fused switches. This arrangement does not require a single
main service disconnecting means. Six separate enclosures
also would be permitted as the service equipment. Where
factory-installed switches that disconnect power to surge
protective devices and power monitoring equipment are included
as part of listed equipment, the last sentence of
230.71(A) specifies that the disconnect switch for such
equipment installed as part of the listed equipment does _not_
count as one of the six service disconnecting means permitted
by 230.71(A). The disconnecting means for the control
circuit of ground-fault protection equipment or for a poweroperable
service disconnecting means are also not considered
to be service disconnecting means where such
disconnecting means are installed as a component of listed
equipment.

Of coarse all of this means nothing if we are talking about a feeder.


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## Shorty Circuit (Jun 26, 2010)

The transformer can deliver 4263 amps but each set of feeders to an MDP is only rated to handle 2280 amps. Even if each MDP is protected at 2000 amps, what protects the feeders against a fault that draws more than 2280 amps but less than 4263 amps between the secondary of the transformer and one of the MDPs? IMO you need an upstream distribution bus rated at 4000 amps with two 2000 amp fused disconnect switches.


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## nitro71 (Sep 17, 2009)

Shorty Circuit said:


> The transformer can deliver 4263 amps but each set of feeders to an MDP is only rated to handle 2280 amps. Even if each MDP is protected at 2000 amps, what protects the feeders against a fault that draws more than 2280 amps but less than 4263 amps between the secondary of the transformer and one of the MDPs? IMO you need an upstream distribution bus rated at 4000 amps with two 2000 amp fused disconnect switches.


What protects POCO wire? Pretty much something will melt. Either their fuse will blow or the service entrance conductors will fail. 

Off hand this intall seems good to me. The OP needs to do his research. 

There are probably some available fault current ratings to take into consideration but I'm not really up on that part of the trade yet.


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## Shorty Circuit (Jun 26, 2010)

nitro71 said:


> What protects POCO wire? Pretty much something will melt. Either their fuse will blow or the service entrance conductors will fail.
> 
> Off hand this intall seems good to me. The OP needs to do his research.
> 
> There are probably some available fault current ratings to take into consideration but I'm not really up on that part of the trade yet.


"What protects POCO wire?"

Fuses on the utility pole cutouts.


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## don_resqcapt19 (Jul 18, 2010)

Shorty Circuit said:


> "What protects POCO wire?"
> 
> Fuses on the utility pole cutouts.


Utility fuses on the primary are not sized or intended to protect the transformer or the secondary conductors of the transformer.


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## Shorty Circuit (Jun 26, 2010)

don_resqcapt19 said:


> Utility fuses on the primary are not sized or intended to protect the transformer or the secondary conductors of the transformer.


They will blow in a short circuit though. Utilities are not required to conform to NEC for installations related specifically to their own networks, they are exempt and allowed to set their own standards. But premesis wiring installed and owned by the user is requird to conform and therefore the feeders on the secondary side of the utility transformer are required to be protected from overload. In this example they may not be. Coud there be two sets of lugs and fuses for each inside the transformer? Then it would seem reasonable. I'm not familiar with this arrangement.

The utilities probably know from experience that the number of feeder failures due to oversizing cutout fuses is so small that it doesn't pay for them to worry about it. In jurisdictions I'm familiar with the standard medium voltage feeder is 2/0 and the voltage is 13.2K. I don't know if they even size the fuses for the transformers they install. They seem confident that their transformers can operate well above their ratings without much risk of failure.


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## don_resqcapt19 (Jul 18, 2010)

Shorty Circuit said:


> They will blow in a short circuit though. ...


Yes, the intent of the utility primary fuses is to protect the primary distribution system. They need to keep the cash registers running.


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## Shorty Circuit (Jun 26, 2010)

don_resqcapt19 said:


> Yes, the intent of the utility primary fuses is to protect the primary distribution system. They need to keep the cash registers running.


They could go to the added expense to protect every one of them from overload too but guess who would pay the extra cost. They must make an acceptable profit to stay in business. That's what the Public Utilities Bureau is for, to see to it that their profit is sufficient to attract market capital without being excessive by setting the rates that they can charge. This is what it means to be a public utility with a monopoly over your customers' supply. You will make a profit. Not a huge one but in a down stock market public utilities are usually relatively safe harbors...but not always.


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## bobelectric (Feb 24, 2007)

Gatspy said:


> I just wanna ask if two 2000A main panel is allowed to be installed in a 1500kVA 13.8kV:208/120 transformer? 12 sets of 500mcm cables to be installed per phase in the secondary side. Does it violate the NEC?
> 
> Thanks..


I would rely on all the knowledge we are spending over there while we let our own Country wither away...


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## Rockyd (Apr 22, 2007)

Shorty Circuit said:


> They will blow in a short circuit though. *Utilities are not required to conform to NEC for installations related specifically to their own networks, they are exempt and allowed to set their own standards.* But premesis wiring installed and owned by the user is requird to conform and therefore the feeders on the secondary side of the utility transformer are required to be protected from overload. In this example they may not be. Coud there be two sets of lugs and fuses for each inside the transformer? Then it would seem reasonable. I'm not familiar with this arrangement.
> 
> The utilities probably know from experience that the number of feeder failures due to oversizing cutout fuses is so small that it doesn't pay for them to worry about it. In jurisdictions I'm familiar with the standard medium voltage feeder is 2/0 and the voltage is 13.2K. I don't know if they even size the fuses for the transformers they install. They seem confident that their transformers can operate well above their ratings without much risk of failure.


Craftsmen in the inside electrical portion of the industry tend to wrp their head around 310.16 as a standard in their standard, when applying amperage to wire. Look at the rest of the charts past 310.16 free air, and high temp wires. Have to think outside of the electrician's "normal amperage mode".

We probably overbuild by up to 300% on most installations in how we do things. The code is used by rookies to electrical engineers. It's purpose is to protect the whole bunch. 

That is why sometimes people get excited when they see #14 protected by a 40 amp OCPD for a motor. It's perfectly legal, and safe.

Most stuff over 600 volts is not accessible to the public, and, is subject to a different set of rules than we are. Life insurance for us as electricians is no greater cost than for most of the public. For lineman the rate is a lot higher. It's all in perception, when in fact, they maybe entirely safe as the rest of us.


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## Shorty Circuit (Jun 26, 2010)

Rockyd said:


> Craftsmen in the inside electrical portion of the industry tend to wrp their head around 310.16 as a standard in their standard, when applying amperage to wire. Look at the rest of the charts past 310.16 free air, and high temp wires. Have to think outside of the electrician's "normal amperage mode".
> 
> We probably overbuild by up to 300% on most installations in how we do things. The code is used by rookies to electrical engineers. It's purpose is to protect the whole bunch.
> 
> ...


First of all electrical engineers have to conform to NEC because in most jurisdictions it's the law. if they violate it their drawings might not be approved by the building department or if they are they might not pass inspection. When there is a failure and an injury or loss of property, the lawyers will look for any excuse to sue and collect money even if a code violation had nothing to do with the loss. Courts may not necessarily be sophisticted enough to make distinctions.

In the event of a utility equipment failure, utilities are only liable to restore service, not consequential damages. Since utilities have allowed capacity to lag behind projected demand, reduced or eliminated redundancy, neglected maintenance, cut costs on personnel and in every other way they can think of to keep rates low, their networks are becoming increasingly unreliable. It's ludicrous for people to talk about "smart networks" when the dumb one they have is badly in need of a major capital improvement. In some areas that's happening but the number of engineers available has dwindled because there is little incentive to work for utilities considering how poorly most of them pay. In fact the general deterioration at all levels is becoming increasingly obvious while the need for adequate and reliable electrical power increases in a society completely dependent on electrical and electronic technology ultimately supplied from the network. 

Insofar as protecting motor feeders with seemingly large OCPDs that is anticipated in NEC. NEC is not a specification or training manual, it is intended for those already trained and experienced. Inspectors can accept equivalent methods if in their judgment they are adequate, the electrical boards who hire them have the final say. That's stipulated in Chapter 80. 

I don't know if networks are overbuilt by 300% but when one man pulls a switch on a piece of equipment in Arizona and 5 million people in Southern California, Arizona, and Mexico are in the dark as they were three weeks ago, or when a single overloaded feeder in Ohio takes out the entire Northeast quadrant of North America as happened in 2004 nearly 40 years after it happend in 1965 and wasn't supposed to be possible there are at least some critical aspects of the networks that are not only underbuilt but underengineered. Fortunately most mistakes that get built, even some of the stupidest ones usually don't have consequences. But no one's luck holds out forever.


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