Transformer Sizing

[dmulhol925]

We have a transformer rated at 600amp on the secondary. Downstream is a breaker set at 900amp trip. Is it correct to say the transformer size needs to be sized for duty load only and not full load of the facility?

 

[kingpb]

So, essentially you have no protection on the LV side.

What is the protection on the HV side set at?

Look in NEC 450 for HV side limits.

But, a breaker with a fixed rating for 900A is odd, I assume it has an adjustable trip unit. if so, turn it down to the required setting.

 

[petersonra]

We have a transformer rated at 600amp on the secondary. Downstream is a breaker set at 900amp trip. Is it correct to say the transformer size needs to be sized for duty load only and not full load of the facility?

There is insufficient information in your question to give you an answer, including what you mean by "duty load".

Transformers are required to have protection on both the transformer primary and secondary and the primary and secondary conductors.

As long as the installation meets all of those requirements, it is acceptable.

What little you have told us makes me suspicious that it might not be compliant, especially if the 900 A CB is the only downstream protection. Do you have a one line diagram of what is actually there that you can post?

i am a little suspicious of the statement that the xfmr is rated 600A secondary. I know of no transformers that are rated that way.

 

[kingpb]

There is insufficient information in your question to give you an answer, including what you mean by "duty load".

The vulture's are circling. :lol:

 

[petersonra]

The vulture's are circling. :lol:

Circling is OK.

Swooping down and biting off chunks of the OP before he expires is prohibited by the rules of the forum.

 

[iceworm]

We have a transformer rated at 600amp on the secondary. Downstream is a breaker set at 900amp trip. Is it correct to say the transformer size needs to be sized for duty load only and not full load of the facility?

I don't know what this means. Can you explain a bit better?

Generally a transformer must be sized for the loads it will see - oversized to handle any internittent loads such as motor starting.

The primary and secondary CBs are sized per 450.3.A or 450.3.B. A secondary CB may not be required. But the secondary conductors and secondary panel require a OCP.

So what exactly are you asking?

ice

 

[petersonra]

Generally a transformer must be sized for the loads it will see - oversized to handle any internittent loads such as motor starting.

where is this in the code?

 

[kingpb]

Transformers do not need to be over-sized, per se, it must be capable of providing the Watts and VARS to the electrical system it serves during starting of loads (which includes motors), such that the voltage drop on the system remains within acceptable limits. Selection of proper impedance is necessary. Simply over-sizing is not necessarily the answer.

 

[petersonra]

Transformers do not need to be over-sized, per se, it must be capable of providing the Watts and VARS to the electrical system it serves during starting of loads (which includes motors), such that the voltage drop on the system remains within acceptable limits. Selection of proper impedance is necessary. Simply over-sizing is not necessarily the answer.

where does it say this in the code?

 

[kingpb]

where does it say this in the code?

It won't because as we all know the NEC is not intended as an engineering design guide, it is minimum requirements to provide safety.

I believe 90.1 makes that quite clear.

 

[GoldDigger]

It won't because as we all know the NEC is not intended as an engineering design guide, it is minimum requirements to provide safety.

I believe 90.1 makes that quite clear.

But I have those sizing rules right here in my shirt pocket somewhere.....

 

[jim dungar]

But I have those sizing rules right here in my shirt pocket somewhere.....

90.1(C) does not say the NEC is not part of the design process, it says that the NEC is not sufficient to be the only reference used.

 

[iceworm]

where is this in the code?

Don't know - never looked for it in the code. I never expected a document that starts out saying:

90.1 Purpose.
(B) Adequacy. This Code contains provisions that are considered
necessary for safety. Compliance therewith and
proper maintenance results in an installation that is essentially
free from hazard but not necessarily efficient, convenient,
or adequate for good service or future expansion of
electrical use.​

and followed by:

(C) Intention. This Code is not intended as a design specification
or an instruction manual for untrained persons.​

to address the need for an electrical system to actually work. Although, my continued employment likely requires that the systems I work on - work reliably. I know that just seems mean as can be on their (my customers) part, but that's the way they are.

Maybe look at 210.11, then go to 215 for the feeders.

Did you have something else in mind?

ice

 

[petersonra]

Don't know - never looked for it in the code. I never expected a document that starts out saying:

90.1 Purpose.
(B) Adequacy. This Code contains provisions that are considered
necessary for safety. Compliance therewith and
proper maintenance results in an installation that is essentially
free from hazard but not necessarily efficient, convenient,
or adequate for good service or future expansion of
electrical use.​

and followed by:

(C) Intention. This Code is not intended as a design specification
or an instruction manual for untrained persons.​

to address the need for an electrical system to actually work. Although, my continued employment likely requires that the systems I work on - work reliably. I know that just seems mean as can be on their (my customers) part, but that's the way they are.

Maybe look at 210.11, then go to 215 for the feeders.

Did you have something else in mind?

ice

does not say anything at all about xfmr size. only about conductors.

 

[kingpb]

90.1(C) does not say the NEC is not part of the design process, it says that the NEC is not sufficient to be the only reference used.

Jimbo, where does it say that? :? My 90.1(C) reads; "This Code is not intended as a design specification or an instruction manual for untrained persons".

Further, 90.1(B); Adequacy. This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance results in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

Could you elaborate on where your wording is found? :roll:

 

[petersonra]

Jimbo, where does it say that? :? My 90.1(C) reads; "This Code is not intended as a design specification or an instruction manual for untrained persons".

Further, 90.1(B); Adequacy. This Code contains provisions that are considered necessary for safety. Compliance therewith and proper maintenance results in an installation that is essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use.

Could you elaborate on where your wording is found? :roll:

I think the essence of what he says is correct even though the code book does not phrase it that way.

part of the design process is to ensure that the system actually works while incorporating the requirements in the code that are mostly oriented toward reducing hazards from the use of electricity.

 

[iceworm]

...part of the design process is to ensure that the system actually works while incorporating the requirements in the code that are mostly oriented toward reducing hazards from the use of electricity.

Where does it say that in the code?
:roll::roll:
ice

 

[iceworm]

does not say anything at all about xfmr size. only about conductors.

I'm completely lost on where you are headed. Why would it matter? I really don't need a code or regulation to tell me to build systems so they will actually work. My customers really don't hire me to build systems that are marginal or underrated.

However, if you need a regulation, maybe look at 110.3.A.5, and 110.3.A.7.

ice

 

[iceworm]

... i am a little suspicious of the statement that the xfmr is rated 600A secondary. I know of no transformers that are rated that way.

Well, a 500kva 480Y secondary would have a secondary FLA of 601.407A - that's pretty close to 600A. I figured it was likely that.

ice

 

[jim dungar]

Could you elaborate on where your wording is found?

My wording is my wording.

The text of the 90.1(C), as you quoted, states "This Code is not intended as a design specification...." it continues with saying it is also not "... an instruction manual for untrained persons".


Remembering the warning about adequacy given in 90.1(A), I try not to 'design to' the NEC but I do make sure that my designs comply with it.