45 KV Transformer Feeding 200 amp panel?

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fellhauer

Member
I have seen many times where a 480 Volt 3 phase service with a 45 kV Step down Transformer feeding a 208 V 3 phase 200 amp panel.

My question is how is calculated possible?

I have had a heated conversation with my boss that 45 kV transformer will hold a 200 amp panel but he is sure that you need a 75 kV transformer for a 200 amp panel.

Please help me prove my point!!! I don't have any information to back my argument.:)
 

rick hart

Senior Member
Location
Dallas Texas
Any reason you think he became a boss? You might try listening to him, especially if you don't have any information of your own. The only resaon a 200A panel is going to be connected is not that it will supply a full 200A. Look closely and I will be wiling to bet that the panel you speak of is fed with 125A conductors or main.
 
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iwire

Moderator
Staff member
Location
Massachusetts
fellhauer said:
I have had a heated conversation with my boss that 45 kV transformer will hold a 200 amp panel but he is sure that you need a 75 kV transformer for a 200 amp panel.

45 * 1000 = 45,000 watts / 208 = 216 / 1.73 = 125 amps of capacity.

Are you sure it's a good idea to supply a 200 amp panel with a transformer that only has 125 amps of capacity?

You could.......if there is a 125 amp breaker between the transformer and the panel and as long as the calculated load is under 125 amps.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
A 45KVA transformer will probably overheat if asked to supply 200A for any length of time.

But how many 200A panels actually have 200A of load running at one time? If you look at the table 450.3(B), in many cases the circuit breaker on the secondary side is permitted to be 200A or larger.

So the important thing is usually the calculated load that the transformer will have to supply, not the capacity of the transformer compared directly with the ampacity of the breaker.

Steve
 

iwire

Moderator
Staff member
Location
Massachusetts
steve66 said:
If you look at the table 450.3(B), in many cases the circuit breaker on the secondary side is permitted to be 200A or larger.

Steve working without a book I seem to recall the largest breaker you could use on the secondary side of a 45 kva would be in the 150 amp ball park.
 

jcormack

Member
Location
Pennsylvania
The type of transformer was not stated. Is it Dry-type, oil-filled, encapsulated? transformers often have multiple KVA ratings, based on how hot you can let them run. Oil-filled typically have 55 deg C and 65 deg C ratings - as well as OA, FA and so on. The standard size pole-mount that the POCO uses here for 200 Amp 240/120 service is a 25 KVA...............And yes I Know the POCO does not use NEC.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
iwire said:
Steve working without a book I seem to recall the largest breaker you could use on the secondary side of a 45 kva would be in the 150 amp ball park.

Bob,
If the primary is properly protected at not more than 125% of FLA, it makes no difference what size breakers are on the secondary. Of, course protection of the secondary conductors is a different matter.
 

rick hart

Senior Member
Location
Dallas Texas
jcormack said:
The type of transformer was not stated. Is it Dry-type, oil-filled, encapsulated? .
The problem is making the evidence fit the claim.
Without specifics that allow the provisions of Article 450 to be ammended, we must assume that this is a general duty transformer serving general loads. Generally speaking- 125A is the answer to the question.
 

iwire

Moderator
Staff member
Location
Massachusetts
jim dungar said:
Bob,
If the primary is properly protected at not more than 125% of FLA, it makes no difference what size breakers are on the secondary. Of, course protection of the secondary conductors is a different matter.

Thanks, I should not work from memory alone.
 

fellhauer

Member
I hate when the boss man is right

I hate when the boss man is right

Thank you

I guess that the engineer who made the drawings for this strip mall must have just been considering that the connected load was under the 125 amp mark and for some reason just protected it at 200 to make myself and a couple of others puzzled. I hate it when hes right! :grin:
 
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