Transformer sizing

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lpelectric

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I have calculated 4 offices and 3 apartments to come up to 596.7 amperes 120/208 3 phase. A 225 KVA transformer will give 624 full load amps at 208. The next size up that I know of is a 300 KVA which will give 832 amps full load. Am I pushing it too close to allow the 225KVA? :smile:

I'm going to require the 300 KVA, but could I permit the 225 and be within code?
 
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bsh

Senior Member
Did you use NEC Article 220 to come up with the 596.7 amp load or is that total connected load? If it is based on Art 220 then you are probably too close to the 225 KVA transformer capacity. The 225 won't give you much future capacity if you are at 96% of the transformer capacity
 

lpelectric

Senior Member
bsh said:
Did you use NEC Article 220 to come up with the 596.7 amp load or is that total connected load? If it is based on Art 220 then you are probably too close to the 225 KVA transformer capacity. The 225 won't give you much future capacity if you are at 96% of the transformer capacity

I used Article 220 (and Appendix D). I checked the guy's figures and he's under what a 225 KVA full load transformer is and he want's to use it, but I want him to use the next size up. I was wondering if I could compel him to go higher.

*since I posted this question, I've been told there will be an additional building planned to be off the transformer, so it looks like we'll have to figure it all over again. :smile:
 

vicdelta

Member
Transformer Capacity

Transformer Capacity

Look at 220-3,220-11,220-18,220-19 & 220-32 for service load calculation,make sure demand factors are applied correctly.Its a good design practice to add 20% more capacity on transformer rating for future loads.But as you know Utility Companies design the service capacity based on Building Demand load you submit for Service request.
 

lpelectric

Senior Member
vicdelta said:
Look at 220-3,220-11,220-18,220-19 & 220-32 for service load calculation,make sure demand factors are applied correctly.Its a good design practice to add 20% more capacity on transformer rating for future loads.But as you know Utility Companies design the service capacity based on Building Demand load you submit for Service request.

Thanks. Very helpful.:smile:
 

kingpb

Senior Member
Location
SE USA as far as you can go
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Engineer, Registered
lpelectric said:
I have calculated 4 offices and 3 apartments to come up to 596.7 amperes 120/208 3 phase. A 225 KVA transformer will give 624 full load amps at 208. The next size up that I know of is a 300 KVA which will give 832 amps full load. Am I pushing it too close to allow the 225KVA? :smile:

I'm going to require the 300 KVA, but could I permit the 225 and be within code?

Your post is confusing me since a 120/208V supply is a modification of a three phase 4 wire system utilizing a single phase three wire open wye configuration. At least according to ANSI/IEEE C84.1.

Therefore, "120/208V 3 phase" is ambiguous.

Perhaps you could clarify?:confused:
 

charlie b

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kingpb said:
Therefore, "120/208V 3 phase" is ambiguous.
Not to me it isn't. Nor, I believe, to most members of our industry. I don't have that ANSI/IEEE standard handy at the moment, so I can't look up what you are describing. Are you saying that that standard is saying that there is a difference between "120/208" and "208/120"? I think both are equivalent ways of saying three phase, four wire, wye.
 

charlie b

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lpelectric said:
I'm going to require the 300 KVA, but could I permit the 225 and be within code?
This question has apparently become irrelevant, given your new information about additional loads. But I think it is worth a reply anyway.

You're going to require . . . ? Please don't.

Could you permit . . . ? Yes you could.

It is not within the Inspector's perogative to want more than the code requires. If the calculated load is within the rating of the transformer, then the code allows use of that transformer. If you are asking whether there is a "don't load beyond 80%" type of requirement for transformers, there isn't. The process of calculating load includes provisions for adding 25% where needed (e.g., largest motor, continuous loads). The process does not include adding extra capacity for future growth or to get the calculated load further from the available capacity.
 

kingpb

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charlie b said:

Not to me it isn't. Nor, I believe, to most members of our industry. I don't have that ANSI/IEEE standard handy at the moment, so I can't look up what you are describing. Are you saying that that standard is saying that there is a difference between "120/208" and "208/120"? I think both are equivalent ways of saying three phase, four wire, wye.

120/208Y volts is not the same as 208Y/120 volts. Just as 120/240 volts is not the same as 240/120 volts.

These voltages are commonly mis-stated along with 480Y/277V, although 277/480V is not as ambiguous since there is not a readily apparent alternative.

According to C84.1 note a to Table 1:

View attachment 890

and note d:
View attachment 891

I do not know the history on when/why these voltages became so acceptably mis-stated. I do know there are advantages of using the correct designations, as it is not necessary to use e.g. 3ph, 4 wire etc. behind the voltage on drawings. Simply stating 208Y/120V is all that is necessary since it means 208V L-L, 120V L-N, and it is a 3 phase 4 wire system.

Unfortunately, in the NEC handbook they mix the voltages as in Article 100
pag 31.

The industry will not change overnite, all I can do is try and make people aware.
 

charlie b

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You may have the "right answer" or the "book answer," but I doubt it is going to overcome the "common answer." I will treat 120/208, 120Y/208, 120/208Y, 120Y/208Y, 208Y/120Y, 208Y/120, 208/120Y, and 208/120 as being synonymous, even though I seldom see more than two of these. I suspect that most of the industry will do the same.

kingpb said:
The industry will not change overnight, all I can do is try and make people aware.
Best of luck with that one. I think you will not succeed until after I succeed in convincing the world that the distance between the bottom of a newborn baby?s feet and the top of his or her head is the child?s ?height,? not ?length.? Care to pool our PAC money on these two causes? :D
 

steve66

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Illinois
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Engineer
charlie b said:
This question has apparently become irrelevant, given your new information about additional loads. But I think it is worth a reply anyway.

You're going to require . . . ? Please don't.

Could you permit . . . ? Yes you could.

It is not within the Inspector's perogative to want more than the code requires. If the calculated load is within the rating of the transformer, then the code allows use of that transformer. If you are asking whether there is a "don't load beyond 80%" type of requirement for transformers, there isn't. The process of calculating load includes provisions for adding 25% where needed (e.g., largest motor, continuous loads). The process does not include adding extra capacity for future growth or to get the calculated load further from the available capacity.

I agree 100% with Charlie. And if he hadn't mentioned it, I wouldn't have realized you were an inspector. When I'm playing the role of "inspector", I don't do the load calculations. I have the project engineer do the calculations and sign off on them. Then I review them to make sure they look reasonable, but I don't go through them with a fine tooth comb. (I know in theory, two different engineers should come up with the exact same load calculation for the same loads. But in practice, I don't think two different people would every get the same answer.)

And if it makes you feel any better about the size of the transformer being so close to the size of the load, NEC calculations are usually very conservative. If the POCO installs the transformer, they usually size it at about half the calculated load.

We just measured the load on a building I had designed. After the initial building was constructed, we did additions and added equipment until the load calculation was at the brink of the service's capacity. But the measured load was only 250 amps on a 900 amp service. I don't know if that is typical, but most buildings don't load the service to their calculated capacity.

Steve
 

infinity

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charlie b said:
You may have the "right answer" or the "book answer," but I doubt it is going to overcome the "common answer." I will treat 120/208, 120Y/208, 120/208Y, 120Y/208Y, 208Y/120Y, 208Y/120, 208/120Y, and 208/120 as being synonymous, even though I seldom see more than two of these. I suspect that most of the industry will do the same.




I would do the same. Upon reading the question I understood that this was the secondary voltage supplied by the 225 or 300 KVA transformer in question.
 

kingpb

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Location
SE USA as far as you can go
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Engineer, Registered
It's ironic that people in this forum professing to follow the letter of the code, and dissect every word so they can exhume the intended interpretation, are also unwilling to accept the black and white text that needs no interpretation; found in the IEEE or ANSI standards. Which, BTW, dictates most of what is found in the illustrious NFPA 70. Standards are developed so that we can all talk the same language.

208Y/120V is a technically different system then 120/208Y volt system. Just because people have unknowingly used voltage designations incorrectly does not now make them correct. It would seem irresponsible for anyone to knowingly use the designations incorrectly.

Charlie, as a professional engineer, I am frankly taken aback by your flippant response. I would think you would be more supportive of the standards seeing how you have published papers in IEEE Transactions.
 

lpelectric

Senior Member
charlie b said:
This question has apparently become irrelevant, given your new information about additional loads. But I think it is worth a reply anyway.

You're going to require . . . ? Please don't.

Could you permit . . . ? Yes you could.

It is not within the Inspector's perogative to want more than the code requires. If the calculated load is within the rating of the transformer, then the code allows use of that transformer. If you are asking whether there is a "don't load beyond 80%" type of requirement for transformers, there isn't. The process of calculating load includes provisions for adding 25% where needed (e.g., largest motor, continuous loads). The process does not include adding extra capacity for future growth or to get the calculated load further from the available capacity.

Points well delivered and well taken, Charlie. I think I got above my "raising" there. I do not have the authority to waive the code (and most of the time I wonder if I have the ability to understand it)..:smile:
 
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