Transformer Sizing

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tpd

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The engineer purchased a 3 phase 45kva transformer with the idea it can be used for future expansion if needed. Problem is he did not get a distribution panel-board. So it is going to feed a 100 amp panel only at this time. If this transformer is properly sized for it's capacity then the secondary conductors will not physically fit in the 100 amp lugs! This is primary protected only. So the question is would it be acceptable to use an undersized breaker (calls for a 70 amp) on the primary so a smaller conductor can be used for the 100 amp panel. Then document that the secondary will need upgraded if it ever becomes necessary to expand. The primary conductors are already installed for the 70 amp protection. This is an industrial installation and I was asked what can be done after they pulled in the conductors and realized the problem. Thanks
 
tpd said:
The engineer purchased a 3 phase 45kva transformer with the idea it can be used for future expansion if needed. Problem is he did not get a distribution panel-board. So it is going to feed a 100 amp panel only at this time. If this transformer is properly sized for it's capacity then the secondary conductors will not physically fit in the 100 amp lugs! This is primary protected only. So the question is would it be acceptable to use an undersized breaker (calls for a 70 amp) on the primary so a smaller conductor can be used for the 100 amp panel. Then document that the secondary will need upgraded if it ever becomes necessary to expand. The primary conductors are already installed for the 70 amp protection. This is an industrial installation and I was asked what can be done after they pulled in the conductors and realized the problem. Thanks
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You can always make the OCPD smaller than the maximum the code allows.

what voltage is the transformer operating at?
 
tpd said:
It is a 480 delta to 208/120 wye.
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Are secondary conductors pulled?

Does the 100A panel have a 100A main breaker?

You can run multiple sets of secondary conductors, each set to a single OCPD rated to protect the secondary conductor set, but you cannot run one large set of secondary conductors and make several taps to it.
 
Smart $ said:
Are secondary conductors pulled?

Does the 100A panel have a 100A main breaker?

You can run multiple sets of secondary conductors, each set to a single OCPD rated to protect the secondary conductor set, but you cannot run one large set of secondary conductors and make several taps to it.
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They haven't ran the secondary conductors yet. The 100 amp panel does have a main breaker and the lugs on it are way to small for the large secondary conductors originally planned for this installation.
So the concern was that if they ran the conductors sized for the 100 amp panel - #2 I believe. And left the 70 amp OCD in the primary side then the current capability on the secondary would far exceed the 100 amp.
 
tpd said:
They haven't ran the secondary conductors yet. The 100 amp panel does have a main breaker and the lugs on it are way to small for the large secondary conductors originally planned for this installation.
So the concern was that if they ran the conductors sized for the 100 amp panel - #2 I believe. And left the 70 amp OCD in the primary side then the current capability on the secondary would far exceed the 100 amp.
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Nominal operating current on the secondary is controlled by the load, and limited in fault condition by the overcurrent protection. As long as neither of those exceed the ampacity of the secondary conductors [beyond permitted], you'll be good to go.
 
tpd said:
The engineer purchased a 3 phase 45kva transformer with the idea it can be used for future expansion if needed. Problem is he did not get a distribution panel-board. So it is going to feed a 100 amp panel only at this time. If this transformer is properly sized for it's capacity then the secondary conductors will not physically fit in the 100 amp lugs! This is primary protected only. So the question is would it be acceptable to use an undersized breaker (calls for a 70 amp) on the primary so a smaller conductor can be used for the 100 amp panel. Then document that the secondary will need upgraded if it ever becomes necessary to expand. The primary conductors are already installed for the 70 amp protection. This is an industrial installation and I was asked what can be done after they pulled in the conductors and realized the problem. Thanks
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Just wondering how long are the Conductors on the 208/120 secondary
 
petersonra said:
I don't believe you are allowed to protect the secondary or its conductors with the primary OCPD in such a case.
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Transformer secondary protection is only required if primary protection exceeds the maximum primary-only protection rating factor of 450.3.

Secondary conductors are protected by the single ocpd at the load end, provided the conductor ampacity and ocpd rating comply with 240.12(C). That is why I asked if the panel had main ocpd.
 
tpd said:
The engineer purchased a 3 phase 45kva transformer with the idea it can be used for future expansion if needed. Problem is he did not get a distribution panel-board. So it is going to feed a 100 amp panel only at this time. If this transformer is properly sized for it's capacity then the secondary conductors will not physically fit in the 100 amp lugs! This is primary protected only. So the question is would it be acceptable to use an undersized breaker (calls for a 70 amp) on the primary so a smaller conductor can be used for the 100 amp panel. Then document that the secondary will need upgraded if it ever becomes necessary to expand. The primary conductors are already installed for the 70 amp protection. This is an industrial installation and I was asked what can be done after they pulled in the conductors and realized the problem. Thanks
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You later said this secondary was 208/120 volts - so at 45 kVA it's rated output current would be 125 amp. What kind of 100 amp panel do you have that won't accept a 125 amp conductor? Most would accept up to a 2/0 conductor at the supply terminals whether they are main lugs or a main breaker, some even a larger conductor.
 
ron said:
240.21(C)(1) supports your statement, at least about the secondary conductors.
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there was a later statement that suggested there was a 100A main on the secondary PB which probably could serve to protect the secondary conductors if it was close enough.

to me if the only problem is that the wires are too big, the easy solution is to splice on smaller conductors.
 
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