3 phase transformer. Dry type out doors and current protection for transformer.

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fastline

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midwest usa
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We have a 75KVA ventilated dry transformer that needs set temporarily. It will supply 2 machines for a bit until separate transformers and load center are installed.

The primary per the 75KVA, will need protected at 90A/480V. Because the secondary side will feed two machines, there is no intention to run both machines at the same time right now. However, I want to ensure the transformer is well protected and I would prefer to have secondary current protection but with two machines connected, that is really not possible without a load center. Is primary protection plenty sufficient?

As well, we will probably need to install this transformer outdoors. We intend to shield it from radiant sun heating and weather with a small roof but want to ensure that is sufficient?

This is an Acme brand floor mount transformer if that helps.

Obviously wiring will be sized appropriate to the loads.
 
fastline said:
We have a 75KVA ventilated dry transformer that needs set temporarily. It will supply 2 machines for a bit until separate transformers and load center are installed.

The primary per the 75KVA, will need protected at 90A/480V. Because the secondary side will feed two machines, there is no intention to run both machines at the same time right now. However, I want to ensure the transformer is well protected and I would prefer to have secondary current protection but with two machines connected, that is really not possible without a load center. Is primary protection plenty sufficient?

As well, we will probably need to install this transformer outdoors. We intend to shield it from radiant sun heating and weather with a small roof but want to ensure that is sufficient?

This is an Acme brand floor mount transformer if that helps.

Obviously wiring will be sized appropriate to the loads.
Click to expand...

The transformer primary can be as high as 125A, and transformer secondary protection would not be required.

If the transformer is outside, the two feeders for the machines can be unlimited length, but must terminate at an ocpd not greater than the ampacity of the conductors. Also, if the machines are inside, the feeders from the secondary need a disconnecting means either outside or inside nearest the point of entrance of the conductors. See 240.21(C)(4).
 
david luchini said:
The transformer primary can be as high as 125A, and transformer secondary protection would not be required.

If the transformer is outside, the two feeders for the machines can be unlimited length, but must terminate at an ocpd not greater than the ampacity of the conductors. Also, if the machines are inside, the feeders from the secondary need a disconnecting means either outside or inside nearest the point of entrance of the conductors. See 240.21(C)(4).
Click to expand...

No secondary OCPD?
 
texie said:
How are the transformer secondary conductors going to be protected without an OCPD?
Click to expand...

david luchini said:
If the transformer is outside, the two feeders for the machines can be unlimited length, but must terminate at an ocpd not greater than the ampacity of the conductors. Also, if the machines are inside, the feeders from the secondary need a disconnecting means either outside or inside nearest the point of entrance of the conductors. See 240.21(C)(4).
Click to expand...

....
 
Yeah, we will have to find either a load center or fused disconnect for each secondary connection to protect the conductors.

Question on sizing though. Each machine has their recommended KVA for them but we also know from experience that the machine will never see that much current and if it did, there would be something radically wrong in which the machine will shut down immediately.

So with that said, I have one machine spec'd at 44KVA, another with 27KVA on their input badge. That is 71KVA and we have a 75KVA transformer. Now, I want to be very clear, each machine has several motors and all of them would have to be on their knees to even approach the KVA on the badge so a fair estimate they will normall run at half of that.

However, each machine has one large milling spindle on a VFD and they start up fast. Nothing like across-the-line but typically about 200% of FLA for acceleration. If both were accelerating at the same time, it would be probable that the KVA could reach as high as 100KVA for 1-2sec periodically.

I am just curious if I am over thinking any of this overload on the transformer and more likely that as long as we are matching up the KVA, it is likely a non-issue? I guess in other words, how is overload handled in a dry transformer? I know oil filled can take a beating as the power company seems to like to drive them at 120% or more!:eek:
 
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