So I want to install a 480/277V 100A main breaker subpanel from a 480V buss duct. I figure a 75kva transformer is appropriate for this. Using Ohms law, I get about 90A rated current for a 75kva at 480. So 90 x 1.25 = 112. So I will fuse the primary and secondary both at 100A. Sound about right?
Transformer size / ocpd?
- Thread starter jes25
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Assuming you are going from a 480 delta to a 480/277 wye.
Your 100 amp sub-panel would be fine... some folks might go 125.
A 100 amp OCP on your buss might cause you a problem due to inrush current.
With a 100 amp secondary, you primary could go as high as 225 amp.
In any event be sure to follow the Art 250 rules as well as Art 450.
Your 100 amp sub-panel would be fine... some folks might go 125.
A 100 amp OCP on your buss might cause you a problem due to inrush current.
With a 100 amp secondary, you primary could go as high as 225 amp.
In any event be sure to follow the Art 250 rules as well as Art 450.
iceworm
Curmudgeon still using printed IEEE Color Books
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Slow poster - Following Augie's reasoning
I would go with:
Primary - 125A minimum to 225A max. Primary conductor ampacity to match or exceed primary OCP. Going lower than 125A leaves one open to spurious trips on power up - something that would really piss off my customers.
Secondary - 100A is fine, 125A is also fine. But you said the panel was "100A main breaker subpanel". Why would you separately fuse the secondary?
ice
Thanks for the replies guys. I haven't had to do this since school, so it's very helpful. So perhaps the best (least expensive anyhow) design is to put in a 100A main lug panel with 125A primary OCPD and no secondary protection. This panel will have only a few lights on it, but the customer asked for a 100A panel.
Is there a limit as to how far I can run the secondary without OCPD?
Yes I planned to order the transformer you cited. I'm not sure how the transformer feeding the buss duct is configured, but it has no neutral in the duct. How the transformer feeding the duct is configured as in delta or wye makes no difference. Correct?
I could procure a 480Y / 480Y as well and it would work correctly still? I realize this would be an unusual transformer.
iceworm
Curmudgeon still using printed IEEE Color Books
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Jes -
You need some closer help than our keyboards. And that's okay - there is no sin in not having experience. You are doing the Engineering. And that is also okay - it's not rocket science. But you are the "engineer of record". Keep that in mind.
The xfm and panel are 3ph. The transformer dows not necessarily have to have secondary protecton. The panel has to have a main - protected at the rating of the panel - 100A panel, requires 100A (or smaller) OCP. The secondary conductors have to have OCP. Go look up the nec section on panelboards. Go look at the 240 section on xfm secondaries. The secondary distance from the xfm to the OCP is dependent on a bunch of stuff.
You want to get a 480D/480Y xfm. Y-Y xfm are hard to handle.
ice
iceworm
Curmudgeon still using printed IEEE Color Books
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Jes -
One other thing to watch for. Ocassionally an xfm is speced as "totally enclosed" as opposed as "ventilated".
If so, the inrush for "totally enclosed" will be a lot higher than for a "ventilated". The totally inclosed requires a much larger primary CB to prevent nuisance trips. Doesn't happen often, but has happened to me often enough that I check. Nuisance trips are really embarasing.
ice
One other thing to watch for. Ocassionally an xfm is speced as "totally enclosed" as opposed as "ventilated".
If so, the inrush for "totally enclosed" will be a lot higher than for a "ventilated". The totally inclosed requires a much larger primary CB to prevent nuisance trips. Doesn't happen often, but has happened to me often enough that I check. Nuisance trips are really embarasing.
ice
petersonra
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I am a fan of making the primary OCPD protection as large as permitted, even if it makes my wires bigger.
I second that emotion.
A 75kva 480d-208y/120 has been one of my most problematic issues that I have have as a former sales and applications engineer.
All to often those in the field are backed into a corner when the panel that they intend to feed the transformer with is limited to 100a frame breakers max. The spike of inrush current is the issue that can cause nuisance tripping of the PRI. breaker at the point which the transformer is energized. The minimum inrush of a standard efficiency 150degC, 75kva transformer is about 10x the PRI. FFLA which is 900a. As such a 100a breaker which has a 1000a max mag. trip would be marginal. With 115 and 80degC, k factor transformers the inrush may increase anywhere from 20-50%.
As petersonra stated the higher the OCPD the better.
Remember that the secondary OCPD protects the transformer from being overloaded, the PRI. OCPD protects the upstream distribution system from a transformer failure due to an internal fault. The pri. OCPD really has little to do with protecting the transformer at all. As such try to use as much of that 250% as you can. The only other consideration is a question of protecting the cable between the PRI. OCPD and the transformer.
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On the other hand, if your OCPD size is limited, making your wires bigger will just make the inrush worse.
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