buffalonymann
Senior Member
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- NC
I need to specify a circuit breaker for the primary side of transformer - using bussmann fault calculator but not sure which values I should use
Fault Current
- Thread starter buffalonymann
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buffalonymann
Senior Member
- Location
- NC
I don't have the one line available so I don't know the size or impedance of the transformer to the MDP. From MPD 800 AMP 480 CB 100k rated travels about 140 ft to another 800 amp breaker 30k rated with parallel 500s. I need to install 200 amp breaker for line side of 225kva xfmr on the load side of the second 800 amp breaker
As a conservative measure, select a breaking capacity for 200 amp breaker equal to that of the second 800 amp breaker.
- Location
- Connecticut
- Occupation
- Engineer
A 200A breaker seems awfully small for the primary of a 225kVA transformer.
Without know the available fault current supplying MDP I don't see how you can obtain an accurate number for the downstream breakers. Using arbitrary numbers, the 140 ft of a 800 amp run run reduce the fault current about 25k.
(I also agree with david in that a 200 amp breaker is normally small for a 225kva 480v transformer)
(I also agree with david in that a 200 amp breaker is normally small for a 225kva 480v transformer)
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kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
Std impedance is going to be around 4.8%, for a 225KVA transformer with std tolerance of +/-7.5%. For Short circuit it's the -7.5% that is used.
Thus for 225KVA @ 4.46%; Max thru fault SC is roughly 5.045MVAsc. Using those numbers can get you in the ballpark for rating requirements at transformer.
Thus for 225KVA @ 4.46%; Max thru fault SC is roughly 5.045MVAsc. Using those numbers can get you in the ballpark for rating requirements at transformer.
I may have misread but I think he is questioning devices on the line side
A single-line diagram could help, IMO.
- Location
- Connecticut
- Occupation
- Engineer
That is what I read, as well.
buffalonymann
Senior Member
- Location
- NC
It is by design; there is no secondary OCPD and I need to limit the secondary current
mayanees
Senior Member
- Location
- Westminster, MD
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- Electrical Engineer and Master Electrician
SC rating for 200-amp breaker
SC rating for 200-amp breaker
To determine the necessary kA rating for the 200-amp primary breaker, look upstream to the transformer that is feeding the panel that houses it. The worst-case short circuit rating for the breaker will be the transformer kVA/voltage/1.732/impedance (Z). So for a 480V, 500 kVA unit, 500/480/1.732/0.05 for a 5% Z = 12.028kA is the highest required short circuit rating. You could also just match the rating for the breakers in the panel, but that assumes that the original installation is correct.
A more precise calculation would include cable lengths and the reported utility contribution. If you need help with that then post the information (in one-line diagram sketch) and I'll run the SKM analysis, or someone will beat me to it.
Peace, and run.
John M
SC rating for 200-amp breaker
To determine the necessary kA rating for the 200-amp primary breaker, look upstream to the transformer that is feeding the panel that houses it. The worst-case short circuit rating for the breaker will be the transformer kVA/voltage/1.732/impedance (Z). So for a 480V, 500 kVA unit, 500/480/1.732/0.05 for a 5% Z = 12.028kA is the highest required short circuit rating. You could also just match the rating for the breakers in the panel, but that assumes that the original installation is correct.
A more precise calculation would include cable lengths and the reported utility contribution. If you need help with that then post the information (in one-line diagram sketch) and I'll run the SKM analysis, or someone will beat me to it.
Peace, and run.
John M
electrofelon
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- Cherry Valley NY, Seattle, WA
Why do you need to limit the SC current at the secondary? It already is quite low.
- Location
- Connecticut
- Occupation
- Engineer
What's the secondary voltage and what is it supplying?
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
buffalony,
I just saw this explanation of the circuit and write to append my post.
Your 200-amp breaker should match the 30kA rating for the 800-amp panel main. That's the easy answer. If you're trying to reduce the rating to something less than 30kA, you could look to use NEC 240.86 Series Ratings and find a breaker that's listed with the model of the 800-amp, 30 kA rated breaker to get you to 30kA.
buffalonymann
Senior Member
- Location
- NC
We are a plastics extrusion company; extruders have long barrels with resistance heaters attached. We use 480/240 delta-delta XFMR to supply single phase heaters. They installed parallel secondary conductors to distribution blocks to power the 3 and 4 kw heaters. They installed a 300 amp breaker on the primary. I am eliminating one of the secondary sets of conductors and so I need to limit the primary current to limit the current on the now single secondary conductors.
buffalonymann
Senior Member
- Location
- NC
I don't know the 30kA rating is engineered. I have a request in to acquire the one line so I can see the XFMR specs and calculate the FC
The breaker needs to have a rating equal to or higher than the available fault current from the source at the breaker. This is independent of what it feeds or its rating.
- Location
- Connecticut
- Occupation
- Engineer
Without knowing all the details, it sounds like the conductors from the distribution blocks to the heaters is a violation of 240.21.
Also, will the 200A c/b properly protect the now single set of secondary conductors per 240.21(C)(1)?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
This is not what protective devices do.
The Amps Interrupting Current or Rating (AIC or AIR) is the amount of fault current the protective device can interrupt/clear without damaging itself. The AIC rating has to be greater than the amount of fault current available on its load side terminal. So the selection of a transformer primary side protective device involves understanding the system parameters and pretty much has nothing to do with the secondary side at all.
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