could someone help me with how to find out the fault current rating for my overcurrent protection devices? I am mounting a 400amp MDP with branch circuits about 5 feet away from the transformer. I need to order my main and the and branch circuit ocpds. How do I do my calculation for this in order to get the correct OCPDs with the right fault current rating on them. Thank you
Fault current rating for OCPDs
- Thread starter JdoubleU
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qcroanoke
Sometimes I don't know if I'm the boxer or the bag
- Location
- Roanoke, VA.
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- Sorta retired........
If it is the utility's transformer ask them.
If it's an in-house transformer, you need the nameplate data including impedance.
If you have that one of us can assist to a degree.
(In some cases engineering input is needed due to motor contribution and other factors)
If you have that one of us can assist to a degree.
(In some cases engineering input is needed due to motor contribution and other factors)
Your fault current is the total available (possible) short circuit current. It is line voltage divided by transformer and cable impedance, assuming you are not feeding any motors; if you are, you also have to account for motors working as generators. You can give us the numbers and we'll tell you what your kAIC is. It'll be a rough estimate.
kingpb
Senior Member
- Location
- SE USA as far as you can go
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- Engineer, Registered
If you want to learn how to do it - Do Google search for a paper about MVA short circuit method.
If you want somebody to tell you, then provide data on this forum; I guarantee you will get a whole bunch of answers, many of which will convolute the hell out of the original question :lol:, and probably leaving yourself scratching your head and wondering why dear god, why! :roll:
If you want somebody to tell you, then provide data on this forum; I guarantee you will get a whole bunch of answers, many of which will convolute the hell out of the original question :lol:, and probably leaving yourself scratching your head and wondering why dear god, why! :roll:
But we will also tell him how to install the transformer (or whats wrong with the install) plus discuss the inspectors in that area and the effect EMF has on the facility...
Carultch
Senior Member
- Location
- Massachusetts
You can calculate the fault current at the transformer secondary with this spreadsheet:
http://www.mikeholt.com/freestuff-shortcircuitcalculations.php
You'll need to know:
1. the transformer KVA
2. the secondary voltage and grid type
3. the transformer impedance percentage
4. power factor of the loads. Note: a closer to 100% power factor means less fault current, so make sure you pick the smallest number that can well-represent your loads.
This spreadsheet also helps you follow the fault current down each of your feeders, based on their lengths and wiring configuration.
It is common that overcurrent devices are series rated. Check with the manufacturer to see whether or not this is the case. If so, you can have load side breakers at low fault current capacity, while your first breakers are of the highest fault current capacity. The essential requirement is that the first breaker trips before there is a possibility of failing the downstream breakers.
One trick you can often use, is to use fuses in order to withstand higher fault currents, and reduce it on their load side. You also run in to coordination issues, in that the fuses have to blow before the downstream breakers can fail.
To keep it simple, calculate for the worst scenario that your imagined system will be loaded with and the highest fault current that the OCPDs will be subjected to. Multiply by some fudge factors, that will be your fault current rating (the OCPD rating should be greater than the max fault available at the point of installation.)
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
Calculate the full load current of the transformer. Divide that by the impedance of the transformer, and that's the rating you're looking for.
So if it's a 45kVA, 208 V sec'y,: 45000/1.732/208 = 125 A
Transformer impedance =3.35%
Minimum SC rating = 125/.0335 = 3728 A
Use 10kA rated OCPDs
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