For this example, we'll use a 3-phase transformer rated at 500kVA, 12470V Primary to 480/277V Secondary.
The first step is to calculate the primary and secondary amps.
There are a number of shortcuts you can use to get you there, but I prefer to break the problem into single-phase pieces and calculate in steps.
1) 500kVA / 3.0 = 166.7 kVA per phase.
2) 12470V L-L / sqrt(3) = 7200V L-N
note: even if the primary is Delta, this approximation still works.
3) 166700 VA / 7200V = 23.2 Amps on the Primary
4) 166700 VA / 277V = 602 Amps on the Secondary
The second step is to pick out a primary fuse.
As a rule of thumb, pick out an "E" rated (Also known as a "backup" fuse) that has a basic ampacity of 150% of the primary current. Where 150% of the primary current doesn't correspond to a standard fuse size, pick the next size up.
In this example, we'll work with the Edison/Cooper brand.
Google "edison e-rated medium voltage 15.5 kV fuses" and open the first PDF that comes up in the search results.
Under the 15.5kV class fuses, we would choose the 40E model MV155F1DBX40E
The 40E simply means it's a 40A rated fuse, with an E-Class curve.
Refer to NEMA for more information on what the "E" means - it's a pretty loose definition and you'll find that different manufacturers E-rated fused can vary quite significantly.
So... the next thing we need to do is check that the transformer inrush current can pass without blowing the fuse.
Look at the chart on the left of page 3 of the above Edison/Cooper PDF.
Now a conservative approach to determining transformer inrush is to simply take the primary current and multiply it by 12, and plot that on the chart at 0.1 second.
On that chart, we see that 278A @ 0.1s is well below the 40E curve. In fact, it just fits below the 25E minimum-melt curve.
If we wanted to minimise the arc-flash hazard on the secondary side, using this model of fuse, we may want to select the 25E for the fastest possible clearing time that still allows the inrush current to pass. The 30E or 40E would also be acceptable, especially if you have large downstream motors or other loads with large inrush currents.
Next, select a circuit breaker for the secondary side.
When choosing a breaker, check carefully the % rating. Breakers <1000A are often "80% rated" meaning, for example, a 100A breaker can only be relied upon for 80A of continuous load current.
A 100% breaker, on the other hand, means just that.
In this case, the easiest choice is a 600A breaker that is 100% rated.
Alternatively, 600/0.8 = 750, you could choose a 750A breaker that is 80% rated, although an 800A breaker is probably the nearest standard size available.
The first step is to calculate the primary and secondary amps.
There are a number of shortcuts you can use to get you there, but I prefer to break the problem into single-phase pieces and calculate in steps.
1) 500kVA / 3.0 = 166.7 kVA per phase.
2) 12470V L-L / sqrt(3) = 7200V L-N
note: even if the primary is Delta, this approximation still works.
3) 166700 VA / 7200V = 23.2 Amps on the Primary
4) 166700 VA / 277V = 602 Amps on the Secondary
The second step is to pick out a primary fuse.
As a rule of thumb, pick out an "E" rated (Also known as a "backup" fuse) that has a basic ampacity of 150% of the primary current. Where 150% of the primary current doesn't correspond to a standard fuse size, pick the next size up.
In this example, we'll work with the Edison/Cooper brand.
Google "edison e-rated medium voltage 15.5 kV fuses" and open the first PDF that comes up in the search results.
Under the 15.5kV class fuses, we would choose the 40E model MV155F1DBX40E
The 40E simply means it's a 40A rated fuse, with an E-Class curve.
Refer to NEMA for more information on what the "E" means - it's a pretty loose definition and you'll find that different manufacturers E-rated fused can vary quite significantly.
So... the next thing we need to do is check that the transformer inrush current can pass without blowing the fuse.
Look at the chart on the left of page 3 of the above Edison/Cooper PDF.
Now a conservative approach to determining transformer inrush is to simply take the primary current and multiply it by 12, and plot that on the chart at 0.1 second.
On that chart, we see that 278A @ 0.1s is well below the 40E curve. In fact, it just fits below the 25E minimum-melt curve.
If we wanted to minimise the arc-flash hazard on the secondary side, using this model of fuse, we may want to select the 25E for the fastest possible clearing time that still allows the inrush current to pass. The 30E or 40E would also be acceptable, especially if you have large downstream motors or other loads with large inrush currents.
Next, select a circuit breaker for the secondary side.
When choosing a breaker, check carefully the % rating. Breakers <1000A are often "80% rated" meaning, for example, a 100A breaker can only be relied upon for 80A of continuous load current.
A 100% breaker, on the other hand, means just that.
In this case, the easiest choice is a 600A breaker that is 100% rated.
Alternatively, 600/0.8 = 750, you could choose a 750A breaker that is 80% rated, although an 800A breaker is probably the nearest standard size available.
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