winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Just a bit of summarizing of other posts.
To answer your original question: It _may_ be possible to put a suitable fuse into your disconnect to properly protect the existing 800A panel. This is what is known as a 'series rating', and cannot be calculated, you need the manufacturer's blessing.
As others note, in their experience the series rating starts with a panel that has a 22 KAIC rating and the series rating goes up from there, meaning you may be out of luck with your 14KAIC panel.
As Jraef notes, during a fault motors can suddenly act as generators and feed energy into the fault. So if you want to use a series rating there is a rather strict limit on the total motor load connected to you system.
Electric-Light brings up an important point. The 'infinite buss' calculation is a quick and easy calculation to do, but it gives you a worst case result. The 'infinite buss' assumes that there is no restriction to current flow on the primary side of the transformer, and give you the fault current right at the transformer terminals.
But in the real world there will be some impedance in the MV lines feeding the transformer, some impedance at the substation feeding those lines, and some impedance in the wires from the transformer terminals to the panel. Prior to jumping through any other hoops, a more accurate calculation _may_ be in order.
This is not a no-brainer, however; depending on the location it may be quite difficult/expensive to get the primary fault current information. On the other hand, if you have 500 feet between the transformer/fuses and the panel, your secondary conductors may be sufficient to limit fault current.
-Jon
To answer your original question: It _may_ be possible to put a suitable fuse into your disconnect to properly protect the existing 800A panel. This is what is known as a 'series rating', and cannot be calculated, you need the manufacturer's blessing.
As others note, in their experience the series rating starts with a panel that has a 22 KAIC rating and the series rating goes up from there, meaning you may be out of luck with your 14KAIC panel.
As Jraef notes, during a fault motors can suddenly act as generators and feed energy into the fault. So if you want to use a series rating there is a rather strict limit on the total motor load connected to you system.
Electric-Light brings up an important point. The 'infinite buss' calculation is a quick and easy calculation to do, but it gives you a worst case result. The 'infinite buss' assumes that there is no restriction to current flow on the primary side of the transformer, and give you the fault current right at the transformer terminals.
But in the real world there will be some impedance in the MV lines feeding the transformer, some impedance at the substation feeding those lines, and some impedance in the wires from the transformer terminals to the panel. Prior to jumping through any other hoops, a more accurate calculation _may_ be in order.
This is not a no-brainer, however; depending on the location it may be quite difficult/expensive to get the primary fault current information. On the other hand, if you have 500 feet between the transformer/fuses and the panel, your secondary conductors may be sufficient to limit fault current.
-Jon
