I have an unusual situation that I am trying to correct. I ran across existing condition that is out of the ordinary. ( at least for me) Have a couple questions I would try to get answered before I see an electrical engineer.
An existing job has one 4 inch 40 ft conduit going from the main service disconnect under the building to an existing FPL manhole. From there, it goes 130 feet underground with 4/0 cu to the FPL transformer(s).
It is being fed by a high leg delta transformer arrangement, a 240/120 volt 3 phase transformer arrangement with 240 volts between A, B, & C phases with 120 volts neutral to A & C, B being the high leg with 208 volts to neutral / ground. ( or it will be,,,the far right leg is connected as
per utility configuration and they carried that through to the one panel ( of four ) which is 3 phase, feeding some 3 phase air conditioners. ( 2 - 60 A 3 pole breaker feeds and a few 20-30 amp 2 pole breakers )( The other 3 panels are a single phase ). The high leg is presently breakers 5/6, 9/10 ect on the one of 4 panels (feeds the A/C's.)
The 4 inch conduit presently contains two pairs of 600 mcm thwn-2 for A & C phases which breaks off into 2 meters, each feeding 2 - 225 amp 10k aic breakers, 3 - 2 pole and 1 - 3 pole feeding the air conditioning panel. The electric room is fairly tight, so there is no room to change the 4 breakers to fusible disconnects. The high leg is fed from the 4 inch conduit with
one 500 mcm conductor and the neutral is one 600 MCM conductor. ( They are set up so they do comply with the 10 25 ft tap rule ), except the high leg fuse is 700 amp being fed with the one 500 mcm thwn-2 cu underground conductor and going on to the first meter/breaker combination. ( each meter is fed by 500 mcm conductors tapped off from an overhead gutter which feeds the meter socket, dual lugs with 4/0 cu on the load side of the meter(s) feeding the disconnects.
The 800 amp disconnect presently has 3 - 700 amp KLU fuses, so I plan on changing the fuse on
the high leg to a 250 amp KLU fuse ( or equivalent ). The A & C legs come out of the disconnect bottom to the top gutter in parallel 600 mcm conductors where they tap off in 500 mcm to A & C legs of the meter. All wire is thhn/thwn-2 cu.
I believe I can use Shawmut Class L A4BQ fuses for new fuses which looks like they will comply
with fault current requirements/curves and allow me to utilize the existing 10 K aic breakers, especially since the a/c panel is only 8 ft away from its feeding breaker.
FPL ( local utility ) sent me a fault current letter stating my feed from the transformers is 53,000K, But I was wondering .....
CAN I repull the underground feed to the FPL manhole and replace the 6 wires there now with 2 pairs of 600 mcm thwn-2 fuses for A & C phases, 1 250 mcm for B phase, and 1 700 mcm for a larger neutral. This way I can re-use the existing conduit without cutting open the concrete floor
( venue is presently open ) and the 6 conductors will meet the 80% derate when figured for thwn-2 at 90 C. It will also allow me to meet the calculated neutral and panel loads and also
the fault current requirements. I will turn this over to an electrical engineer once I get these questions answered, but it does bring up some interesting questions and I can go in with a possible solution...
Thanks and open for comment, DHSVCS
An existing job has one 4 inch 40 ft conduit going from the main service disconnect under the building to an existing FPL manhole. From there, it goes 130 feet underground with 4/0 cu to the FPL transformer(s).
It is being fed by a high leg delta transformer arrangement, a 240/120 volt 3 phase transformer arrangement with 240 volts between A, B, & C phases with 120 volts neutral to A & C, B being the high leg with 208 volts to neutral / ground. ( or it will be,,,the far right leg is connected as
per utility configuration and they carried that through to the one panel ( of four ) which is 3 phase, feeding some 3 phase air conditioners. ( 2 - 60 A 3 pole breaker feeds and a few 20-30 amp 2 pole breakers )( The other 3 panels are a single phase ). The high leg is presently breakers 5/6, 9/10 ect on the one of 4 panels (feeds the A/C's.)
The 4 inch conduit presently contains two pairs of 600 mcm thwn-2 for A & C phases which breaks off into 2 meters, each feeding 2 - 225 amp 10k aic breakers, 3 - 2 pole and 1 - 3 pole feeding the air conditioning panel. The electric room is fairly tight, so there is no room to change the 4 breakers to fusible disconnects. The high leg is fed from the 4 inch conduit with
one 500 mcm conductor and the neutral is one 600 MCM conductor. ( They are set up so they do comply with the 10 25 ft tap rule ), except the high leg fuse is 700 amp being fed with the one 500 mcm thwn-2 cu underground conductor and going on to the first meter/breaker combination. ( each meter is fed by 500 mcm conductors tapped off from an overhead gutter which feeds the meter socket, dual lugs with 4/0 cu on the load side of the meter(s) feeding the disconnects.
The 800 amp disconnect presently has 3 - 700 amp KLU fuses, so I plan on changing the fuse on
the high leg to a 250 amp KLU fuse ( or equivalent ). The A & C legs come out of the disconnect bottom to the top gutter in parallel 600 mcm conductors where they tap off in 500 mcm to A & C legs of the meter. All wire is thhn/thwn-2 cu.
I believe I can use Shawmut Class L A4BQ fuses for new fuses which looks like they will comply
with fault current requirements/curves and allow me to utilize the existing 10 K aic breakers, especially since the a/c panel is only 8 ft away from its feeding breaker.
FPL ( local utility ) sent me a fault current letter stating my feed from the transformers is 53,000K, But I was wondering .....
CAN I repull the underground feed to the FPL manhole and replace the 6 wires there now with 2 pairs of 600 mcm thwn-2 fuses for A & C phases, 1 250 mcm for B phase, and 1 700 mcm for a larger neutral. This way I can re-use the existing conduit without cutting open the concrete floor
( venue is presently open ) and the 6 conductors will meet the 80% derate when figured for thwn-2 at 90 C. It will also allow me to meet the calculated neutral and panel loads and also
the fault current requirements. I will turn this over to an electrical engineer once I get these questions answered, but it does bring up some interesting questions and I can go in with a possible solution...
Thanks and open for comment, DHSVCS