I will be replacing 4 fuse boxes in a 3 unit residential building with 60 amp CB panels. A central boiler provides the heat and the hot water tank is gas. The property is served 120/249 volts from an underground utility system through a 1 ½ inch conduit. When the utility undergrounded the area several years ago they installed a 480Y/277 volt 3 phase distribution system. They also installed 120/240 volt transformers in their vaults to pick up the existing 120/240 volt services. The utility requires that any new or rewired services be 480 volts so that they can eventually remove the 120/240 volt transformers. The utility proposed that I install a concrete pad in my back yard and they would install a 480/120/240 volt transformer on the pad. I would pull code sized conductors from the transformer to the new switchgear. Unfortunately I do not have room for the transformer because of mandated parking spaces. I requested a 100 amp 480 volt single phase service and my request was granted. The utility will disconnect the existing 120/240 volt service in their vault and reconnect it at 480/277 volt after I have installed the meter base on the existing 1 ½ inch conduit riser. The available fault current is 33,000. My plan is to install a 50 KVA 480/120/240 volt dry type transformer in the basement of the building. The equipment layout will be: Utility – Meter – 100 amp safety switch – transformer – 200 amp main lug panel with 4-60 amp 2 pole breakers – 4- 60 amp main lug panels. Cabling will be: 3#2 CU THHN from the meter base to the safety switch and from the safety switch to the transformer. 2-3/0 and 1-2/0 CU THHN from the transformer to the 200 amp panel, and 3#6 CU THHN from the 200 amp panel to each 60 amp panel. Grounding: The 100 amp safety switch is the grounding point. 1#6 BC from 2 ground rods, the water and gas pipes, the transformer and each 60 amp panel. 1#4 BC from the 200 amp panel to the safety switch. If any of you guys would care to make comments it would be much appreciated.
One thing to think about with the drytype is it's rated temperature rise. There are typically three ratings: 80°C, 115° and 150°C. The higher the rating—the hotter it runs under load, with some electrical energy turned into heat.
I wouldn't hesitate to contact the transformer manufacturer’s' rep and ask them to weigh the advantages and costs of the three ratings. An initially higher-cost, lower temperature rise transformer may be cost effective over its expected life in less heat generated in the transformer room, and potential energy savings. Plan on clearances around the transformer to be no less that specified on the transformer label. Higher-rise units may require increased clearances—remember these instructions are part of the device’s NRTL listing. [It is surprising the amount of heat a ventilated dry-type transformer can produce.] If in a year’s time more electricity is used for air conditioning than heating, the lower rise unit can have an additional advantage. Aside from temperature rise is the rating of the transformer insulation system. 220°C-rated insulation is desirable and fairly standard.
Most transformer termination kits come in two styles—compression and setscrew type. With normal vibration and heat cycling, the compression type are better if you have or can borrow the right tooling.
Several above- and below-normal 2½% voltage taps are typically furnished with a 50kVA unit.
Ask too about the noise from the drytype in the intended location with respect to the building occupants. Unlike temperature rise, noise levels do not change significantly with load.
#84790 - 05/07/0308:41 AMRe: No kidding! 480 volt res. service
In 1995 a city councilwoman told the utility they had to provide 200 amps of 120/240 volts to any customer who requested it. She was voted out of office in 1996 and the utility never followed through with her demand. The economic vitality of the area has deteriorated over the years. The city is spending millions upgrading the main arterial through the area with new, wider sidewalks. Any small shop that rewires is required to provide a transformer for 120/240. Renters cannot afford it and the result is fires waiting to happen. The area is Seattle's University district near the UofW.