Mackinac Island is located at the northwest extremity of Lake Huron, 4 miles east of St. Ignace, Michigan, across the Straits of Mackinac. It is a heavily visited summer resort area. More than 600 people reside on the Island year round. White limestone cliffs topped by cedar trees line the 2,300-acre Island’s 8-1/2 miles of shoreline, that surrounds the forested interior of the Island. Its commercial area has developed from an old fishing and trading village to an area with a variety of shops and large modern hotels.

Edison Sault Electric Company has served customers on Mackinac Island since 1932. As the Island has developed and the demand for electric power has grown over the years, Mackinac Island customers have become a very important part of the businesses and residents served by Edison Sault.

Information about the Mackinac Island service area:

• Edison Sault presently serves approximately 900 customers on the Island during the peak summer season.

• The nature of the electric power requirements of customers on the Island are uncommon for electric utilities to serve. Customer load predominately consists of electric heating and air conditioning. The Grand Hotel alone has three 3,000-ampere, 480-volt services to supply its needs. All-electric, residential homes with 400-ampere to 800-ampere, 120/240-volt services are common. (Typical homes with natural gas heating utilize 100-ampere to 200-ampere services.) Further, the few remaining domestic water heating loads on the Island that are served with propane-fired boilers are gradually being converted to electric loads.

Development on the Island also has driven electric load growth. For example, the recent expansion of the Island’s school added nearly 1 megawatt of load.

• Summer peak power demand for 2000 was in excess of 10 megawatts.

At one time, the power requirements for the Island were supplied by several diesel generator units located on the Island. As the Island’s economy grew and development expanded on the Island, the requirement for electric power continued to rise. Special-purpose, submarine-type electric power cables were installed from St. Ignace to the Island to enable Edison Sault to supply a portion of the Island’s power requirements from the Company’s diesel generator plant located at St. Ignace.

In the mid-1980s, the diesel power plants could no longer efficiently supply the Island’s growing load. The cost of delivering fuel to the Island had become prohibitive. Further, the diesel power plant on the Island increasingly became inconsistent with the unique character of the Island. By 1989, all diesel generators on Mackinac Island and at St. Ignace had been removed from service.

Present Service Facilities. Mackinac Island is presently served by two three-phase, submarine cable circuits (with a seventh submarine cable acting as a spare), operating at 13,200 volts, installed across the Straits of Mackinac in an east-west direction from the mainland at St. Ignace to the Island. Please refer to Appendix 1.1 for an overview of these facilities.

The two submarine circuits use individual cables with 25,000-volt ethylene propylene rubber insulation system and 4/0 copper conductors. (Although 15,000-volt insulation would normally be chosen for cables operating at 13,200-volts, the higher voltage insulation was selected to provide additional operating margin.) The cables are also covered with a stranded steel wire, concentric neutral, armor jacket to protect them from possible damage should a ship anchor in the Straits for safe harbor. The cables are over 3 miles long, shore to shore.

• The first circuit, designated as 26N1, was supplied by the Okonite Company and was installed in 1983. Three individual cables are used for this three-phase circuit. This circuit replaced a circuit installed in 1939 that used smaller 1/0 size copper conductor and an oil-filled, paper insulation system with a protective steel armor jacket.

• The second circuit, designated as 26N2, was supplied by the Kerite Company and was installed in 1988. Three individual cables are used for this three-phase circuit. When this circuit was installed, a spare cable was also installed to facilitate restoration of service should one of the six cables used in the two circuits fail. This circuit replaced a circuit that had been installed in 1965, which used smaller 1/0 size copper conductor and a crosslinked polyethylene insulation system with a protective steel armor jacket.

As a result of the recent power outage and repair work, the land-based portion of these two cable circuits has been replaced with standard 350 MCM underground distribution cable at both St. Ignace and on the Island, as the majority of the cable failures occurred in this portion of the circuits. The replacement facilities at St. Ignace also included two new 336.4 MCM overhead circuits.

From the Riser Station, two 4/0 size, the submarine cable (primary distribution circuits) ran up the nearby bluff where they connected to the distribution circuits on the Island.

The Switching Station served to replace the Block House and Riser Station facilities. Its location was chosen to hide the facilities from immediate public view and thus maintain the Island’s turn-of-the-century atmosphere. The station also provided needed space for equipment for inspection, maintenance and construction activities. Please refer to Appendix 1.3 for a view of the Switching Station.

At St. Ignace, the submarine cables connect to a 69-13.2 kV, 10 MVA transformer at St. Ignace Substation, which is located close to the shoreline. This is a load-tap-changing transformer with a 14 MVA capacity utilizing its fan cooling equipment. Please refer to Appendix 1.4 for a view of the substation.

As a result of the recent power outage and repair work, the land-based portions of these two submarine cable circuits have been replaced. Standard 350 MCM underground distribution cables were spliced onto the submarine cables at the shoreline and then routed to new overhead circuits, using 336.4 MCM conductors, which connect to the St. Ignace Substation.

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