>>> burning in central california is growing virtually unchecked tonight as its impact is being felt more than 100 miles away , far from the fire fight, the city of san francisco is under a state of emergency tonight because its water supply and hydroelectric power come from a reservoir near the fire zone. 2,700 firefighters are now on the lines backed by a relentless air assault , but working again to rugged landscape and exhaustion they managed to contain only a tiny piece of this fire. nbc's tom costello is in groveland, california, to bring us the latest. tom, good evening.

>> reporter: hi, lester. public utility crews say they are confident they can maintain water and power to san francisco . this station is serving as an information point so residents can get real time updates on exactly what the fire's status is now. 5,500 homes remain threatened and this fire is in very steeper it roin rain.

>> still holding out.

>> reporter: from the cockpit of a national guard c-130.

>> got you in sight, coming in position mind you.

>> reporter: to blackhawk helicopters, the view of the down-to-dusk aerial mission to attack a fire that has grown into the country's most serious, the challenge getting the water drop from so-called bambi buckets on just the right spot to give the ground crews a quick advantage. meanwhile on the ground, homeowners like careen and matt are on edge, leaving their home when the flames get too close and returning home when the winds seem to be in their favor.

>> twice we went with friends and it got bad for pine mountain lake.

>> reporter: this family visiting from germany didn't understand the radio reports until they got here.

>> we headed out and asked somebody and they told us that in the north i think there is a forest fire .

>> reporter: thick smoke has now drifted all the way to reno, nevada, forcing the cancellation of some outdoor events. at 200 square miles the rim fire is mostly centered in the standis lass national forest and yosemite national park . the burn area so large, it's visible from space. this fire is moving through these forests at about 2,000 degrees, incinerating all the vegetation and turning the ground into this white-gray ash. it will take decades for this forest to grow back. at 3:00 a.m . we caught up with sacramento firefighters, 14 hours into a 24-hour shift. frank black was exhausted.

>> the body level is just up so it's just hard to sleep, at least for me it is.

>> reporter: while base camp offers hot food, showers and beds 24/7, firefighters are tonight wondering how long it will take to contain a 200 square mile fire. at the moment this fire is only 5% contained. fire commanders continue to tell us they are confident that they will keep this fire away from the tourist areas of yosemite, that and yosemite valley . the fire line is about 20 miles from that