Measure all combinations including red-white, blue-white, and red-blue. You are likely to find (respectively):
120, 120, 208
120, 208, 240 (or 208, 120, 240)
You may not tap off of that cable to feed ordinary receptacles without first going through a (perhaps newly installed) subpanel with 15 and/or 20 amp. breakers or unless the breakers serving that cable are no more than 20 amps which defeats (and wastes) the use of the fat conductors.
If this is a 3-phase circuit's cable, you are not obliged to use all 3 phases in a subpanel. Particularly if one of the phases has a higher voltage (like 208) to neutral, you might cap that one off and use just the other two.
Do not use 208 volts for appliances that do not specfically accept that.
If your home has a second meter for the water heater, you are not supposed to tap off of that feed (red and blue?) for anything else.
Last edited by AllanJ; 11-26-2007 at 09:16 AM.