Operating Voltages Revisited J. Michael Mooney A regular topic of discussion in HOME POWER is the selection of operating voltages for the AE home. Excellent writings in recent issues by Windy Dankoff have done much to identify problems and suggest solutions. In 1985 the power loading for a full-size, fully equipped, fully independent AE home was over 6 KWH, today it is under 5.5 KWH, and will plummet to near 4 KWH by 1990. Selection of operating voltages should be considered with a view down the road to our destination. The amp/capacity, or "ampacity" for DC systems of reasonable size has long been established. As loads go up, the voltage level must be raised or the wire must be increased times the square of current. Expressed in terms of A/H/Day and KWH/DAY consumed, the table below describes the upper daily load limit for each operating voltage. INSERT TABLE 1 Since our full-size wilderness home is to consume about 5.5 KWH per day, the formula suggests a minimum operating voltage of 24 VDC. We also see that we are crowding the upper limit for that voltage. In 1882 as we proceeded to wire our nation for electricity, the same problem surfaced. Amp/capacity or "ampacity" requirements seemed destined to drive voltage levels higher than wanted, or needed, to power appliances. Thomas Edison, though a DC advocate, solved the dilemma by devising the three wire "WYE" (240-neutral-240) circuit which feeds AC powered homes to this day. The technique allowed voltage levels to be raised from 120 to 240 volts in order to meet ampacity requirements, then split into two 120 volt "legs" for appliances. Heavy loads such as boilers, electric stoves, motors, compressors, etc. were powered at the 240 volt level. Lighting, well sockets, and portable appliances were powered at the 120 volt level. We have come full circle in that our "off the shelf" DC appliances favor the 12 volt level, and ampacity is calling for 24 volts. DC does not lend itself to Edison's wye circuit, but we will accomplish the same result by simply splitting our photovoltaics, battery bank, and load distribution to produce a dual voltage (12 & 24 VDC) system. The use of readily available 24 VDC lighting fixtures and a 24 VDC inverter will enable us to power main lighting and all AC loads from the 24 volt power bus. Wall sockets, small appliances, and table lamps will be powered at the 12 VDC level. INSERT TABLE 2 Low voltage and cordless appliance technology is rapidly shifting loads away from the inverter bus, and onto the 12 volt DC bus. In three years time the number of appliances on the list will grow and the overall load will shrink. By 1990, the 12 VDC bus will be at 1500 watts, the 24 VDC bus at 2500 watts, (4 KWH total). Very few appliances will then be powered through the inverter.