HVAC Actuator controls (4-20mA or 0-10V?)

Well there are a number of good reasons to use a 20 ma current controlled device.

1. Yes , if there is enought voltage to drive the current , then you can connect all of the actuators in series , they each provide a voltage drop ( about 3 volts ) in the 20ma loop.
2. Noise immunity, these are low impedence devices and as such they are less succeptible to votage induction from a stray source. They are usually immune to a 120VAC conductor a fraction of an inch away.
3. Accountablilty, if you connect a 20 ma indicator light in series with all the actuators , then you know that the power supply is good and that you have continouity in your system.
4. Usually they are built with an LED that drive a photodetector , the LEDs usually want 4-20 ma to light up. Its a legacy thing also. back from the old telegraph days I suppose.

Steve

One of the major advantages of the old 20 mA current loop as opposed to RS232 is the distance. The 20 mA loop is supposed to be good for up to about 1/4 mile versus perhaps 100 feet or so for RS232 if you're lucky

I'd say 20mA loop runs as far as you want....As long as the voltage source at the beginning can cope with supply enough voltage for 20mA (of course you would use a real current source anyhow). One of the source I know (back from early computer experience) is use of 20mA loops for driving TELETYPE printers! In industrial automation, 4-20mA is still used a lot (must be the FORTRAN of the field busses I gues :-)). And RS232 can (with good cabling ) be operated also much further than 100feet, not? And use of old-fashioned modems as repeaters used to work too.

regards, Marco