> From: "Nick Evans" <[email protected]> > Subject: US supply Voltage questions > > I wonder if anyone can help me with a couple of practical questions concerning > USA supply voltages. > > 1. Does anyone know what the normal maximum current rating is on a 110V > supply for a domestic socket outlet.
The US outlets I'm looking at in a catalog are all rated NEMA 5-15R or 15 amps. You should always look for NEMA rated plugs and sockets for anything. > 2. Are there any situations (for example in commercial installations) where a > single phase supply of greater than 110V can (would normally be supplied)? It depends and it's not really 110VAC. It could be as high as 120VAC in some circumstances. Somewhere I believe I saw the US line voltage available to the consumer (you and me) listed as officially 117VAC. Fairly slow high line conditions can occur depending upon what your power grid is connected to. I believe there was a situation where the power grid for a community had an electric foundry attached. At about 1 am in the morning, the foundry would shut down. Since the power company had tried to adjust for the increased load while the foundry was running, the line voltage was driven above 150 VAC for a brief period of time. At a much lower level, this type of thing happens in your house all the time. > If so what would be the normal maximum current > supply for this increased voltage. Roughly the more voltage, the less current. But, I'll probably get nailed on that one. Power in equals power out. So if you need 115 watts from the line, that requires 1 amp of current for roughly 115VAC. Drop the line voltage up or down and the current should run in the opposite direction. Going to 120VAC the current should drop to 115W/120VAC = approx. .95 amps. What you really want to be concerned about are brownout conditions. If the voltage drops, the current should rise. Say our 115VAC drops to 105VAC. To maintain the 115 Watt requirement the current should rise to 115W/105VAC = 1.1 amps. If the line powering the motor in your air conditioner or frig begins to brownout or droop, the motor can stall. Thus, the motoring effect drops to zero. Now, the only resistance through the motor is the wire itself.
