rkarlqu@hpscdc.HP.COM (Rick Karlquist) (05/10/88)
Does anyone out there know how the "official" power line voltages
are arrived at. It seems that in the beginning, there was 110
volts (at least all the old timers talk about 110) and now you
often see 115 volts listed as nominal. However, you often see
equipment marked as 120 volts or surprisingly 117 volts. Where
do they come up with an odd ball number like 117 anyway. My
guess is its 115 + 2%, but that's not based on anything. Will
the real power line voltage please stand up!
Rick Karlquist
Hewlett-Packard
rkarlqu%hpscdc@hplabs.hp.comsj1f+@andrew.cmu.edu (Steven Kent Jensen) (05/11/88)
The funny 117 comes from the fact that AC voltage is not constant, it
varies as a sine wave. Therefore the root-mean-square (basically the average
of the absolute value) voltage is the peak voltage divided by the square root
of two. peak voltage on normal lines is about 165V. As to the other numbers,
I do not know why there is a shift (I can see rounding 117 to 115 or 120, but
not 110).
Steven Jensenrsd@sei.cmu.edu (Richard S D'Ippolito) (05/19/88)
In article <5770004@hpscdc.HP.COM> Rick Karlquist writes: >Does anyone out there know how the "official" power line voltages >are arrived at. It seems that in the beginning, there was 110 >volts (at least all the old timers talk about 110) and now you >often see 115 volts listed as nominal. However, you often see >equipment marked as 120 volts or surprisingly 117 volts. Where >do they come up with an odd ball number like 117 anyway. My >guess is its 115 + 2%, but that's not based on anything. Will >the real power line voltage please stand up! How they were arrived at, I don't know. But here, the utility tariff requires them to provide 120v +- 5% AT YOUR METER. All numbers such as 110, 117, 115, etc., are either manufacturers ratings for the equipment allowing for some voltage drop due to expected house wiring losses (or the thin cords they put on them!) or come from ignorant electricians. Single phase pole transformers are actually center-tapped with the tap grounded at the pole. Therefore, your dryer gets 240v, not 220. By the way, in three phase systems, the line-to-line voltage is 480v, making the line-to-ground 277v (480/square root 3). There are also certain three-phase systems that provide 208v line-to-line (which is 120v line-to-neutral with a "Y"-connected secondary), but you will not normally encounter them in a residential building. Rich