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From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: sci.electronics
Subject: Re: How are power line voltages determined?
Message-ID: <5516@aw.sei.cmu.edu>
Date: 19 May 88 15:39:15 GMT
References: <5770004@hpscdc.HP.COM>
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Reply-To: rsd@ae.sei.cmu.edu.UUCP (Rich D'Ippolito)
Organization: Carnegie-Mellon University, SEI, Pgh, Pa
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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