Re: 110, 115, 117, 120 or 125 VAC ???

From: 	Jim Lux[SMTP:jimlux-at-earthlink-dot-net]
Sent: 	Saturday, January 03, 1998 12:05 AM
To: 	Tesla List
Subject: 	Re: 110, 115, 117, 120 or 125 VAC ???

> I am curious about mains voltage and equipment ratings.  I have 
> transformers, computers, and all kinds of appliances that can't seem to 
> agree on what voltage they would like to be given.   110, 115, 117, 120 
> and 125 seem to be the most popular values, but why so many subtle 
> variations?  Are these different standards for different parts of the 
> country?  My home's service clearly measures 120.0 VAC, but my newest 
> variac is calibrated for 115V input.  That's why I ask.
> Ditto for the 220, 225, 230, 240 VAC appliances.
This is a combination of tradition and where you measure the voltage. For
example, the power company refers to a 480V service, but the motors you
hook up are rated at 460V. This is because they know there is some voltage
drop in the wires from the transformer (480) to the motor (460). In the
above case, some 20 Volts, or about 5%.

The NEC requires that wires be sized for a maximum of 2% voltage drop at
rated current. The remaining part of the variation has to do with the
"voltage regulation" of the service transformer, i.e. the resistive losses
under load.

You have no doubt noticed that the output of almost any transformer
(particularly the "wall wart" type) drops noticeably from no-load to rated

As a practical example, I built a system that ran some 100 amps worth of
460Volt motors. When I went out to the site to test it, I hooked the VOM up
to the line, and was appalled to read 510+ volts. Then, when they turned on
the roller coaster that my stuff was next to, the line voltage dropped
right down to 480V, at the panel. After the distribution with several
hundred feet of feeder, etc. the motor voltage was 462 Volts, at rated

That is why they are called "nominal" voltages.....