208 120 and 480

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Old 01-13-11, 02:19 PM
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Question 208 120 and 480

In my facility I have a 20-yr-old panel (Square D nice QMB type) labelled three-phase 208/120. In a couple of the panel's disconnects I measure 120 from lug to ground but between any two of the three lugs in a disconnect I get 212-215 instead of the 208 I expected. Is this variation within a normal range? I used a second tester and got the same.

This only matters as we will be ordering a piece of equipment with a pretty tight tolerance.

My service entrance is labelled 4wy 120 480 (don't know why it says 120 instead of 277) and runs through a sealed cabinet which I'm guessing houses a transformer to step it down to the adjacent Square D panel labelled as noted.
 
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Old 01-13-11, 02:41 PM
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Originally Posted by clemdva View Post
between any two of the three lugs in a disconnect I get 212-215 instead of the 208 I expected.
Generally +/- 10% of nominal voltage is within tolerance for your incoming service. Most of the time it will be within 5%, but 10% would still be acceptable for most power companies during special circumstances. You could contact an engineer with your power company for your company's specific policy, although I'm certain that 4-5V over nominal will be within spec.

This only matters as we will be ordering a piece of equipment with a pretty tight tolerance.
You may want to consider a line conditioner and/or UPS system if incoming power quality is critical.
 
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Old 01-13-11, 03:29 PM
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Originally Posted by clemdva View Post
My service entrance is labelled 4wy 120 480 (don't know why it says 120 instead of 277) and runs through a sealed cabinet which I'm guessing houses a transformer to step it down to the adjacent Square D panel labelled as noted.
If the cabinet hums then it is a transformer. The hum will be pretty noticeable. There should be a label on it. If not it could just be a CT cabinet.

Talk to the manufacture about just how picky the machine is on your voltage. Depending on the load of the grid, voltage can vary from day to day or even by the hour.
 
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Old 01-13-11, 05:57 PM
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Your service is a 208/120 Volt 3 phase 4 wire. The cabinet is most likely a CT (current transformer) cabinet for metering purposes and is most likely fastened shut with tamperproof fasteners and is also most likely sealed. You may need a UPS system for a critical piece of equipment. The Square D QMB equipment is a fusible panelboard. You maybe should be checking what size switch you need for the new equipment as Square D fusible switches seem to become obsolete rather quickly. If they are obsolete, you can still most likely get a good used switch. See if there is a Series number on the switch, that, and the catalog number, is important in identifying Square D switches.
 
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