3 phase HELP


Old 06-12-05, 12:12 PM
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Question 3 phase HELP

Need some help understanding 3 phase 208, 240, and 460. I have a panel that I need to replace and was wondering if they are 2 pole or 3 pole breakers and whats the difference. They are loose and are throwing quite a bit and the bus bar looks fine just the breakers and wire has been hit by lightning a lot and I guess that is the problem. Intend to replace the wire and breakers but need some information and learn more about 3 phase power in the process. The panel that i need to replace breakers is listed 208/120 4 wire. 3 black wires go to all the breakers. How can I tell if they are 2 pole or 3 pole and what is the difference? When having 208 3 phase how do I get a single phase circuit? Take one of the hot leads and a neutral? Is it possible to power 3 phase lights from 208 3 phase? What type of lights should I use and do I need a contactor?

I know it is a lot of questions but I need to clean up that panel and put up some lights in the process.

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Old 06-12-05, 02:02 PM
Join Date: Sep 2000
Location: Tujunga, CA, USA
Posts: 209
First, you really need a qualified electrician to look at your system. He (she) can give you the most economical and efficient solution to your problems. Also, 3-phase power is more dangerous than single phase. 277/480 really wants to jump out and bite.

120/208 3-phase is a wye configuration. (Wish I had a picture). It has 3 phase conductors and a neutral. If you measure voltage between any two phase conducuctors, you get 208 volts. If you measure voltage between any phase conductor and neutral, you get 120 volts. By the way, 120/240 3-phase is a delta configuration and it is a lot differnt.

The number of poles on the breaker depend on what connects to it. For instance, 3-pole breakers may be used to power a subpanel or power things with 3-phase motors motors (air conditioning), 3-phase water heaters, etcetera. 2-pole breakers are used like 2-pole breakers in a house for things that require 208 single phase (you see a number of things that say they use 208-240 volt; this means they can work on 208 or 240 volts). Lighting loads and general purpose 120 volt circuits can use single pole breakers.

I have never seen 3-phase lights. It is possible to run 3 120 volt light circuits with a 3-phase breaker on a 120/208 system (you cannot have three on a 120/240 volt delta 3-phase system). On 120/208, this uses 3 phase conductors and one neutral wire. There are 277 volt lights but they are single phase lights used on 277/480 volt systems.

Correctly wiring a 3-phase system needs an expert, it is not a do it youself project. Harmonics and excess amperage can become a problem on the neutral wire. Working with a 3-phase system require more care and saftey. Like any multiwire circuit, if you turn off a single pole breaker, the neutral wire for that circuit may be conducting electricity which can still kill you.
Old 06-12-05, 03:09 PM
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Phil H , thanks for the reply. We have a old 3 phase system at our shop and need to do some maintenance on it and I am just trying to get an understanding of what we have. Electrician is coming in a couple weeks.

I understand the difference in poles and breakers better now. One question about 3 phase power though .. can you get single phase out of any flavor of 3 phase whether it be 208,240, or 460? I know you can on 208 and 460 but can you get it out of 240?

Anyway we have 800 amps of 3 phase coming from the pole and have various 208 and 460 panels. Do we have two different transformers to get those different voltages? I would think so somewhere but I do not see anything but the large transformers on the pole and the the wire goes underground and feed 5 or six large fuse panels on the side of the building and then on to various sub panels around the 5-6 shop buildings we have.

We do have 277 lights fed off of one of the 460 panels.

Another question about the breakers and them being loose. Lightning has hit one panel in particular over and over again in the last few years. Will that make breakers loose? Does it wear down the bus bar? There is a lot of room to put new breakers at the bottom of the panel .. is that ok or should the entire bus bar or guts of the panel be removed and replaced too?

Thanks for your help,
Old 06-12-05, 06:11 PM
Join Date: Oct 2004
Location: USA
Posts: 719

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