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#177033 - 04/19/08 09:56 PM
Re: Is this legal
[Re: gfretwell]
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renosteinke
Moderator
Registered: 01/22/05
Posts: 3052
Loc: Reno Nv USA
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Off the top, it looks good to me.
Panels are generally not listed for serving single phase circuits when used with a delta service. Using the panel to feed a separate single phase panel is just one code compliant way .... splitting the feed at the meter, or using a gutter, have their own issues.
Which brings up wire sizing. I can't imagine any way the #3 would draw more than 100 amps.
Side note: Every such service I've seen has actually had only two transformers (open delta). The (much smaller) second transformer also limits the power available to that 208v wire.
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#177036 - 04/20/08 12:19 AM
Re: Is this legal
[Re: gfretwell]
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renosteinke
Moderator
Registered: 01/22/05
Posts: 3052
Loc: Reno Nv USA
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Looks OK to me .... especially if there are only 2 transformers, there's no way that small wire is ever going to pull more than 100 amps.
Indeed ... and I know this sounds 'backwards' .. but the PoCo might not want parallel 250's on the 'high' leg.
On 'this side' of the meter, the main concern - it seems to me - is that we prevent any possible accidental 208v single-pole circuits. There's no issue with 2-pole circuits ... those will be 240v, no matter what buss they come from. Because of the difference in transformer sizes, a conscientious sparky would make sure to draw his 240 single-phase loads from "A" and "C" legs - but I don't think that's a requirement of any code.
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#177052 - 04/20/08 08:29 PM
Re: Is this legal
[Re: wa2ise]
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renosteinke
Moderator
Registered: 01/22/05
Posts: 3052
Loc: Reno Nv USA
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I suppose you've hit on why things are the way that they are ....
Three phase is pretty unusual in residential areas. It's almost unheard of to a house itself; before the PoCo will supply three phase to a house, there is a tremendous amount of paperwork.
When three phase is an afterthought - that is, the PoCo didn't expect for there to be a need for three phase at the site, and the three phase loads are specific - then the PoCo is likely to save a few dollars, and create it using only two transformers. Again, a large part of the paperwork the PoCo will want will focus on exactly how the delivered power will be distributed.
Unless he can see the transformers up on a pole, there is no way for a sparky to tell if he is dealing with a "delta" or "open delta" service. The only indication that a leg is overloaded will be the voltage drop during peak loads.
For economy, the PoCo likes 'open delta' three phase services. That's one less transformer to buy. By any other criteria, the "Wye" style is much preferred : three transformers, every leg the same voltage to ground.
For the "dryer" example, we look back to the difference between an "appliance" panel and a"power" panel. A "power" panel, in practice, lacks a neutral - so there will be no 120v loads. This is one case where it pays to read all that fine print on the panel labels 
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#177054 - 04/20/08 08:55 PM
Re: Is this legal
[Re: renosteinke]
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electure
Moderator
Registered: 12/24/00
Posts: 3615
Loc: Fullerton, CA USA
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The setup is like many older commercial services.
SoCA Edison hasn't used them in decades, but many are still in service.
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#177089 - 04/21/08 10:19 PM
Re: Is this legal
[Re: George Little]
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WESTUPLACE
Member
Registered: 03/29/04
Posts: 115
Loc: Houston, TX USA
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There is a neighborhood in sw Houston built in the 1950's that has lots of homes with 3p. I see lots of odd breaker panels. like a panel with a 3p main lug but with only 1 3p breaker spot, 4 2pole breakers and a split buss sub feed for lighting. I also seen mostly 2p main lug (also split buss) with a 30 or 60a fused switch for the 3p. Most of these homes used to have AC with a water chiller (tower) outside and pumps and compressor (non hermetic) in the utility room. Homes are only single story ranch 2000-3100 sf. Lots of the AC units been replaced with 1p unit mostly on undersize wiring due to replacing a 3p with a single phase using the existing 3 p ckt like 5ton unit on 12.
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