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Typical load centers here are rated 125 and 200. I occaisionally see a 60 amp 4 space unit.

We do have some 3 phase 120/240 delts services here in the older neighborhoods. The used then strictly for the AC units in the olden days (50's/early 60's)
 

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My house has a 120/208 3 phase wye, but it's very rare to see 3 phase to a house. As stated above, I've seen a few 240 deltas with single phase panels and a delta breaker feeding an A/C unit.

I've seen 60, 100, 125, 150, and 200 to a house, but I suspect the 150s are really 200 amp panels with 150 amp mains.

Rob
 

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It seems the vast majority of old fuse boxes I run into are 60 A or 125 A. Old breaker panels are often 100 A. I see 150 A boxes occasionally. And most services installed since the late 70's are 200 A. I have installed 200, 400, 600, 800, and 1200 A services on residences.

But the majority of residential services I come across are 100 or 200 A.
 

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the big McMansion's typically have 3 phase, but it's pretty rare for a conventional residence to have it.
 

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Three phase seems to be limited to certain areas. No need for it in neighborhoods so the utilities save the money be not having to put it on the poles.
 

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I see 3 phase in some older neighborhoods feeding several houses. The houses services are of course, split phase.

Even a McMansion probably won't need 3 phase. Unless your name happens to be Billy Gates and you bathe in $$$.
 

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Come to think of it, I never seen a house around here have a three phase service, unless the owner has a shop that has three phase, and the house gets fed off that.

There are some apartments in downtown areas that get what they call network service, which is two hots and a neutral, but off of a 208Y/120 4 wire secondary. (hots are any two random legs of the three)

There's even a few houses in town that only have 120 volt service.
 

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The reason so few houses are 3 phase is simply cost.

To supply single phase, the POCO needs two primary lines. Either one hot and the neutral, or two hots. There is one transformer. The secondary (from the transformer to the meter) needs to be 3 conductor.

If the service is 3 phase, a minimum of 3 primary lines are needed. If all 3 lines are hots, the service can be either an open delta, a closed delta, or a wye. The open delta needs two transformers, the closed delta and the wye need 3 transformers. The secondary here is 4 wires.

If the service is a delta, the secondary will have a 'high leg'. There will be 240 volts between any two hots, two of the 3 hots will be 120 volts to neutral, the 3rd hot will be 208 to neutral.

If the service is a wye, there will be 208 (not 240) volts between any two hots, and all 3 hots will be 120 volts to neutral.

I would strongly not recommend installing a 3 phase panel in a house that's fed by a delta system. A 3 phase wye would be fine though. The reason being that sooner or later, someone will install a single pole breaker on the high leg, and send 208 volts to a 120 volt device. With obvious results! A single phase panel with a delta breaker (feeding an A/C unit) is fine. The only place the high leg exists here is the 3 phase A/C unit.

A 3 phase delta panel installed in a commercial/industrial building is OK, usually persons installing breakers in these buildings are more experienced, but I've still seen plenty of stuff blown up by the high leg here as well!

The only reason I can think of to install a 3 phase service to a house is because the house is huge, or there is 3 phase equipment that's not practical to run from a phase converter.

Rob
 

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i've seen a house with a 3 phase delta panel indoors...they stil; had the red tabs on the buss for the stinger leg....this is a disaster waiting to happen, some diyer or handyman is going to come along and hook a circuit up to the stinger
 

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i've seen a house with a 3 phase delta panel indoors...they stil; had the red tabs on the buss for the stinger leg....this is a disaster waiting to happen, some diyer or handyman is going to come along and hook a circuit up to the stinger
Then the Mr Handyman will find out rather quickly why it is called the bastard phase.
 

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I've only seen one house with a 3 phase service - 1600 amps.

30,000 sq ft "single family" with a pool and outbuildings.
 

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I've only seen one house with a 3 phase service - 1600 amps.

30,000 sq ft "single family" with a pool and outbuildings.
i've seen quite a few homes with 3 phase service, some delta, some wye....i just don't understand why they didn't use a trough and then feed a single phase panel, that was the strange thin...this house was about 2000 sq ft, and the old a/c comp. was 3 phase delta
 

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Discussion Starter · #17 ·
For resi service, 60A, 200A and 1200A, 240v 1ϕ center tapped

How would I find out the available short circuit current
or the minimum required interrupting capacity of the main resi. breaker for this service?

It seems to be between 22,000A and 200,000A. . .?

I'm trying to work backwards to this
http://en.wikipedia.org/wiki/Thévenin's_theorem
equivalent impedance.
 

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How would I find out the available short circuit current
or the minimum required interrupting capacity of the main resi. breaker for this service?

It seems to be between 22,000A and 200,000A. . .?

I'm trying to work backwards to this
http://en.wikipedia.org/wiki/Thévenin's_theorem
equivalent impedance.
The interrupting capacity of a service is determined by the available fault current. Fault current being determined by the internal impedance of the transformer, the resistance of the connecting conductors, and the service voltage. And whether the short is an arcing fault or a bolted fault.

The power company engineer is the person to ask about any particular service's available fault current
 

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Discussion Starter · #19 · (Edited)
The interrupting capacity of a service is determined by the available fault current. Fault current being determined by the internal impedance of the transformer, the resistance of the connecting conductors, and the service voltage. And whether the short is an arcing fault or a bolted fault.

The power company engineer is the person to ask about any particular service's available fault current
Yeah, it's sure looking that way.
For either bolted or arcing fault, for ball park, 10KA to over 65KA interrupting rating seems common, with
http://books.google.com/books?id=CI...=X&oi=book_result&ct=result&resnum=7#PPT69,M1
100kA for 100A or 200A service. Ray C. Mullin even talks about 240 KA.

240v/100kA = 2.4 milliohm source impedance gives a 24 mV change with a 10A load. Worst case, 240 mV drop at the panel with a 10A load with the 10KA rating.

I think this is close enough for a pass/fail spec for resi. system impedance. If there is a bad connection these values will probably be way higher, so the good readings are hopefully clumped and the bad readings are scattered but all of them way higher.

Using an elec. dryer or elec. water heater as a test load will be even more accurate.
One of these days I should check my own house (which has no symptoms). Real world numbers beat theory any day.:)
 

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A single phase transformer has to be huge to have more than 10,000 amps of fault current.

A typical pole mount transformer will have about 7% impedance, a pad mount will be slightly less.

A 100KVA transformer operating at 240 volts will have a full load current of 416 amps. If the impedance is 5%, the short circuit current will be 8320 amps. This assumes an infinite source (full voltage at the primary), and no losses in the secondary conductors, or across the main breaker.

The POCO around here will do a fault current study free of charge. They will tell you how much short circuit current is available at their meter. In my experience, a 120/208 or 120/240 service has to be pretty good sized to come up with 10,000 amps.

Rob
 
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