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Old 08-20-2009, 08:44 PM   #16
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wire sizing


If the POCO won't connect a 480 volt single phase service, they will connect a 3 phase one. However, a minimum of 3 primary wires are needed. Look on the pole you want to come off of. If there are only two wires, single phase is all that's available. If there are 3 or 4, 3 phase is an option.

If you can get a 480 volt 3 phase service, the voltage drop issue would be much better, even with smaller wire. This service size would be 70 amps.

You'd need 3-400 MCM and 1-3/0. The transformer at the house would be a 45 KVA, I'd use a dry one. Get rainshields for it if it's installed outside. You'll need a 3 phase panel, with a 150 amp main breaker. (A Square D QO342MQ150 is one example, there are plenty of others). Don't use a single phase panel on a 3 phase system. A 150 amp 120/208 3 phase system gives about the same amount of power as a 200 amp 120/240 single phase one.

3-400's and 1-3/0 will fit comfortably in a 4". Depending on who's pulling, you could likely go 300' between pull boxes.

Rob.

P.S. One big advantage to a 3 phase system is if you have a well pump, 3 phase ones are FAR more reliable than single phase ones. So are A/C units.

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Old 08-20-2009, 08:54 PM   #17
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What you could do is propose to POCO that you open up the 3000' trench for them to drop their primary in. If you have access to a trencher that will go deep enough (36" min here) but preferred to be 48", this could reduce the labor side for them if all they have to do is drop the wire in.

I've never done a primary extension/install. But on the secondary here I did for an upgrade to 400 amp service. I opened the trench and dropped in conduit, POCO ran the wire, installed a new transformer and pad at 0 cost to me.
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Old 08-20-2009, 09:55 PM   #18
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Originally Posted by micromind View Post
If the POCO won't connect a 480 volt single phase service, they will connect a 3 phase one. However, a minimum of 3 primary wires are needed. Look on the pole you want to come off of. If there are only two wires, single phase is all that's available. If there are 3 or 4, 3 phase is an option.

If you can get a 480 volt 3 phase service, the voltage drop issue would be much better, even with smaller wire. This service size would be 70 amps.

You'd need 3-400 MCM and 1-3/0. The transformer at the house would be a 45 KVA, I'd use a dry one. Get rainshields for it if it's installed outside. You'll need a 3 phase panel, with a 150 amp main breaker. (A Square D QO342MQ150 is one example, there are plenty of others). Don't use a single phase panel on a 3 phase system. A 150 amp 120/208 3 phase system gives about the same amount of power as a 200 amp 120/240 single phase one.

3-400's and 1-3/0 will fit comfortably in a 4". Depending on who's pulling, you could likely go 300' between pull boxes.

Rob.

P.S. One big advantage to a 3 phase system is if you have a well pump, 3 phase ones are FAR more reliable than single phase ones. So are A/C units.
How did we got to a 3-phase 480v service???? For a residence no less???

Micro, this is a typical primary/secondary residential service. I think you are making way more of a big deal out of it than need be.
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Old 08-20-2009, 11:36 PM   #19
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You're probably right Petey, I was just coming up with what I thought were possible alternatives to a $20,000 POCO hook-up.

Rob

P.S. I realize that VERY few houses have 3 phase services, mine does though.
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Old 08-21-2009, 05:55 AM   #20
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Even so Micro, it's not the 3-phase issue. 480v is still not nearly enough to counter the voltage drop seen in a 3000' service.
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Old 08-21-2009, 07:01 PM   #21
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Here are the figures I used;

400 MCM AL has a resistance of 0.0529 Ohms per thousand feet. Figuring 3000' out and 3000' back, the total resistance is 0.3174 Ohms. Because it's a 3 phase system feeding a 3 phase load, the actual resistance will be a bit less. More on this later.

The current of a 45 KVA 3 phase transformer at 480 volts is 54 amps. This will result in about 17 volts drop.

The current of this transformer at 120/208 volts is 125 amps. If the loads are balanced among the 3 phases, the net power available is pretty close to 200 amps at 120/240 volts single phase. Assuming exactly 480 volts at the POCO pole, at full load, the voltage at the panel will be about 116/203. If the transformer is tapped at 2.5%, the no load voltage will be about 123/211, and full load will be about 119/207.

In a single phase system, one wire conducts, and the other returns. Which wire conducts and which returns changes 60 times a second. In a 3 phase system supplying a 3 phase load, one wire conducts and the other two return. The reason I didn't consider this in the voltage drop calculations is because this effect will be mostly lost due to voltage drop across the transformer itself.

The chance of a house ever drawing 125 amps from 3 legs or 200 amps from two legs is pretty much zero. A good-sized A/C unit starting can come close, especially with other loads on simultaneously, but for all practical purposes, it'll never happen. The actual voltage at the panel (assuming exactly 480 at the POCO pole) will run somewhere between 120/208 and 118/205. Certainly, you'll notice the lights dim when the A/C kicks on, but you'd also notice it with almost any other system as well.

True enough, if this system is installed, the owner is responsible for maintenance from the meter to the house. If the POCO installs a service at the house, they're responsible for it. The meter is the demarcation point in any system.

Rob

P.S. I'm not sure if the 480 3 phase service, trenching, conduit, wire, and a transformer can be installed for $20K. Maybe, but I sort of doubt it. If it's anywhere close, I'd recommend the POCO route. That way if anything goes wrong, it's one phone call vs. a bunch of troubleshooting, digging, etc.
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Old 08-21-2009, 08:37 PM   #22
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True enough, if this system is installed, the owner is responsible for maintenance from the meter to the house. If the POCO installs a service at the house, they're responsible for it. The meter is the demarcation point in any system.
This is not the case in most areas I know of. NYS for sure.

Regardless of who installs it, in NY the customer is responsible for EVERYTHING underground. From the primary tap down to the house. The only things they do not own and maintain are the meter itself and the transformer.
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Old 08-21-2009, 09:13 PM   #23
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That's interesting; around here the POCO is responsible for everything on their side of the meter. Anything downstream of the meter is the owners responsibility.

On overhead services, we have to provide the meter base, the mast, and the wire in the mast. If it's pole-mount, we have to provide and install the pole. We're not allowed to mount anything on their poles.

If it's underground, we have to provide and install conduit to their specifications. Primary is always 4", and 4' deep. Commercial/industrial secondary is one 4" per 400 amps + 1 spare, and if the main is 2500 amps or larger, it's a trench-duct, not conduit. 200 amp resi is 1-3" + a spare, 400 amps is 1-4" + a spare. 3' deep. Over 400 amps is treated as comm'l. They provide and install the wire. All conduit has to be inspected by the POCO prior to backfill. 6" of approved sand below, and 12" above the conduit. After backfill, the POCO inspector and the electrician meet and pull a mandrel through the conduit. This insures it isn't crushed by backfill.

Interesting how different local areas are.

Rob
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Old 08-21-2009, 10:04 PM   #24
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That's interesting; around here the POCO is responsible for everything on their side of the meter. Anything downstream of the meter is the owners responsibility.
For single phase, the POCO comes to me. For 3-phase, I have to go to them.

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