3phase, 3wire to feed single phase condominums

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mwbas

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Have a project where the contractor did not provide a neutral conductor for a 480V,3phase service. The service conductors leave an outdoor utility transformer and enter a bussed wireway inside the building. The service conductors feed multiple apartment units via bus taps. There are multiple services in the room which are fed 480V,3p,4W off the same utility transformer.

The 3phase bussed wireway is used to tap 2 phases at a time to feed single phase apartment units.

Without that neutral wont the phase conductors back to the utility transformer overheat since there will be a large unbalanced load with nowhere to go?

I have looked in the code but cannot find any help there about the neutral being required.
 

charlie b

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Welcome to the forum. Let me pose a few questions:

Where is the service disconnecting means? Are the Neutral and Ground buses bonded to each other at that location? Is there a grounding electrode connected to that same point? In what manner is each unit supplied with power (i.e., I doubt each unit gets a 480 volt feed), and where are the transformers? Is the center-wye point of each transformer tied to planet Earth, and is a neutral derived at that point for use by each unit's panel?


If there is no neutral (i.e., grounded) conductor in a 120/240 volt single phase system, you get the same very dangerous situation that would happen if the neutral wire becomes open. A load on Phase A and a load on Phase B will be essentially in series with each other. That creates a voltage divider, since there is no neutral wire to carry back the unbalanced current. The voltage across each load will be determined on the basis of their relative resistance values. One light might have 60 volts across it, and a vacuum cleaner might have 180 volts across it.
 

jim dungar

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Have a project where the contractor did not provide a neutral conductor for a 480V,3phase service. The service conductors leave an outdoor utility transformer and enter a bussed wireway inside the building. The service conductors feed multiple apartment units via bus taps. There are multiple services in the room which are fed 480V,3p,4W off the same utility transformer.
Something is wrong here. If there is no neutral brought to the building how is each service fed with 4 wires?

The 3phase bussed wireway is used to tap 2 phases at a time to feed single phase apartment units.
How many wires are actually run to each apartment? 2 hots + 1 neutral + 1 ground?
What voltage is fed to each apartment 480V? Is there a transformer in each apartment?

Without that neutral wont the phase conductors back to the utility transformer overheat since there will be a large unbalanced load with nowhere to go?
Neutrals are required only if there are loads connected to them. It is perfectly acceptable to have circuits with only 2 hots and a grounding (green wire).
 

wawireguy

Senior Member
I'm thinking about how fast I could make coffee on 480v with my 120v coffee maker, bet it could brew a cup fast!
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I'm thinking about how fast I could make coffee on 480v with my 120v coffee maker, bet it could brew a cup fast!
Yeah, and since watts is volts times amps, it'll only use 1/4 the current. Think of the money you'd save! :wink:
 

hillbilly1

Senior Member
Location
North Georgia mountains
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Owner/electrical contractor
I think what the OP is saying is that there is a busway from the 480 four wire service that is only using three wire bus, with a 480 volt single phase tap for each apartment, where a single phase 120/240 volt stepdown transformer is used. No neutral would be needed with this set up on the primary side.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
I know you know thats wrong, but for those who think this is right - its not, its wrong. Sooner or later someone will quote this phrase in a serious topic, and believe it to be so...
Oh, so ya wanna get all serious on me, eh? :wink:

Okay, if one were to double the voltage to a constant impedance the current would also double. And, since power = volts x amps, the resultant power would quadruple. (This is the science behind audio-amplifier bridging, by the way.)
 

Smart $

Esteemed Member
Location
Ohio
Oh, so ya wanna get all serious on me, eh? :wink:

Hmmm... I think db was commenting on your statement about the money that would be saved...
LarryFine said:
Yeah, and since watts is volts times amps, it'll only use 1/4 the current. Think of the money you'd save! ;)
Same amount of power = no money saved :rolleyes:

LarryFine said:
Okay, if one were to double the voltage to a constant impedance the current would also double. And, since power = volts x amps, the resultant power would quadruple. (This is the science behind audio-amplifier bridging, by the way.)
While that is good in basic theory, it only remains true if each section of the bridged amplifiers have an output impedance equal to or less than half of the constant impedance load.
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Hmmm... I think db was commenting on your statement about the money that would be saved...
Since electricity is sold by the KwH, four times the power would theoretically do the same work in 1/4th the time, costing the same.

Same amount of power = no money saved :rolleyes:
Absolutely. My entire post was a joke, by the way. :cool:

While that is good in basic theory, it only remains true if each section of the bridged amplifiers have an output impedance equal to or less than half of the constant impedance load.
Modern transistor amps have outputs with impedances of 1/10th of an ohm and lower, which translate into very high damping factors.

An amp that is capable of doubling its output power into half the impedance will quadruple when bridged, if the power supply is able.
 
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