three phase meter with only two legs

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mike1061

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Chicago
I've just seen something I've never seen and don't understand. In Chicago, a commercial space with a three phase service to feed only a a/c unit. The service had three wires, three hots. 240/120 with the high leg and no neutral. No ground connection either. The meter has both 120 volt phase conductors connected to the meter. The high leg passes through to the disconnect. There is a tap conductor running back to a "5th lug?" It is a special lug that connects to the meter in a style like the other connectors. Does that work like that? Is that phase considered metered like that? It has an Edison marked tape on it but no Edison tag to seal the meter box.
My plan is to replace the service with a new one. I plan on 100 amp 120/240 3 phase. I plan on a driven rod and a water pipe ground. No neutral. Can I reuse the meter and box? Is there anything else I should know?
Sorry. The meter and disconnect will be inside. The driven rod will be outside.
Thanks
Mike
 
Well there's definitely something you will have to figure out in this scenario.


For one, you cannot have a 240/120 3ph high-leg service without a neutral... which is where you get both the high-leg measurement and 120V legs reference.

If it actually is a 240/120 3ph high-leg servivce, Code requires the grounded conductor (neutral) to be brought to the service equipment whether it is used or not.

Water pipe electrode (WPE) req'd if present, as are others that exist. WPE is req'd to be supplemented by rod electrode... but if you can't prove rod electrode is less than 25 ohms to earth, you have to supplement it with a second rod electrode.

Not certain if Chicago Code has any amendments which affect the above....
 
mike1061 said:
I've just seen something I've never seen and don't understand. In Chicago, a commercial space with a three phase service to feed only a a/c unit. The service had three wires, three hots. 240/120 with the high leg and no neutral. No ground connection either. The meter has both 120 volt phase conductors connected to the meter. The high leg passes through to the disconnect. There is a tap conductor running back to a "5th lug?" It is a special lug that connects to the meter in a style like the other connectors. Does that work like that? Is that phase considered metered like that? It has an Edison marked tape on it but no Edison tag to seal the meter box.
My plan is to replace the service with a new one. I plan on 100 amp 120/240 3 phase. I plan on a driven rod and a water pipe ground. No neutral. Can I reuse the meter and box? Is there anything else I should know?
Sorry. The meter and disconnect will be inside. The driven rod will be outside.
Thanks

Mike
Click to expand...


I believe that is similar to what is called a Delta breaker. If it's what I'm thinking, it used to be common to have 3ø power to a residence. Only the high leg was connected to the lug on the side and usually the 3ø was for HVAC equipment.
Not sure if they used same setup for commercial/industrial.
What you have may be different. I just thought it sounded like what I described.
 
I used a grounded pipe for another system to get the voltages. Thanks for the reply. quote's
Smart $ said:
Well there's definitely something you will have to figure out in this scenario.


For one, you cannot have a 240/120 3ph high-leg service without a neutral... which is where you get both the high-leg measurement and 120V legs reference.

If it actually is a 240/120 3ph high-leg servivce, Code requires the grounded conductor (neutral) to be brought to the service equipment whether it is used or not.

Water pipe electrode (WPE) req'd if present, as are others that exist. WPE is req'd to be supplemented by rod electrode... but if you can't prove rod electrode is less than 25 ohms to earth, you have to supplement it with a second rod electrode.

Not certain if Chicago Code has any amendments which affect the above....
Click to expand...
 
I think your seeing it right. I still would like to know if I can use the existing meter box. Thanks for the reply.
Little Bill said:
I believe that is similar to what is called a Delta breaker. If it's what I'm thinking, it used to be common to have 3ø power to a residence. Only the high leg was connected to the lug on the side and usually the 3ø was for HVAC equipment.
Not sure if they used same setup for commercial/industrial.
What you have may be different. I just thought it sounded like what I described.
Click to expand...
 
I have seen an identical setup when a 3 phase service was added to a residence alongside the single phase service to power an A/C unit that the owner had cobbled together from an old 5HP refrigeration compressor.. The meter was labelled "3 phase 2 wire" and the high leg did not pass through the meter. Works fine as long as the only load is 3 phase. I was kind of surprised that the POCO would run an unmetered wire into a house.
 
Any current through the high leg will be matched by current in the other two legs and the neutral.
But it does seem that if the neutral is also not metered there will be a problem determining the high leg current.
I do agree that as long as the only loads are either 120V to neutral or balanced three phase the result will be good enough to satisfy POCO. But there are many ways a dishonest customer could work the system to his advantage. A high leg to neutral load would be one of them.
 
GoldDigger said:
Any current through the high leg will be matched by current in the other two legs and the neutral.
But it does seem that if the neutral is also not metered there will be a problem determining the high leg current.
I do agree that as long as the only loads are either 120V to neutral or balanced three phase the result will be good enough to satisfy POCO. But there are many ways a dishonest customer could work the system to his advantage. A high leg to neutral load would be one of them.
Click to expand...

A high leg to neutral load for sure, free power, but I'm not sure how 120V to neutral loads would affect the metering.
 
jstjohnz said:
A high leg to neutral load for sure, free power, but I'm not sure how 120V to neutral loads would affect the metering.
Click to expand...
120V to neutral loads will be correctly metered in the same way that a two wire meter can handle line to neutral loads on a 120/240 single phase three wire system.
 
GoldDigger said:
120V to neutral loads will be correctly metered in the same way that a two wire meter can handle line to neutral loads on a 120/240 single phase three wire system.
Click to expand...

I may be way off base here but,
If I have a 1A 3 phase load, then power would be 1.7*240 (phase to phase voltage) =408W (if my math is right).

If I have a 1A load on phase A to neutral, and a 1A load on phase C to neutral, (assuming B is the high unmetered phase) then my power is 240 watts. In both cases wouldn't the meter read the same since it sees 1A on each metered leg?
 
With a three phase load the net current in the A and C lines would be greater than 1A and also would not be in phase with the A to C voltage. I have not tried to see what net effect that would have on the power reading.
It is certainly possible that it would be less than the full three phase power, but POCO might well be willing to accept that as long as the three phase load is small compared to the single phase 120/240 load.
 
GoldDigger said:
With a three phase load the net current in the A and C lines would be greater than 1A and also would not be in phase with the A to C voltage. I have not tried to see what net effect that would have on the power reading.
It is certainly possible that it would be less than the full three phase power, but POCO might well be willing to accept that as long as the three phase load is small compared to the single phase 120/240 load.
Click to expand...
I don't see how OP's going to get any 120V load (or 208V anything) without a neutral run to the service. :blink:
 
mike1061 said:
The service serves only a a/c unit. Nothing else.
Thanks
Mike
Click to expand...
In that case, you could even use a single phase meter with only one CT and apply a multiplier to the meter reading. (Unless the A/C is a significantly unbalanced load, say from the presence of a single phase fan and three phase compressor.)
 
Smart $ said:
That's not a fact in evidence per OP.
Click to expand...

There is another service to the building that supplies all of the loads other than the one three phase A/C.
Sorry if that changes anything. I was trying to keep the discussion to the three phase meter I didn't understand.
Thanks
Mike
 
curt swartz said:
If this is a grounded system you are required to bring the neutral to the service.
Click to expand...

Why would you want an ungrounded service? Would one even be legal in a store front? Would it change if there was another service, with 120 volt loads. Then would you then have to ground this service? I believe it would have to be bonded to the other service, as all systems and any metal that might become energized, have to be bonded together. Then you would be connected to ground that way.
Thanks
Mike
 
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