VOLTAGE DROP FROM SERVICE TO UTILITY TRANSFORMER - HOW MUCH?

[Ragin Cajun]

To make an involved story short, I have a new project where the utility changed their rules and noW require the contractor to run the secondary conduits AND cables from the transformer to the building services!

My problem is what % voltage drop am I allowed in this run. The distances range from 200' down to 120', at 120/208V, 3 ph. LONG RUNS! The NEC allows for no more than 5% voltage drop for both branch and feeders. It's implied that this applies to the interior of the building.

What is allowed for the service conductors from the service to the transformer? Previously that was the purvey of the utility, but now it's me! And depending on what I have to design to, the conductor size and quantity per phase can become huge.

What can I use for voltage drop for the service conductors???

 

[shortcircuit2]

I think that it is important to consider VD from the source.

 

[charlie b]

I would size the secondary conductors based on the transformer's full load secondary current. I would then calculate VD along those conductors. If that would result in low voltage at branch circuit outlets, it would be time to contact the utility company about lower impedance transformer or about voltage taps.

 

[Ragin Cajun]

The project is a large apartment building. One of the services is 180' from the power company transformer to the meter center with a 635A calculated demand load at 120/208V, 3 ph. There are a total of 7 services around the "U" shaped building.

The contractor installs the conduit and pulls the cables, terminates them at the meter center, but the power company terminates them at the transformer.

The contractor/builder requirement to provide the secondary cables is "new" and costing the builder well over $100,000. Cheap @#$% utility!

The utility is serving the whole building from a single transformer.

 

[augie47]

FWIW, that's been SOP here on commercial projects for years.
Note that the NEC does not limit voltage drop except for a few specific cases. The 5% is in an Informational and not enforceable.
In some areas it is required by "green" codes but not by the NEC.
You might check your engineered drawings and see if they have a specification. Other than that I would figure it at 5% or less based on your calculated load.
Is the 635 amps for 1 service or the total load ??

 

[Hv&Lv]

Whats odd is now it falls under NEC with the POD at the XF.
if we fed this it would probably be fed with 350MCM AL and be no issue. We have several fed this way

 

[Ragin Cajun]

Total demand load on the building is 1000 KVAd, or 2801 Ad.
Total connected load is ~~ 2365 KVAc, or 6616 Ac
72 1, 2, and 3 BR units plus common area loads.
For the meter center with 635Ad and 190' to the transformer, the voltage drop is 1.31% with 4 - 750 MCM al per phase.

RC

 

[augie47]

It looks like on your 200' run (635 amps) a 500 kcmil AL (or parallel 250s) would give you a 5% drop

 

[MyCleveland]

The project is a large apartment building. One of the services is 180' from the power company transformer to the meter center with a 635A calculated demand load at 120/208V, 3 ph. There are a total of 7 services around the "U" shaped building.

The contractor installs the conduit and pulls the cables, terminates them at the meter center, but the power company terminates them at the transformer.

The contractor/builder requirement to provide the secondary cables is "new" and costing the builder well over $100,000. Cheap @#$% utility!

The utility is serving the whole building from a single transformer.

How to you get that approved ?

 

[Knightryder12]

How to you get that approved ?

That's standard here in Florida.

 

[electrofelon]

For 200', I would just use the standard minimum conductor size and not even think about calculating voltage drops.

 

[Julius Right]

According to ANSI 84.1 American National Standard for Electric Power Systems Voltage Ratings (60 Hertz) and Equipment 5.1.1 Range A—Service Voltage :
Electric supply systems shall be so designed and operated that most service voltages will be within the limits specified for Range A. The occurrence of service voltages outside of these limits should be infrequent.
That means the Utility has to supply between maximum and minimum voltage at the connection point.
See Table 1 for these limits.
If the service is in no-load stage the voltage has to be not more than maximum. If the service point is full load, then the voltage has not to be less than minimum.
In my opinion, the transformer- full load- supply voltage has to be 5% more than the rated. In this case a 5% voltage drop from here up to service point will get the rated voltage. If the transformer voltage will be the rated with 5% voltage drop on cables the service voltage is still in limits[minimum]. If the transformer voltage will be the rated but the no-load is required at service point the voltage here will be 5% more but still the maximum allowed.

 

[Ragin Cajun]

According to ANSI 84.1 American National Standard for Electric Power Systems Voltage Ratings (60 Hertz) and Equipment 5.1.1 Range A—Service Voltage :
Electric supply systems shall be so designed and operated that most service voltages will be within the limits specified for Range A. The occurrence of service voltages outside of these limits should be infrequent.
That means the Utility has to supply between maximum and minimum voltage at the connection point.
See Table 1 for these limits.
If the service is in no-load stage the voltage has to be not more than maximum. If the service point is full load, then the voltage has not to be less than minimum.
In my opinion, the transformer- full load- supply voltage has to be 5% more than the rated. In this case a 5% voltage drop from here up to service point will get the rated voltage. If the transformer voltage will be the rated with 5% voltage drop on cables the service voltage is still in limits[minimum]. If the transformer voltage will be the rated but the no-load is required at service point the voltage here will be 5% more but still the maximum allowed.

Julius,
Thanks for the ANSI quote.
I aim to limit the voltage drop on the main service feeders to no more than 1.5%. The SER cable from the meters to the unit panel boards will be sized for a 1.5 - 2% drop. The units are small so the 1-1/2 - 2% for branch circuits is adequate.
Thanka all for your input and comments