Service Drop to Commercial Facility

[eeee]

I counted 150 T-5 flourescent lights in my 25,000 square foot facility, plus other lights that I did not count emanating from 3 phase 4 wire lighting panels. Now, I will assume unity power factor and 208/120volts and we all know that each circuit breaker from the 3 phase lighting panel will feed one of its three phases to its respective lights so that the lights in effect are being fed from a single phase off of the 3 phase lighting panel. Assuming 70watts per light, the current yields the following: I=70wattsX150lights/120volts (single phase so it is not necessary to devide by the square root of 3)=87.5amps for a 120volt single phase flourescent lights layout. That is just the lights, not to include a 15RLA air conditioner and a couple of other window air conditioning units, two 20 amp pieces of equipment (one operating at 120volts and the other operating at 480volts), three other 3 amp pieces of equipment, a space heater, 8 computers and some other items.

The electricians measured approximately 30amps on all three phases with 5 amps on the neutral with their ampmeter from the secondary of the set of three 100KVA ground mounted pots, which yields a 3phase, 120/240volt, 300KVA transformer they say is wye-wye connected.

I have a load study which indicates the same amp load for the building also peformed by a consultant.

I would devide the above equation by 277 if the flourescent lights were 277/480volts. Then 70X150/277=38 amps. Still it is not possible to get the 35 amps for the 3 phase reading off all three phases of the transformer secondary with 5 amps on the nuetral. Also, how can there be a nuetral when I only counted 3 wires going in to the building on the aerial service drop entering in to the weatherhead.

How can this be??

As eluded to above the other issue is that both the electricians and the load survey done by a consultant state that it is a wye-wye connection from the transformer when I only counted 3 wires going in to the building from the service drop (meaning there is no ground comming in to the building from the transformer). Wouldn't this have to be a wye-delta configuration.

 

[bob]

Re: Service Drop to Commercial Facility

lighting panel. Assuming 70watts per light, the current yields the following: I=70wattsX150lights/120volts (single phase so it is not necessary to devide by the square root of 3)=87.5amps for a 120volt single phase florescent lights layout.
I would devide the above equation by 277 if the florescent lights were 277/480volts. Then 70X150/277=38 amps.

You have to divide both results by 3.
87.5/3 = 29 amps per phase and 38/3 = 13 amps per phase.

Still it is not possible to get the 35 amps for the 3 phase reading off all three phases of the transformer secondary

I'm not sure what you asking.

[ December 07, 2005, 01:43 PM: Message edited by: bob ]

 

[eeee]

Re: Service Drop to Commercial Facility

Bob,

I have a 3 phase, 208/120volt lighting panel that is feeding T-5 flourescent lights. I don't know if the ballasts on the lights are 120volt or 277volt. If they are 120volts and 70 watts each and there are 150 of them, you state the amps is (70WattsX150)/120volts/3=29amps.

The 150 lights are spread out over all 3 phases of the 3 phase lighting panel and that is why I believe you indicate that I should divide by 3. That means that all 150 lights consume 29 amps total in that the entire 3 phase lighting panel is only disbributing 29 amps to all 150 120 volt flourescent lights?

Another way to do this calculation I then assume is to consider how many lights consume one phase. If it is 150/3=50lights, than amps=70wattsX50lights/120volts=29 amps for one phase and 29 amps for each of the three phases and this is the anwer you got so it checks. Thanks for that.

My other question is this. If I see 3 wires comming from my transformer and entering the building weatherhead (the service drop), that would be a delta connection, right?

Comming off of my primary is 3 wires and a ground making it wye. The secondary of my transformer feeding the weatherhead (the service drop to the MDP) is only 3 wires. Without the 4th wire (nuetral), it must be a delta, right?

 

[don_resqcapt19]

Re: Service Drop to Commercial Facility

eeee,

which yields a 3phase, 120/240volt

That is not a wye system voltage. 208/120 would be the voltage if it is a wye system.
Don

 

[jbwhite]

Re: Service Drop to Commercial Facility

120/240v delta, or 110/208 wye, you still need a fourth wire to get 110v loads. if this were delta, only two out of every three breakers could be used for 120v lighting loads.

you did say flourcent lights, so it is doubtfull that they are 240volt tap.

are you sure that this is 3 phase?

 

[jbwhite]

Re: Service Drop to Commercial Facility

correct to myself. looks like you have a 480v delta to 110/208 wye transformer in the building.

 

[eeee]

Re: Service Drop to Commercial Facility

Don, that is a very interesting point you bring up. The electricians document that the transformer is a 3 phase, 120/240volt wye-wye connected transformer. Don, you say that a wye system voltage would be 208/120 volts. My load study shows the voltage as 208volts. I assume the electricians made an entry error here.

I still only see 3 wires for my service drop. This means no nuetral. Would you agree the secondary must be a delta?? If it is a delta, wouldn't it then be 208/120 volts or would it be 240/120 volts?? I may not be able to get in to the transformer yard to verify right now since it is fenced and most probable locked.

 

[eeee]

Re: Service Drop to Commercial Facility

I just learned that all phase to phase voltages for a delta connection are 480 volt after reviewing some elementary math. Sorry, should have known that.

 

[jbwhite]

Re: Service Drop to Commercial Facility

Originally posted by eeee:
I just learned that all phase to phase voltages for a delta connection are 480 volt after reviewing some elementary math. Sorry, should have known that.

I hope you mean on your project, cause i wire 240 delta all the time. mostly corner ground, or no reference to ground.

 

[eeee]

Re: Service Drop to Commercial Facility

The electricians must have thought it to be a wye secondary. Problem is that only 3 wires were leaving the secondary of the transformer. It must be a delta. The description of the service would be 480volt. There is no phase to nuetral voltage with delta.

 

[jbwhite]

Re: Service Drop to Commercial Facility

then where are you getting the neutral for your lights?

 

[bob]

Re: Service Drop to Commercial Facility

eeee
I think everyone is confused as to your voltage.

I have a 3 phase, 208/120volt lighting panel that is feeding T-5 florescent lights. I don't know if the ballast on the lights are 120volt or 277volt. If they are 120volts and 70 watts each and there are 150 of them, you state the amps is (70WattsX150)/120volts/3=29amps

If you only have 3 wires connected to the buiding then you have a delta service. You need to clear up what the voltage is. If it is 480
then you will need a 480 delta to 120/208 wye transformer to get the 120 volts you seem to need. If the service is delta you can not have 120 or 277 volts available under normal conditions.
Is this location in the US?

[ December 07, 2005, 04:42 PM: Message edited by: bob ]

 

[infinity]

Re: Service Drop to Commercial Facility

Originally posted by jbwhite:
120/240v delta, or 110/208 wye, you still need a fourth wire to get 110v loads. if this were delta, only two out of every three breakers could be used for 120v lighting loads.

you did say flourcent lights, so it is doubtfull that they are 240volt tap.

are you sure that this is 3 phase?

As Don mentioned, it would be a 120/208 volt system not a 110/208 volt system.

 

[eeee]

Re: Service Drop to Commercial Facility

My observations were not good. I was looking at the weatherhead from the transformer yard, but noticed another wire connects with the service drop from the primary aerial. There are 4 wires entering the facility so it is Wye-Wye. My applogies. I should have looked at the weatherhead on the facility.

 

[eeee]

Re: Service Drop to Commercial Facility

I have read where wye-wye connections are rare and cause harmonics and balancing problems.

Maybe I could change this configuration to a Delta-Wye for the replacement transformer. That should be ok I would think and I understand the transformers can be altered on the inside to make a wye entrance on the primary side as seen from the substation converted to a delta entrance as configured on the inside of the transformer.

 

[thinfool]

Re: Service Drop to Commercial Facility

Originally posted by eeee:
I have read where wye-wye connections are rare and cause harmonics and balancing problems.

This is not a problem in a solidly grounded system. Nor is it rare.
We brought this up to a utility that we worked with several years ago. They made it crystal clear that wye-wye was 'their' standard. They were bigger than us....so we played by their rules. (Tampa Electric, Florida)

 

[bob]

Re: Service Drop to Commercial Facility

I have read where wye-wye connections are rare and cause harmonics and balancing problems.

This is not true. The fact that you have a yy connections does not cause harmonice. Equipment connected casues the harmonics.

Maybe I could change this configuration to a Delta-Wye for the replacement transformer.

you can order the transformer delta-Y.

That should be ok I would think and I understand the transformers can be altered on the inside to make a wye entrance on the primary side as seen from the substation converted to a delta entrance as configured on the inside of the transformer.

You can not change the primary. If you order a pad mtd transformer
just ask for a delta y connection.