Calculating full load on a utility transformer

[EdCampbell]

Hi, Firstly let me explain my situation. I am an electrician but my focus is mainly in low voltage and production machinery. I live in Mexico city and well my neighborhood has a small problem. We live on a street that is about 800 meters long and it has a 112.5 kva utility transformer in the middle deviding the street in half more or less. I have counted close on 114 utility meters from one end to the other, with at least one property having two 3 phase meters. Now we are suffering major voltage drops and the property I live on is right near one end of this street. I have measured an average voltage of between 114 and 108 volts, but is drops below 100 volts up to 97 volts. We are in the process of petitioning the utility company to put in another transformer. However I need help in calculating the full load of each phase. In reality I am looking at an average of 38 meters on each phase. Ok, just on the 3 phase side I know I should get about 300 amps per phase, but dividing that between 38 houses that puts it at less than 10 amps max per house which is too low. Just putting 10 houses using 10 amps each over a distance of 300 meters will drop the voltage considerably seeing as the main cable supplying the houses is Alluminium. So can anyone help me with understanding how you calculate the maximum number of users per phase giving that each house can use up to a maximum of 30 amps not constantly and all at once but for a short while. The worst thing is that just in my little corner of the street I have counted close on 50 utility meters. Also what is the maximum distance between the transformer and the final house. I cannot see what size the feeder cables are but I am working on maybe 3/0 or above. The thing is that for the first 150 meters the cable is quite thick and for the last 150 meters it is thinner, not sure why this was done, but to me it seems illogical.

 

[xptpcrewx]

Hi, Firstly let me explain my situation. I am an electrician but my focus is mainly in low voltage and production machinery. I live in Mexico city and well my neighborhood has a small problem. We live on a street that is about 800 meters long and it has a 112.5 kva utility transformer in the middle deviding the street in half more or less. I have counted close on 114 utility meters from one end to the other, with at least one property having two 3 phase meters. Now we are suffering major voltage drops and the property I live on is right near one end of this street. I have measured an average voltage of between 114 and 108 volts, but is drops below 100 volts up to 97 volts. We are in the process of petitioning the utility company to put in another transformer. However I need help in calculating the full load of each phase. In reality I am looking at an average of 38 meters on each phase. Ok, just on the 3 phase side I know I should get about 300 amps per phase, but dividing that between 38 houses that puts it at less than 10 amps max per house which is too low. Just putting 10 houses using 10 amps each over a distance of 300 meters will drop the voltage considerably seeing as the main cable supplying the houses is Alluminium. So can anyone help me with understanding how you calculate the maximum number of users per phase giving that each house can use up to a maximum of 30 amps not constantly and all at once but for a short while. The worst thing is that just in my little corner of the street I have counted close on 50 utility meters. Also what is the maximum distance between the transformer and the final house. I cannot see what size the feeder cables are but I am working on maybe 3/0 or above. The thing is that for the first 150 meters the cable is quite thick and for the last 150 meters it is thinner, not sure why this was done, but to me it seems illogical.

The fact you have low voltage is reason enough to increase capacity. No need to calculate the loading in my opinion. I would make the case that equipment is being damaged by having inadequate voltage at the service. Unless you know all the wire lengths, sizes, connections, and loads the maximum distance cannot be calculated.

 

[EdCampbell]

The fact you have low voltage is reason enough to increase capacity. No need to calculate the loading in my opinion. I would make the case that equipment is being damaged by having inadequate voltage at the service. Unless you know all the wire lengths, sizes, connections, and loads the maximum distance cannot be calculated.

Ok, but do you think that only increasing the capacity would suffice. To me it would be better to put in another transformer supplying only our end as there are still a number of empty plots in that section and guaranteed someone will start to build thus in the end over loading the system once again.

 

[augie47]

I would agree with you but without specifics there is no way to know for sure but the problem is likely two-fold, an undersized transformer and a voltage drop due to cable size/length,
Power companies have a whole different approach to load than our Codes. There are power co guys here can you you a better idea but taking away the 3 phase loading, a 112.5 kva can supply a lot of residential loads.

 

[xptpcrewx]

Ok, but do you think that only increasing the capacity would suffice. To me it would be better to put in another transformer supplying only our end as there are still a number of empty plots in that section and guaranteed someone will start to build thus in the end over loading the system once again.

Adding another transformer is what I mean by increasing the capacity. Factors affecting voltage drop will be a combination of voltage regulation from the transformer impedance and primary distribution line and the secondary cable voltage drop resulting from long lengths with multiple tap-offs. The undervoltage condition obviously gets worse with increased loading (demand from all the customers at the same time) and is why you are seeing the voltage sag down really low.

I’m not sure what standards apply in Mexico City to service limits, but it might be leverage for you to find out. In the USA we have ANSI/NEMA C84.1 which establishes recommend limits of service and utilization voltage.