Oversizing transformer capacity.

[INGMRS]

Hello,

I have a doubt regarding the way of sizing a transformer according to NEC:

¿Why is so oversize?,

Here in Costa Rica we have as law the version 2014 of the NEC, and always is a discution with the utility company the size of the transformers, this due to these companies use a different methodology of calculation, and normally this methodology give a capacity that is less than the half of the NEC calculation or even less.

¿Is the same in USA?

After to do the installation, and check the maximum demand of energy, have you realiced that is really less than the calculated?

I have learnt a way to calculate the size of the transformer using and old formula of the "Rural Electrification Administration” (REA).

DM= FA *FB, where:

FA= N*[1-0.4*N+0.4*root(N²+40)]

FB=0.005925*(E)^(0.885)

Where:

N= Number of consumers

E= kWh average per consumer per month.

To calculate FA and FB, you can use the following formulas:


CF=(FA)/((3.29)(N))

DF=((3.29)(N))/(FA)

Where:

CF= Coincidence Factor
3.29, is a FA value for one consumer.

Example of application:

if there are 406 consumers wich average consume is of 205.9 kWh, is possible to calculate FA and FB according to the formulas, the result wourl be:

FA=414
FB=0.661

DM= 274KW.

My question is:

what metodology of calculation do you use, and why?

Thansks in advace,

I really appreciate any response about this topic.

Regards,

 

[Jraef]

Here, the power utilities are not required to follow the same rules as the rest of us, so to them, the NEC is just a general guideline at best. Their rules on transformer sizing are more about risk assessment and inventory costs.

 

[bobby ocampo]

There is an ANSI standard on the Loading Guide for Transformer

C57.91-1995 - IEEE Guide for Loading Mineral-Oil-Immersed

 

[INGMRS]

Here, the power utilities are not required to follow the same rules as the rest of us, so to them, the NEC is just a general guideline at best. Their rules on transformer sizing are more about risk assessment and inventory costs.

Hi,

Thanks for your response,

Here in Costa Rica is the same, but my doubt is with respect to the people that calculate the capacity of the transformers in multifamily dwelling, or comercial centers like MALLs, what do they use?, the NEC requirements according to the currents or some other method?, May be a method as the one used by the utility company, similar than the one that I posted before?

Thanks in advance for your response,

Regards,

 

[INGMRS]

There is an ANSI standard on the Loading Guide for Transformer

C57.91-1995 - IEEE Guide for Loading Mineral-Oil-Immersed

Hi,

Thanks for your response,

I am not sure that these standard shows you how to calculate the size of the transformer according to a demand load.

I think as i remember you can find the parameters to test he machine, and its procedures, but not to methology of calculus for the size of the transformer.

Thanks
Regards,