To size a Transformer for a restaurant do I count the continuous load at 100% or 125%

charlston

Member
In the test for load calculation of a restaurant:
To size the transformer for the 120V lighting and other 120V equipment do I count the lighting and continuous load at 125% or the transformer can be sized at 100% of the load no matter if is is continuous or not?
 

jumper

Senior Member
For the calculated load for conductor sizing, OCPD, etc your continuous loads are at 125%.

Transformers are sized for actual connected load. Sizing the tranny depends on efficiency of tranny and future design.
 

kwired

Electron manager
For the calculated load for conductor sizing, OCPD, etc your continuous loads are at 125%.

Transformers are sized for actual connected load. Sizing the tranny depends on efficiency of tranny and future design.
Efficiency of tranny? If you have a 50kVA tranny don't you have 50KVA output available regardless of efficiency of the tranny, the difference being on the input power vs output power?
 

jumper

Senior Member
Efficiency of tranny? If you have a 50kVA tranny don't you have 50KVA output available regardless of efficiency of the tranny, the difference being on the input power vs output power?
I am referring to peak efficiency. Usually somewhere between 88% and 95% IIRC. It varies with models.
 

infinity

Moderator
Staff member
I thought that the manufacturer has figured all of that out and the KVA rating is the maximum rating of the transformer?
 

kwired

Electron manager
I am referring to peak efficiency. Usually somewhere between 88% and 95% IIRC. It varies with models.
Ok, but unless you have a fairly steady load 24/7 that peak efficiency level might be somewhat meaningless I would think.
 

jumper

Senior Member
I thought that the manufacturer has figured all of that out and the KVA rating is the maximum rating of the transformer?
Not sure, but I thought that was the max output, not the max efficiency load.

Many transformers are 95% so it is kinda moot.

 

infinity

Moderator
Staff member
Not sure, but I thought that was the max output, not the max efficiency load.

Many transformers are 95% so it is kinda moot.

What about in terms of the OP's question, if the calculation came to 50 kva would you need larger than a 50 kva transformer?
 

kwired

Electron manager
Like I said 50 kVA transformer should be rated for 50 kVA, continuous. Efficiency would just mean that it gives off less waste heat at whatever that peak efficiency rating is. This no different than a 10 HP motor is rated to output 10 HP. Efficiency can and does make it draw more than 10 HP for input power.
 

jumper

Senior Member
What about in terms of the OP's question, if the calculation came to 50 kva would you need larger than a 50 kva transformer?
I size the tranny to the actual connected load not the calculated load. I do not factor in for continuous loads.

IOW, if the lighting load is actually 5000VA, I use that number for tranny.

For branch circuit, feeders and service size, I factor in 125% for continuous loads.
 

topgone

Senior Member
I size the tranny to the actual connected load not the calculated load. I do not factor in for continuous loads.

IOW, if the lighting load is actually 5000VA, I use that number for tranny.

For branch circuit, feeders and service size, I factor in 125% for continuous loads.
On "delicate loads", I sometimes look at the voltage flicker a transformer gives when big motor loads are started.
 

jumper

Senior Member
On "delicate loads", I sometimes look at the voltage flicker a transformer gives when big motor loads are started.
I usually shoot for the following:

Constant load min 50%
Average load 75%
Peak load max 90%+

Basically a rule of thumb that I was taught, seems to work.
 
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