xfmr/ panel sizing

[JoeStillman]

As an aside, when selecting your OCP device for your transformer routinely selecting 250%, albeit allowed by Code, can significantly increase your installation costs. Not only having to select a conductor sized to that OCP device but the 250% can often lead you to a larger frame, more expensive breaker.

I try to aim for 150% on the primary breaker. I can't always prove that you won't trip it on inrush, but experience tells me it's a more reasonable design choice. It feels wrong having the same size wire on the primary and secondary.

 

[JoeStillman]

Yours is a design choice, not an NEC requirement.
Say your load requirement is 38kVA, you would need to install a 50kVA unit. But why also spend the money on a larger breaker and conductors than what your load is? The extra capacity will not be used by the planned load.

Right! But a lot depends on where we are in the life-cycle of the installation. I hate to design a building that doesn't have any spare capacity on day 1.

 

[jim dungar]

Right! But a lot depends on where we are in the life-cycle of the installation. I hate to design a building that doesn't have any spare capacity on day 1.

The future loading should be part of the design load calculation.

 

[JoeStillman]

The future loading should be part of the design load calculation.

Future loading is when my 38" pants still fit me after Christmas next year.

 

[Tulsa Electrician]

Five fridges at 5 amps ?
Mini fridge?
Name plate will help. If not use max circuit VA for each. If not you may have another tripped breaker.
Stove and oven sounds like a kitchen not break room.
220.56 may apply. Using max circuit va and table 220.56 will give you a more accurate estimate since you have no name plates.

An example,
Would be a single 20 amp circuit with no other outlets for the microwave and fridges. 16 amps @ 120v or 1920 va. That would be 7@1920 then use table. This would give you 13440*.65 or 8789.76 va @ 120 volts.

A more realistic number for microwave and basic fridge is 12 amps.

Watch your unbalanced load if you use a 15 KVA.

For stove use a good number since you most likely are running a 40 or 50 amp circuit . Use a 8 kw rating or 8 kva. For a wall mounted oven 4 kva

For your water heater use 4500 watt for a 240 volt basic model.

Receptacle loads ok for a break room use for vending units and general use. Pop machines go up to 12 amps per unit.

These will get you closer with out name plates and known loads.

Also add 125% for any load considered continues.

Now you can size primary and secondary using the transformer you choose in respect to the estimate load.

I come up with 36.85 KVA( 153.55 amps) based on the example I provided.

This is why name plate is important. Other wise head to artical 220.

 

[TexasEE14]

so just to be clear. My secondary conductors to the panel are GREATER than 25 foot long. What table/section should I use to calculate the secondary conductor size? Im a bit confused on this.

 

[augie47]

transformer secondary conductor should be sized with an ampacity no less than the overcurrent device it supplies.
If we are talking 175 amps, 2/0 CU or 4/0 AL
In your case, you would need to add an over current device to maintain the 25ft max tap rule.

 

[jim dungar]

so just to be clear. My secondary conductors to the panel are GREATER than 25 foot long. What table/section should I use to calculate the secondary conductor size? Im a bit confused on this.

Look at the rules in 240.21(C) for OCPD sizing and locations. Select your conductors based on protection by your chosen OCPD.