480v-120/208v transformer sizing for lighting panel

Status
Not open for further replies.

joshtrevino

Member
Location
Beaumont, TX
I have seen several threads indicating that a transformer serving a lighting panel should be sized to the anticipated load on the panel.

Does NEC have any specifications on transformer sizing, or is the sizing of transformers discretionary to the designer? Please site the specific NEC article if this is addressed in the Code.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I have seen several threads indicating that a transformer serving a lighting panel should be sized to the anticipated load on the panel.

Does NEC have any specifications on transformer sizing, or is the sizing of transformers discretionary to the designer? Please site the specific NEC article if this is addressed in the Code.

I am not sure there actually is such a code section.

Are you asking if you can size it less than the anticipated load?
 

joshtrevino

Member
Location
Beaumont, TX
I have calculated the load per NEC, and am planning to size the transformer based on the load calculation. I just wanted to verify that I am not missing any specific NEC requirements. You confirm what I have found - that there is no Code specifications on transformer sizing.
 

kwired

Electron manager
Location
NE Nebraska
It must have a minimum size that is larger than the connected load as calculated using art 220. Many people will install larger transformers to allow for future loads to be added, but there is no code or rule that says just how much larger to size it in this circumstance.

Say you calculated a load of 19kVA you would likely install a 25 KVA transformer as it is the next larger standard size. If you knew there was a good chance of additional load in the future you may install a 37.5 or even a 50 as they are next standard sizes but they will cost significantly more, so you would make this consideration based on information of how likely the possible future need is to happen.

Now if you are talking smaller transformers than what I mentioned especially smaller than 15 kVA, there are not as big of jumps between standard sizes, but the price also does not jump quite so much from one size to the next.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It must have a minimum size that is larger than the connected load as calculated using art 220. Many people will install larger transformers to allow for future loads to be added, but there is no code or rule that says just how much larger to size it in this circumstance.
I agree this is probably how he should size it but I can't find any specific requirement in the code that it actually be done that way.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
It must have a minimum size that is larger than the connected load...

There is no code language that addresses this situation.

Clearly the transformer secondary conductors, and their protection, need to be sized properly, which includes load consideration.
Also there is no doubt the transformer and its primary needs to be protected, and the primary conductors must be sized so they are protected.

Where does the sizing of the transformer, itself, come into play?

As long as the primary protective device protects the transformer from being overloaded, based on manufacturers ratings and accessories, we meet the requirements of 110.3.

In a similar vein, there are no NEC guidelines for sizing a motor compared to its load.
 

kwired

Electron manager
Location
NE Nebraska
There is no code language that addresses this situation.

Clearly the transformer secondary conductors, and their protection, need to be sized properly, which includes load consideration.
Also there is no doubt the transformer and its primary needs to be protected, and the primary conductors must be sized so they are protected.

Where does the sizing of the transformer, itself, come into play?

As long as the primary protective device protects the transformer from being overloaded, based on manufacturers ratings and accessories, we meet the requirements of 110.3.

In a similar vein, there are no NEC guidelines for sizing a motor compared to its load.

I agree there is no code that directly tells us minimum size of a transformer. Indirectly we must still size conductors to carry the load, the transformer secondary is going to carry the load as calculated and the primary is going to carry same VA multiplied by primary to secondary voltage ratio. If you protect the transformer and you protect the conductors, you still result in having a transformer capable of carrying the load.

In my earlier example of calculated load of 19kW and a 25 kW transformer, nothing would be wrong with supply conductors sized and protected for primary current at 19kW, and a transformer rated higher than 19kW
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Retired Electrical Engineer - Power Systems
I agree there is no code that directly tells us minimum size of a transformer.

The primary conductors must be sized to carry the full load rating of the transformer. There is no requirement that the transformer be able to carry the computed secondary load.

Using a properly protected 15kVA transformer to feed a computed secondary load of 45kVA is not against code, as long as the secondary conductors are sized and protected correctly for that load.

This is one of the actual places where the NEC is not a design manual.
 

kwired

Electron manager
Location
NE Nebraska
The primary conductors must be sized to carry the full load rating of the transformer. There is no requirement that the transformer be able to carry the computed secondary load.

Using a properly protected 15kVA transformer to feed a computed secondary load of 45kVA is not against code, as long as the secondary conductors are sized and protected correctly for that load.

This is one of the actual places where the NEC is not a design manual.

So I can not supply a transformer with smaller conductors if I reduce overcurrent protection to the point those conductors are still protected from overloading?

I have done that a few times for temp power when a larger than necessary transformer was readily available. Now I may end up going to NEC hell.:ashamed1:
 
Status
Not open for further replies.
Top