Question on Load Calculation

[KWH]

I am feeding to a step down transformer from a 480 volt panel to 240 panel, when figuring connected load on 480 panel do you use the connected load of the step down panel or figure a different way.

Thanks

 

[Dennis Alwon]

I am feeding to a step down transformer from a 480 volt panel to 240 panel, when figuring connected load on 480 panel do you use the connected load of the step down panel or figure a different way.

Thanks

I believe you can size the secondaries to the load however tap rules in article 240 may apply

See if this helps http://ecmweb.com/code-basics/transformer-secondary-conductors

 

[Smart $]

I am feeding to a step down transformer from a 480 volt panel to 240 panel, when figuring connected load on 480 panel do you use the connected load of the step down panel or figure a different way.

Thanks

You use the same VA (volt-ampere) load values, as some additional demand factoring may kick in on the 480V panel. There are those out there that simply carry the demand VA, but that is not the proper way IMO.

 

[Carultch]

I am feeding to a step down transformer from a 480 volt panel to 240 panel, when figuring connected load on 480 panel do you use the connected load of the step down panel or figure a different way.

Thanks

240.21(C) is the article on transformer secondary conductors.


In general, transformer secondary conductors need to have an ampacity for the overcurrent device that protects them. Next size up rule doesn't apply, until you get behind the secondary OCPD.

For 1-ph/2W and 3Ph/3W delta-to-delta transformers, the primary OCPD can protect the secondary conductors at its amp rating multiplied by the voltage ratio.

For any other configuration of transformer, secondary OCPD is required, and secondary conductors must have an ampacity for it. There are also context rules based on how far the conductors extend, in 240.21(C).

 

[kwired]

VA is still VA, all that has changed is volts which will directly and proportionally effect amps. There are inefficiencies in the transformer but they are usually considered negligible.

This reply based on "when figuring connected load on 480 panel" mentioned in the OP.

 

[kingpb]

Size them to carry the full VA of the transformer.

 

[Smart $]

Size them to carry the full VA of the transformer.

The question posed is about calculating the load on the 480V panel. Sizing conductors is a completely separate consideration.

 

[kingpb]

The connected load is the full VA rating of the transformer. Everything should be sized based on that.

Why would you install a transformer, then limit it's output by under sizing the panel, breakers, cable, etc?

 

[kwired]

The connected load is the full VA rating of the transformer. Everything should be sized based on that.

Why would you install a transformer, then limit it's output by under sizing the panel, breakers, cable, etc?

Where this is apparently supplying a general purpose lighting/appliance panelboard - good point, why not just run conductors/equipment that can carry full rating of the transformer.

If it were supplying fixed equipment - there may be all sorts of reasons to provide the transformer they did.

30 kVA may not have been enough but 45 is next standard size? If it is a fixed load or there is slim chance of adding more load later, why run more conductor then it needs? Other conditions of the install may make difference in the decision here as well.

 

[Smart $]

The connected load is the full VA rating of the transformer. Everything should be sized based on that.

Why would you install a transformer, then limit it's output by under sizing the panel, breakers, cable, etc?

Where this is apparently supplying a general purpose lighting/appliance panelboard - good point, why not just run conductors/equipment that can carry full rating of the transformer.

If it were supplying fixed equipment - there may be all sorts of reasons to provide the transformer they did.

30 kVA may not have been enough but 45 is next standard size? If it is a fixed load or there is slim chance of adding more load later, why run more conductor then it needs? Other conditions of the install may make difference in the decision here as well.

I'm not saying you can't figure it that way, but it is truly not the connected load. That would be connected plus future potential.... and that isn't even considering demand. Also there is no mention of load types. It is commonplace to upsize trannies for motor loads.

 

[iwire]

The connected load is the full VA rating of the transformer.

That is a design choice not a requirement.

Why would you install a transformer, then limit it's output by under sizing the panel, breakers, cable, etc?

That is really irrelevant.

 

[kwired]

If you have a short run between transformer and the panel - JMO why not supply it with conductors that can handle the full load rating of the transformer. If you have a long run - I can understand, but also remember you have tap rules that apply to secondary conductors in most cases where your secondary has a neutral. But also if concerned about cost on a long run, why not make the higher voltage primary side the long run and place the transformer close to the panel it supplies?

 

[kingpb]

That is a design choice not a requirement.

"Most systems work best if they are kept simple rather than made complicated; therefore simplicity should be a key goal in design and unnecessary complexity should be avoided."

Wouldn't it be the simple approach to use VA of transformer as the size, seems derating complicates the issue. Just say'in! Oops, wait that was your quote. :roll:

 

[iwire]

Wouldn't it be the simple approach to use VA of transformer as the size, seems derating complicates the issue.

We are not derating anything.


If some one wants to save a buck and underwire a transformer they have on hand that's as easy as wiring it for full capacity.

Now how undersized can you make the primary and still energize the transformer remains a question.