5kva transformer

[Jpflex]

I’m writing this on lunch so I don’t have a lot of time for details but our engineer says 5kva transformer from 480 delta primary will be enough to supply a good size lab building over 1000 ft for standard 120 to 240 volt receptacle coming out of a 200 sub panel

I calculated 5000/ 240/ 1.732 or around 12 amperes

Many 2 branch circuits will be needed

What do y’all think?

 

[augie47]

If it's 5kva it is probably single phase so your math would not apply.
In any event, you would need a load calculation to determine the required power.
# of receptacles, lighting load, HVAC, if any etc.
5 kva is awfully small but not impossible.

 

[Besoeker3]

5 kva is awfully small but not impossible.

I'm inclined to agree. What would a lab require? We are residential. An an electric fettle alone is 3 kW uses that much.

 

[petersonra]

seems pretty small to me too, but you need to figure out what the load will be. 5kva (assuming 240/120 system) is basically two 20 amp 120 V circuits.

 

[Jpflex]

If it's 5kva it is probably single phase so your math would not apply.
In any event, you would need a load calculation to determine the required power.
# of receptacles, lighting load, HVAC, if any etc.
5 kva is awfully small but not impossible.

The source is a 3 phase delta but step down transformer will use two phase legs on primary to deliver secondary single phase 120/240volts for lab (also concerned about imbalance)

My confusion is also when it comes to main bonding jumper or system bonding jumper being located at source or only at one location to avoid objectionable current

The delta transformer separately derived source will be system ungrounded by owners choice besides transformer being cabinet grounded

Do we treat second step down transformer from 480 to 120/240 as starting over and ignore delta grounding setup not being grounded - meaning we ground secondary winding of second transformer and derive a neutral and place system bonding jumpers and main bonding jumpers here and main panel?

 

[Jpflex]

I'm inclined to agree. What would a lab require? We are residential. An an electric fettle alone is 3 kW uses that much.

I understand that a load calculation needs to be done but the business I work for never knows what equipment they will need all at once. The seem to add equipment as they go. There will be a computer system, venting system, 240 volt particle analyzer, 20 branch circuits, lighting etc

 

[Jpflex]

seems pretty small to me too, but you need to figure out what the load will be. 5kva (assuming 240/120 system) is basically two 20 amp 120 V circuits.

So even though the source is coming from a three phase delta do we omit using the sqrt of 3 for computing secondary current?

Therefore 5,000 VA / 240 = 20.8i?

Also I don’t get what effect of using 2/3 phase conductors as opposed to 3/3 phase and output of total current, balance, etc in this regard

 

[jim dungar]

So even though the source is coming from a three phase delta do we omit using the sqrt of 3 for computing secondary current?

Therefore 5,000 VA / 240 = 20.8i?

Also I don’t get what effect of using 2/3 phase conductors as opposed to 3/3 phase and output of total current, balance, etc in this regard

Single phase has one line-line voltage. Three phase has three line-line voltages.
Your transformer has only 1 line-line voltage on its input and so it is single phase and thus uses those formulas, regardless what it is connected to.

Yes, using single phase loads on three phase systems results in unbalanced loading, but it is rarely an issue as long as the load is small compared to the source. For example a 10A load imbalance is not likely to have much impact on a 200A source.

 

[Choice_Gorilla]

If they know their needs are going to change and they have no idea what kind of equipment they are going to end up with in there I would be trying to talk them into a larger transformer. I’m a big fan of future proofing with someone else’s money. It’s more expensive now, but not as expensive as having you come back, demo the old equipment, and install new equipment.

 

[petersonra]

If they know their needs are going to change and they have no idea what kind of equipment they are going to end up with in there I would be trying to talk them into a larger transformer. I’m a big fan of future proofing with someone else’s money. It’s more expensive now, but not as expensive as having you come back, demo the old equipment, and install new equipment.

Or maybe they just want something cheap and dirty for now. It is a design decision, not a code issue if the load calculation supports the sizing of the circuits.

It might make some sense to run the 480 V conductors to the lab and set the transformer there. You could run 3 wires and a ground in conduit. make them all #10, and it would not cost much more. Then if they needed to down the road they could replace the transformer at the lab with a larger one with most of the cost being the new transformer.

#10 wires would support a 25 kva 3 phase xfmr.

 

[augie47]

The source is a 3 phase delta but step down transformer will use two phase legs on primary to deliver secondary single phase 120/240volts for lab (also concerned about imbalance)

My confusion is also when it comes to main bonding jumper or system bonding jumper being located at source or only at one location to avoid objectionable current

The delta transformer separately derived source will be system ungrounded by owners choice besides transformer being cabinet grounded

Do we treat second step down transformer from 480 to 120/240 as starting over and ignore delta grounding setup not being grounded - meaning we ground secondary winding of second transformer and derive a neutral and place system bonding jumpers and main bonding jumpers here and main panel?

When it's a SDS for practical purposes ignore the primary other than the incoming equipment ground (The transformer only requires the phase conductors..it cares less if the primary source is a delta or wye system)
Single phase or 3 phase, follwo 250.30 fro grounding the secondary/