Thread: TRANSFORMER SECONDARY GROUNDING

1. TRANSFORMER SECONDARY GROUNDING

Hello everyone. I will be working with a 10KVA transformer that will step down 460V to 230V. I understand per NEC 450 that if I size my OCPD at 125% of the rated primary current (which is 20A for this transformer), then I do not need to fuse the secondary. What risks do I take by not fusing the secondary? Do the fuses have to be at the transformer or can the by at the service disconnect?

Also, I am used to working with smaller control transformers (1...2..3KVA) where we ground one of the secondary phases. This 10KVA transformer has 120/240 available on the secondary, but I will only use the 240V. Do I still need to ground one of the secondary phases? I understand that the secondary is electrically isolated from the primary, and that any ground faults on the secondary will need the grounded secondary phase....I am just not sure if I need to always have a secondary phase grounded, or only for lower secondary voltages.

2. Notice that 240.21(C) does not give blanket permission to not protecting the secondary in a two-wire to two-wire transformer connection. The transformer ratio must be taken into account.
As far secondary grounding, if your choose not to ground the secondary 250.21(B) would come into play.

I think an argument can be made that 240.20(B)(1) would require you to use the mid-point as a ground, however, I have seen it ignored and a two-wire secondary only being used.

3. Thank you, that makes a lot of sense. It seems like I would only need the ground if I needed a reference from phase to ground.

Is there a rule in the NEC that says I have to derate my transformer by 20%? My transformer needs to supply two compressors with RLA's of 8A each, a heater which draws around 9A, and also a .5 KVA control transformer. This comes out to around 27A.

27A * 230V = 6210VA

My two choices for a transformer are 7.5KVA and 10KVA. They are 480V transformers so I will need to derate by .95 so basically a 7.19KVA and 9.58KVA. If I multiply the derated 7.5KVA by .8 I get 5752VA, which is less than what I need, so I should use the 10KVA.

Does anyone else have a better way to size the transformer? I would prefer to use the slightly cheaper, slightly lighter 7.5KVA if possible.

4. My experience has been with motor loads, you need a little "spare", but that's certainly uneducated
input. Luckily we have guys here that can give you a good answer.

5. What is a good rule of thumb for the "spare". Should all of the motor loads be multiplied by 1.25, or just the largest motor load multiplied by 1.25, or all of the loads multiplied by the 1.25? Or should the transformer just be derated by .8?

6. I add up all of my motor RLA's, multiply by the line voltage, then multiply the result by 1.2% and I get 4637VA
I also have a 500VA control transformer, and a heater that is consuming 2001VA.

The grand total VA that I have for my machine is 7138VA.

I would like to use a 7.5KVA transformer. The transformer is rated at 7.5KVA at 480V, so operating it at 460V I derate that by .95 and get 7188VA.

Should I be comfortable using the 7.5KVA transformer that derates to 7188VA with a load of 7138VA? The only "extra" that I have allotted for the VA is the 1.2X's power consumption for the motors.

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