Transformer Secondary Fusing Question

[fifty60]

This thread is similar to another thread I have open but hopefully the moderators let it ride. I am dealing with a 7.5KVA transformer that will convert single phase 460V to single phase 230V.

The equipment is normally 208V single phase, so we fuse both lines going to all of the motor loads, heater loads, and transformer. Now that I have a transformer stepping down the voltage to 230V, I am not sure how to handle the connections to the loads.

Would I still fuse both lines going to each load? 2 fuses on a single phase motor, 2 fuses for a heater? 1 fuse for L1 (which is one phase of the secondary of the 7.5kva transformer) and 1 fuse for each load for L2 (which is the the other phase from the 7.5KVA transformer).

If I ground one phase of the secondary, do I have to remove the fuses from the loads on that phase of the secondary?


I am also trying to install a receptacle on the transformer. How should I treat the secondary? 1Pole, 1neutral, and 1 ground? Or 2phases and a ground?

 

[petersonra]

in general, all ungrounded lines are required to have overcurrent protection.

 

[fifty60]

I'm not 100% certain on whether or not I should ground the 1 phase of the secondary of this transformer.

Is it acceptable to leave it ungrounded and fuse both phases of the secondary, as well as both lines supplying single phase loads from the secondary?

The transformer does have centertap for 115V that is not being used...

 

[david luchini]

I'm not 100% certain on whether or not I should ground the 1 phase of the secondary of this transformer.

Is it acceptable to leave it ungrounded and fuse both phases of the secondary, as well as both lines supplying single phase loads from the secondary?

See 250.20(B). Your system is not Required to be grounded (so it would be acceptable to leave it ungrounded.) But 240.21(B) would require Ground Detectors to be installed.

 

[fifty60]

I need a little help with the definitions in 240.21. The feeder would be the conductors from the secondary of the transformer all the way to the fuses (intended specifically as secondary fuses) at the equipment. From these fuses I connect directly into a distribution block.

Feeder Taps would be anything that is connected to the distribution block up to the fuses for the branch circuits...

 

[jim dungar]

I would ground the transformer secondary so that it is a 120/240 system. Then protect both ungrounded conductors the 240V circuits. The neutral/ground point would not extend past the transformer.

My experience is that this will be 'expected' by most people that service this equipment.

 

[fifty60]

so if I ground the center tap at the transformer, I do not need to install a ground detector if I fuse the ungrounded conductors?

 

[petersonra]

so if I ground the center tap at the transformer, I do not need to install a ground detector if I fuse the ungrounded conductors?

you only need a ground detect circuit on ungrounded systems. it would be pointless on a grounded system.

 

[fifty60]

I have read the code sections and know how to proceed. Looking at Exhibit 250.4 shows an example of my system. No to just figure out the receptacle....since no phase is grounded, I would want the 2P+G receptacle and not the 1P + 1N + 1G receptacle.

 

[petersonra]

I need a little help with the definitions in 240.21. The feeder would be the conductors from the secondary of the transformer all the way to the fuses (intended specifically as secondary fuses) at the equipment. From these fuses I connect directly into a distribution block.

Feeder Taps would be anything that is connected to the distribution block up to the fuses for the branch circuits...

I think you would be better off considering these to be transformer secondary conductors and not a generic feeder, although it may also be a feeder. Take a look at the requirements of article 450. They are more on point.

 

[Smart $]

I need a little help with the definitions in 240.21. The feeder would be the conductors from the secondary of the transformer all the way to the fuses (intended specifically as secondary fuses) at the equipment. From these fuses I connect directly into a distribution block.

Feeder Taps would be anything that is connected to the distribution block up to the fuses for the branch circuits...

Secondary conductors from transformer to first set(s) of fuses.

Feeder after each set of fuses, if not last set of fuses before load (making those branch circuit conductors).

Feeder taps after distribution block and ahead of branch circuit fuses.

 

[Smart $]

I have read the code sections and know how to proceed. Looking at Exhibit 250.4 shows an example of my system. No to just figure out the receptacle....since no phase is grounded, I would want the 2P+G receptacle and not the 1P + 1N + 1G receptacle.

Correct. 2P + G, rated 250V, and depending on circuit rating, an ampere rating not less than fuse rating.