Transformer and the OCPD

[Bassmangsj]

I design electrical systems to be used with welding robots. The problem I am having is with the ocpd supplied with the robot. The robot itself requires 208-3 phase. On the robot panel is a 30 amp 3-pole breaker that is supplied by robot manufacturer. If the robot is to be installed in a facility that does not have 208 vac I install a transformer with the robot.

The robot manufacturer says to install the transformer downstream of the 30amp breaker. Then 208 vac will be sent directly to the robot controller using a "plug" that fits the high power unit of the robot, so I can not go through another fuse or cb.

I have 3 transformers I can use based on the size of the robot. All are 480/240 delta to 208 wye, 18kva, 10kva, or 4 kva.

Am I wrong to assume the 30amp breaker is too large for the primary of this transformer when I can not protect the secondary because of the "plug" required for the high power unit of the robot?

I figure the primary of the 4kva at 480v should at 6amps (125% of 4.8A), the 10kva at 15amps, the 18kva at 30amps.

Thanks in advance for any advice.

 

[jim dungar]

Do you have an overcurrent device upstream, of the 30A one included with the robot, that could be sized correctly?

Is the transformer being supplied by the robot manufcturer as part of their equipment?

 

[Bassmangsj]

Upstream options

Upstream options

The upstream would be either a bus plug disconnect and fuses or other circuit break from a 480 panel which is the end users responsibility for installation.

They always come back and ask us what size cb/fuse/bus plug to supply the 30amp breaker with.

So I could say then to use 6amp fuses to supply the 30amp breaker? It seems backwards to me.

 

[Bassmangsj]

Also, to answer the second question: I do supply the transformer with the robot and we tell the customer to run their 480 or 240 supply to the top of the 30amp breaker that is installed on the robot control panel.

 

[augie47]

My first thought would be that if your transformer is installed down-steam of the robot main, it should be included as part of the listing and labeling of the machine and should be under the rules of the NRTL listing the machine.

If the transformer falls in the venue of a NEC install, it should meet the requirements of 450.3(B) with proper OCP on the primary or secondary.
From what you say, you are providing no secondary protection, so the primary protection should be 125% {or 167% or 300% --see 450.3(b)}.
If it is a NEC regulated install rather than NRTL, you might also need to address 250.20(B) or 250.21(B).

 

[lauraj]

Wouldn't a wye secondary require secondary overcurrent protection according to 240.4(F)?

 

[jim dungar]

My first thought would be that if your transformer is installed down-steam of the robot main, it should be included as part of the listing and labeling of the machine and should be under the rules of the NRTL listing the machine.

I can't speak for the OP, but I don't think I have ever heard of an industrial robot with an NRTL listing.

 

[augie47]

240.4(F) would cover the conductors, 450, the transformer.
I agree with you that 240.4(F) may well play a key. That's why I reference 250.20(B). It appears he is not using the neutral and I think it may be a bit gray as to if he must use (attach) the XO tap.
We have had situations locally where the only transformers available were wye but XO was not desired and the install was allowed to
ignore XO (questionable call)

 

[Bassmangsj]

Thanks for the responses.

The robots and the robot controllers are ul listed.

I recently started doing this job, and looking back on past prints it differed on how the transformers were utilized. I can tell you, the neutral from the secondary was not used ever. I have been starting to tap the x0 to use for control power for the rest of the system if I have to add a plc, etc. Most prints had 200vac single phase sent to the PLC or other DC power supplies, which is standard practice overseas. According to NFPA79 (at least in the 97 edition) 120vac is the max for control power. There is a 1kva 120 vac single phase secondary on the transformer also, but I feel that is not enough to power all the controls plus a couple fl lighting fixtures if the customer wants it.

I have to put together a list of codes to show that putting the transformer ahead of that 30 amp breaker is the better idea.

Thanks so much for the help.