NEC rules and use of autotransformer

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learning99

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Greetings,
I'm trying to understand a machine's use of an autotransformer, and whether or not it "meets code". This industrial machine is powered by 480 delta 3phase (3 wire). The machine is designed with several motors running off 480, 3p, and a 3phase autotransformer. The autotransformer input is 480 and the output is tapped at 400V, 3phase. 400V single phase is used to power a cartridge heater (4KW). 230V single phase is derived from 400V and the autotransformer 0V reference (I guess the end of the autotransformer). 230V is used for printers, a PLC power supply etc. I'm new to autotransformers and this area of NEC..... Is it "ok" to have this ungrounded 230V? It is "ok" to have a heater load off the autotransformer? NEC refers to ground detectors being used in ungrounded systems.... How does that fit in? I should add this machine gets installed in MFG facilities where only 'qualified' technicians/electricians would work on it.
Thanks in advance for your help and patience.
Learning99
 
Something doesn't quite sound right in the setup you've described.

You say the input is a 480V delta (3 wire), but you describe 230V on the output of the autotransformer.

Usually with a delta, you would have two buck-boost transformers connected in an open delta configuration. These would "buck" two of the 480V legs to 400V across the two transformers, and the third "open delta" leg would also see 400V.

What you describe sounds more like a wye setup which would use three buck-boos transformers in a wye configuration. These transformers would "buck" the voltage from 277 to 230V. On the output side, you would have a 400/240V wye.

As long as the transformers were properly rated for the load, I don't see why you couldn't have 230V or 400V single phase loads connected.
 
Are you certain it's a autotransfomer as opposed to delta primary with a secondary at the voltage you state ?
Is the transformer a integral part of the machine ?
Is the control cabinet assembly U.L or other NRTL lsited ?
 
Thanks for responding. Appreciate the help.
The machine in question is rather small, with a footprint of 6ft x 4ft. The 3wire delta power comes to the machine from the facility. We have not say in how that service is wired. the 3 wire delta 480 comes to the machine. the AT is inside the machine. The AT input is connected to the incoming 480. the output is tapped to 400V. A single phase resistance heater load is connected to the 400. 230V single phase devices (plc power supply, sometimes small motor) are connected between a 400 leg and the 0V AT line which is ungrounded (and I've learned is not and cannot be called neutral in the schematics). So we have ungrounded 230V, single ph and 400V single phase inside the machine. Does this describe the circuit better?
Many thanks,
learning99
 
NEC rules do not normally apply to anything beyond your wiring terminating at the machine.
The transformer and interior wiring are covered by a different set of requirements.
Hopefully the machine has a U.L. or other NRTL label which would provide some assurance that the internal wiring is to some standard.
I'm shooting from the hip, but to me it sounds like a specialty transformer wit windings designed for that specific machine.
 
Thanks! Think you hit the nail on the head... I wasn't sure where NEC stopped, and where other standards took over. Today, started looking at a zig zap AT to create that internal ground... Thanks again for your help. Learning99
 
learning99 said:
Thanks for responding. Appreciate the help.
The machine in question is rather small, with a footprint of 6ft x 4ft. The 3wire delta power comes to the machine from the facility. We have not say in how that service is wired. the 3 wire delta 480 comes to the machine. the AT is inside the machine. The AT input is connected to the incoming 480. the output is tapped to 400V. A single phase resistance heater load is connected to the 400. 230V single phase devices (plc power supply, sometimes small motor) are connected between a 400 leg and the 0V AT line which is ungrounded (and I've learned is not and cannot be called neutral in the schematics). So we have ungrounded 230V, single ph and 400V single phase inside the machine. Does this describe the circuit better?
Many thanks,
learning99
Click to expand...

I'm afraid something still doesn't add up in what you've described. If you have only a 3 wire input, you cannot have 400V and 230V on your AutoTransformer output. You would need to have a 4 wire input to the AT to achieve the output that you have described. Maybe you have a small isolating transformer as Gus suggested.
 
Remember the AT is 3 phase. So I checked and the 400V single phase is off 2 legs of the AT output. The 230V single phase is between one leg of the 400V and the ungrounded "0V". So I think 230 comes from the 400 / 1.7 ... make sense? so now we get 400V and 230V without a grounded reference.... I don't know if NEC cares about this since it is inside an interlocked cabinet (can't open without turning off power)... or if it is just not a preferred practice... thoughts? thanks again, learning99
 
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