210.9 auto transformer

[Jpflex]

I read in a electrical educational book that autotransformers should not have their windings grounded because the peimary and secondary windings are physically contacting each other.

However, nec says different. Specifically NEC says that leads should not derive from a transformer unless one lead is a grounded conductor

 

[hbiss]

An auto transformer has one side of its primary connected to neutral (grounded conductor). There is no isolation between the primary and secondary because both windings are in series. The "output" of an auto transformer is between the neutral and one side of the secondary. So, the load has a grounded conductor connected to it.

Look at a schematic of an auto transformer and see for yourself.

-Hal

 

[hbiss]

However, nec says different. Specifically NEC says that leads should not derive from a transformer unless one lead is a grounded conductor

They are talking about a regular transformer with isolated primary and secondary. Because there is no electrical connection between the primary and secondary, one side of the secondary needs to be connected to the neutral (grounding conductor).

-Hal

 

[jaggedben]

I read in a electrical educational book that autotransformers should not have their windings grounded because the peimary and secondary windings are physically contacting each other.

However, nec says different. Specifically NEC says that leads should not derive from a transformer unless one lead is a grounded conductor

The NEC does not say that. The NEC only talks about grounding conductors for separately derived systems, not transformers. An autotransformer is not a separately derived system.

I also seem to recall a section that explicitly prohibits grounding at an autotransformer, but I can't find it at the moment.

 

[jaggedben]

I also seem to recall a section that explicitly prohibits grounding at an autotransformer, but I can't find it at the moment.

Sorry, it's 210.9, as you mentioned in the subject line.

What this is basically saying is that the grounded conductor must be the same conductor on both sides of the transformer. You don't ground at the autotransformer because the grounded conductor is already grounded elsewhere in the system.

 

[wwhitney]

I read in a electrical educational book that autotransformers should not have their windings grounded because the peimary and secondary windings are physically contacting each other.

That means that an autotransformer is not an SDS, and it is not appropriate to do any additional earthing on the "secondary" side of the autotransformer. The earthing on the "primary" side already takes care of the secondary side.

However, nec says different. Specifically NEC says that leads should not derive from a transformer unless one lead is a grounded conductor

That just means if you have say a 120/240V supply (neutral is grounded), you can't use an autotransformer to turn a 2-wire 240V circuit into a 2-wire 400V circuit (to make up a number; such a circuit would have one leg 120V to ground and the other 280V to ground). But you can use an autotransformer to turn a 2-wire 120V circuit into a 2-wire 100V circuit, if say you need to run some Japanese equipment.

Cheers, Wayne