Control transformer: neutral/ground

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mike abbott

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I have a customer that is concerned about how they are deriving a neutral for 120V power.

They have a 240V, 3-phase, 3-wire bus which feeds several machines. Each machine has a cord drop consisting of 3 phases and a ground, but no neutral. Each machine has a small 3KVA, 1-phase transformer to step down from 240V to 120V. The neutral and ground on secondary side of the transformer are bonded. They use this 120V for motor control AND also for 120V convenience receptacles.

NEC 430.74(B) discusses control transformers and allows this to be used for motor control voltage. However, I cannot find in the code anything allowing or prohibiting this 120V secondary side to be used for 120V receptacles.
 
Re: Control transformer: neutral/ground

I can't quote you chapter and verse from NEC on the fly here. The first thing that strikes me as 3kva is too small for a convenience circuit. Any overcurrent protection? How many receptacles? The only time I have seen this is in a factory assembled control panel. The receptacle was only for use of a diagnostic PC or laptop. :roll:
 
Re: Control transformer: neutral/ground

Originally posted by mike abbott:
I have a customer that is concerned about how they are deriving a neutral for 120V power.
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What you describe is pretty common. Most of the time I would use a much smaller xfmr. The power is normally only used for control, lighting, and conveneinece outlets for powering computers and other similar equipment.

I often use 500Va or even less. 3kva is big.
 
Re: Control transformer: neutral/ground

It's that how you get a neutral on the secondary side of a transformer by grounding it?

You never feed a neutral to the primary side of a transformer in a 3 phase system.

Help me out here, Im learning.
 
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