215.11 Neutral for Autotransformer

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elec_eng

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I have 416V 3ph loads to serve but the building power supply is 480V 3ph. So I am planning to use a 480V to 416V 3ph autotransformer to get 416V 3ph. There are no 1-ph loads so I don't need a neutral. However, 215.11 seems to require a neutral connection for a feeder derived from autotransformer.

There are two exceptions to this section but they specifically lists voltages for those exception, 280 to/from 240 and 480 to/from 600. Since my application voltages are not listed in those exceptions, do I have to pull neutral for my autotransformer?

Seems like 215.11 for 1-ph application and the exceptions are covering 3-ph situation but those exceptions list very specific voltages.

Any thoughts?
 
We’ve been thru that discuss. This is one of the VE items from the EC and autotransformer is supposed to be cheaper and take less space. So now the question in hand is how to design/install this a code compliant way.
 
ActionDave said:
I'm only familiar with hooking up a buck boost to an individual piece of equipment, but they way I read 215.11 it's just saying you have to be connected to a grounded system.
Click to expand...

Dave, can you elaborate what that means? Also, then why does it provide (2) exceptions?
 
elec_eng said:
I have 416V 3ph loads to serve but the building power supply is 480V 3ph. So I am planning to use a 480V to 416V 3ph autotransformer to get 416V 3ph. There are no 1-ph loads so I don't need a neutral. However, 215.11 seems to require a neutral connection for a feeder derived from autotransformer.

There are two exceptions to this section but they specifically lists voltages for those exception, 280 to/from 240 and 480 to/from 600. Since my application voltages are not listed in those exceptions, do I have to pull neutral for my autotransformer?

Seems like 215.11 for 1-ph application and the exceptions are covering 3-ph situation but those exceptions list very specific voltages.

Any thoughts?
Click to expand...

You didn't mention what your loads consisted of and I think that matters.

For a feeder, the code is pretty clear, you need the neutral provided, unless you are making a small percentage voltage change, and listed two common ones 240-208V & 600-480V. 480 to 415V is a pretty small change as well, but not one pinned to the list.

For a branch circuit to a single machine, that might not be considered a feeder. Many world marketed CNC mahines come with a autotransformer for connection in any market, some inside the machine frame, and others shipped as a floor model. Once the branch circuit hits the machine disconnect, the NEC rules end there.

On the units I have connected like this, no neutral connection was required by the Mfg. All machine loads were 3Φ, and the center point and frame was connected to the EGC.

An example of a multitap unit by a local Mfg. stating that its optional for a neutral or EGC connection.
dongan-3Φ Auto.jpg

And if the load only consisted of 3Φ motors rated 380-415 @ 50Hz, those can typically run on 480V 60Hz without a problem, if a 20% speed increase is not an issue with the driven load. The V/Hz ratio is the same, so it just runs a bit faster with higher V/Hz.

MTW
 
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elec_eng said:
Dave, can you elaborate what that means? Also, then why does it provide (2) exceptions?
Click to expand...

215.11 Circuits Derived from Autotransformers. Feeders shall not be derived from autotransformers unless the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

It says the autotransformer has to be fed from a grounded system. I don't see anything that requires you to bring a neutral to the equipment and I've never used a neutral on a buck boost. Since you don't derive a new neutral you need to supply the buck boost from a neutral/grounded system to be able to clear a fault is my guess for the reason why the requirement exists.
 
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