Autotransformer and Class I, II, III

Jerramundi

Senior Member
Location
Chicago
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Licensed Residential Electrician
Question #1) Is an Autotransformer still considered to have (1) Primary Side and (2) Secondary Side since the two coils are no longer isolated?

I want to say NO given that the two portions are wired as a single coil, but then again given that the "Primary Side" and "Secondary Side" taps cover different lengths of this coil, depending on whether you're "bucking or boosting," and thus have different measurable outputs, I lean back towards YES.

Question #2) What Circuit Class, if any, is the "Secondary Side" of a BB Autotransformer?


Technically speaking, the "Secondary Side" circuit is "...supplied from a source that has a rated output of not more than 30V and 1,000VA" (725.41(A)), which would make it a Class I - Power Limited Circuit.

Although when wired as an autotransformer, the output is going to clearly measure more than 30V...

It could also be considered a Class I - Remote Control and Signaling Circuit, as it does not exceed 600V (725.41(B)), although not your typical example
of one (e.g. motor control).

I'm trying to discern the proper classification, if any, because the idea was to run the boosted voltage circuit in an existing conduit with other existing power and light 120V circuits and just want to make sure I'm doing this right.
 

Jerramundi

Senior Member
Location
Chicago
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Licensed Residential Electrician
Scratch the Class I - Power Limited.... the 0.5kVA BB has a 16/32V "rated output," so clearly not Power Limited.... but still an interesting question (I think) in regards to the difference between (1) "...supplied from a source that a RATED output..." versus (2) "...MEASURABLE output..."

Although I supposed the BB would still have to be rated accordingly to allow for being wired as an autotransformer, even those the label clearly limits the secondary taps to 16/32V.
 

jim dungar

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Wisconsin
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Retired Electrical Engineer - Power Systems
The standard 'buck-boost' transformer is really a two winding transformer (with primary and secondary windings) which are connected into an autotransformer configuration, so be careful when looking at nameplates and catalogs.

I consider an auto transformer to have High Voltage and Low Voltage connections as opposed to primary and secondary.
 

Jerramundi

Senior Member
Location
Chicago
Occupation
Licensed Residential Electrician
The standard 'buck-boost' transformer is really a two winding transformer (with primary and secondary windings) which are connected into an autotransformer configuration, so be careful when looking at nameplates and catalogs.

I consider an auto transformer to have High Voltage and Low Voltage connections as opposed to primary and secondary.
Interesting thought. I ask because whether or not it is considered to have a "Primary Side" and "Secondary Side" could, for example, impact over current protection requirements.
 

jim dungar

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Location
Wisconsin
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Retired Electrical Engineer - Power Systems
Interesting thought. I ask because whether or not it is considered to have a "Primary Side" and "Secondary Side" could, for example, impact over current protection requirements.
In which case the primary side would be the input side, regardless if is the LV or HV.
 

Jerramundi

Senior Member
Location
Chicago
Occupation
Licensed Residential Electrician
From Art 450 it appears the Code uses "input" and "output" terminology.
Interesting. I'll have to look into that in regards to, for example, over-current protection requirements.
It seems straightforward, but when you get into circuit classifications and their relevant requirements, most seem to use the terms primary and secondary.
 

Jerramundi

Senior Member
Location
Chicago
Occupation
Licensed Residential Electrician
Any thoughts about the circuit classification of the secondary/output? (e.g. 136V w/ no more than ~18-19A) and whether or not I could run that in the same conduit with power and light circuits?
 

GoldDigger

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Placerville, CA, USA
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Any thoughts about the circuit classification of the secondary/output? (e.g. 136V w/ no more than ~18-19A) and whether or not I could run that in the same conduit with power and light circuits?
IMHO the Class of the autotransformer output (or of an isolation transformer wired as Buck or Boost) has to be determined based on the voltage at the output terminals, not the secondary voltage of the transformer. Once one end of the secondary is connected to a non-power-limited source the game is over.
As for running with light and power, presumably the output circuit IS light and power also, not power limited, so no problem.
 

Jerramundi

Senior Member
Location
Chicago
Occupation
Licensed Residential Electrician
IMHO the Class of the autotransformer output (or of an isolation transformer wired as Buck or Boost) has to be determined based on the voltage at the output terminals, not the secondary voltage of the transformer. Once one end of the secondary is connected to a non-power-limited source the game is over.
As for running with light and power, presumably the output circuit IS light and power also, not power limited, so no problem.
Interesting. So you're going by the final output voltage total created by the BB and NOT the nameplate rating of the BB secondary. Makes sense as that's the voltage carried by the subsequent circuitry. The only thing about it that trips me up is the explicit text of 725.41(A) which uses the terminology "rated output," which makes me look at the 16/32V on the nameplate.

Then again I suppose the BB would have to be rated for the buck or boost scenario to be applicable for such an installation. I just don't see it on the nameplate, only the Square D Online Calculator. I've installed these before and there are directions that come with them to aid you in completing the connections for the desired buck/boost result. I would just think it should be nameplated on the enclosure.

Granted the nameplate rating of the BB secondary goes up to 32V, which effectively negates the subsequent circuitry as Class 1 Power Limited (i.e. greater than 30V). But I'm still stuck on it being Remote Control or Signaling (i.e. up to 600V) although not a typical example of such.

I've eliminated Classes II and III via Chapter 9, Table 11(A). Appreciate the input 👍
 

Jerramundi

Senior Member
Location
Chicago
Occupation
Licensed Residential Electrician
Just for clarification sake, "rated output" (e.g. under Class I Power-Limited Circuits) is NOT the same thing as an "Inherently Limited Power Source" as mentioned in Chapter 9, Tables 11(A)-(B) (e.g. built in OCPD), correct?
 

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
One more thing: Whether or not a limited power transformer is Class II or Class III depends on its output voltage and current. But a transformer with an output limited in the Class II or Class III range is NOT a Class II or III power source unless it is labelled as such.
 
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