551.20 Auto transformers

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Cavie

Cavie

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SW Florida
What is the reasoning behind the NEC not allowing the use of Auto transformers in RV's? Hughes, the maker of Autoformers, states that they do not use park power to operate. The park I am seasonal at has a serious voltage drop problem that I must use one or move. The whole park was rewired 4 years ago with no regard to voltage drop. No proper increase in wire size. Mine is not in the RV but attached to the service pedestal. Can't run the A/C without it.
 
The one Hughes uses has automatic circuitry that bypasses the boost when voltage level is acceptable. A standard buck boost does not, so voltage can spike and stay high.
 
Something that automatically changes level as incoming voltage changes I would say is more of a "regulator" than an straight "autotransformer".

A straight autotransformer will buck or boost incoming voltage by a certain percent regardless of changes in that incoming voltage.

When there is little or no load lets say your voltage is 117 volts. When your AC kicks on lets say it drops to 105.

If you used a straight autotransformer to try to correct this you can maybe get a desired 119 when AC is running, but when AC is off you might be seeing 129 volts which may not be so good either. Then if loading of other units impacts what voltage you see you have even less control of the situation.
 
Yes, Hughes boosts 10% until and nominal voltage is reached. I don't know what it is. Then it switches to 2% boost after that. Mine is a straight boost trans. I get 138 volts to my trailer at idle. Turn on the A/C and it drops 10-12 volts. Add Mr Coffee 8 volts. Toaster? You guess it 10 volts. My question is why the NEC ban on auto transformers. Hughes calls theirs an autoformer.
 
Cavie said:
Yes, Hughes boosts 10% until and nominal voltage is reached. I don't know what it is. Then it switches to 2% boost after that. Mine is a straight boost trans. I get 138 volts to my trailer at idle. Turn on the A/C and it drops 10-12 volts. Add Mr Coffee 8 volts. Toaster? You guess it 10 volts. My question is why the NEC ban on auto transformers. Hughes calls theirs an autoformer.
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Here is Hughes' take on this: https://hughesautoformers.com/run-your-autoformer-with-confidence-a-note-regarding-nfpa70/
 
Interesting read, I didn’t know about the fight to ban auto transformers. I have a relatively old one, but never took it apart. I just assumed they were using an auto transformer, and using a contactor to shunt the boost winding when it was not needed. I’ve seen that done commercially to dim metal halide lighting, except the buck connection is shunted until the fixture is fully lit. Peschel used to make an auto transformer dimmer that had a set of brushes that ran up and down an open face transformer to adjust voltage output. They had problems with the brushes burning the face when the rectifiers were blown due to surges. They had several fires due to that, and the customer we had that used them finally removed all of them.
 
Here’s what it looks like inside. Noticed they are using the ground as the return for the park power indicator light. They do have six wires coming off the transformer, two heavy, four light gauge. Two relays and a printed circuit board. The pc board appears to control the larger relay and the boost light. Can’t get the larger relay out to see what they are doing there.3D3C578D-832E-4BD0-AC25-14FC570CC088.jpeg
 
hillbilly1 said:
Here’s what it looks like inside. Noticed they are using the ground as the return for the park power indicator light. They do have six wires coming off the transformer, two heavy, four light gauge. Two relays and a printed circuit board. The pc board appears to control the larger relay and the boost light. Can’t get the larger relay out to see what they are doing there.View attachment 2552145
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Is there any NRTL marking on these things?
 
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