NFPA regulations and Surge Guard/ Hughes autoformer type units

[Ray,IN]

This post on irv2.com got my attention.  Apparently in 2020 these units may be banned due to fire risks. Hope it isn't too late for RV industry groups to voice their opinions..

[Kirk W]

Interesting line of thought. The idea doesn't make sense since the power pedestal should have a circuit breaker that limits the current draw to 30 or 50a if working properly. The autoformer does not change that in any way but simply boosts the output voltage up by incresing it's current draw in amps but it is still limited by the park's circuit breaker since it is down-stream from that breaker. I can't believe that anyone who understands basic electricit and knows how a park pedestalis wired would make so stupid a rule, but things like that do sometimes happen. 

[Darryl&Rita]

Think farther up the conductor, Kirk. I can sort of see the argument of stressing already stressed infrastructure, but the true answer would be to have proper infrastructure in place, in the first place.

Canadians have a very similar electric code to the NEC, but most of the country doesn't see the temp/humidity combination of so many areas of the US. This gives us the advantage of lower electric loads, so our infrastructure sees less stress.

[Kirk W]

25 minutes ago, Darryl&Rita said:

Think farther up the conductor, Kirk. I can sort of see the argument of stressing already stressed infrastructure,

But if the circuit breaker limits to 30a or 50a the load on that infrastructure is exactly the same no matter what those amps are used for or how they are used. If you take that auto-transformer out of the equasion, an ac motor running an air conditioner draws more amps as the voltage drops so that step up in voltage only increases the amp draw by the amount allowed by the breaker, exactly the same as any other use of that 30a. I have been an electrical tech for many years and the laws of basic electricity and of physics do not change at all from the auto-transformer. While it may be true that the current load is higher when using the autoformer, the limit is still exactly the same so if the circuit is overloaded it could be just as overloaded without that device. 

[TXiceman]

Like Kirk said, the 30 or 50 amp circuit breaker will limit the load at the pedestal to the breaker limit.  The NFPA is blowing smoke to appease the under designed campgrounds.  Rather than attack the root problem, they are trying to limit a temporary solution to the problem.

The simple solution is to mount the Hughes in the basement,  What they cannot see will be on no concern.

Ken

[Dutch_12078]

It's interesting that this is an issue now. NFPA 70 (NEC) Article 551.20(E) Recreational Vehicles and Recreational Vehicle Parks - Combined Electrical Systems, has stated "Autotransformers shall not be used." for many years.

[Darryl&Rita]

What I'm saying is the increased current draw, on multiple RV sites, has to affect the feeder line current. This, then affects the neighbors, and leads to brown-outs and black-outs. One could assume that a park with 10, 30 amp sites must have a 300 amp service, but you know they don't. Due to derating, as allowed by the NEC and CEC, they might have as small as a 100 amp service. Poor decisions of the past is now coming home to roost. Don't forget, code is the bare minimum allowed, not what should be installed. 

[Bill Joyce]

Autotransformers are used outside of RVs. A friend helps at a  machine shop that has 240 three phase  power and uses an autotransformer to boost from 208 to 240.  

Edited August 13, 2019 by Bill Joyce
typo

[Kirk W]

1 hour ago, Bill Joyce said:

Autotransformers are used outside of RVs. A friend helps at a  machine shop that has 240 three phase  power and uses an autotransformer to boost from 208 to 240.  

You are absolutely right. In fact they are pretty common in power supplies of equipment that needs very stable voltages. I have worked with many of them over my previous career. Power to any motor is based on amps times volts equals watts. A motor that draws 1200 watts will need 10a if the voltage is 120 but if that voltage drops to 110 the current rises to 11a and 100v it must use 12a. 

Edited August 13, 2019 by Kirk W

[Lou Schneider]

The problem is voltage rise along the neutral, which is the same as voltage loss along a hot conductor and raises the load end of the neutral above ground.

Since the input and output neutrals are tied together in an autotransformer, any voltage rise on the incoming neutral will be passed on to the output neutral.  Cascade several autotransformers in a row where the neutral is only grounded at the source panel and you can have a lot of voltage on the neutral by the time you reach the end of the line. 

A better solution when designing a power system is to use a full transformer instead of an autotransformer at the intermediary points to keep the input and output neutrals separate, then use a local ground at the transformer to return the output neutral to zero voltage. 

This is what the NFPA is addressing, not using an autotransformer at the final point of use.

I think the iRV2 post is confusing final point of use autotransformers with those installed further upstream in the distribution system where full transformers are more appropriate.

 

Edited August 13, 2019 by Lou Schneider

[Kirk W]

6 hours ago, Lou Schneider said:

I think the iRV2 post is confusing final point of use autotransformers with those installed further upstream in the distribution system where full transformers are more appropriate.

That would explain a lot about the code change since the Huges Autoformer is at the final use point, supplying only 1 RV and should never be cascaded. 

[Ray,IN]

That irv2.com thread is closed due this ongoing thread: http://www.irv2.com/forums/f103/autoformers-in-the-news-455709-2.html

From what little I remember from school, these devices do not have a net loss or gain, it's kinda an even trade-off.

Edited August 14, 2019 by Ray,IN