Bbuck boost sfmrs

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NE (9.06 miles @5.9 Degrees from Winged Horses)
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I have been asked about using buck boost xfmrs in an RV park. Service is 120/208 wye and comes from an existing drop. They want to boost to 240v. I believe refering to 551.72 requiring "...shall be supplied from a branch circuit of the voltage class and rating of the receptacle." Two of the six pedistals would require 50A 125/250V receptacles.

Other than this, I see no reason to boost the voltage, and if needed I would use a 208 to 120/240 single phase transformer not buck boosts. Acme shows no wiring connection that includes the neutral in a BB. I believe the line to neutral voltage would jump to about 133v. Comments?

 

boater bill

Senior Member
Location
Cape Coral, Fl.
I would use the 208-120/240 volt single phase transformers. The RV parks are similar to marinas in the plug design and voltages and that is probably the best way to go.

Hope this helps.
 

tshea

Senior Member
Location
Wisconsin
The first thing to ask is How Many sites need to be 240V? If 1 or 2 then a buck boost will work. If all then consider a 208-240V transformer.
If you have access to the wiring diagram of the buck-boost transformer, look it over. It will not affect the neutral.
The receptacles will be fine.
The real problem is calculating the correct buck boost transformer since the load is unknown.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
ptonsparky said:
Two of the six pedistals would require 50A 125/250V receptacles.
Don't they now?

Other than this, I see no reason to boost the voltage, and if needed I would use a 208 to 120/240 single phase transformer not buck boosts.
Is there no concern with loading only two of the phases with the entire load?

For that matter, won't there be some existing 3-ph feeder rearranging required?
 
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
LarryFine said:
Don't they now?

Not sure by what your comment means. .2 * 6 = 1.2 rounded up to 2 (551.71)
Is there no concern with loading only two of the phases with the entire load?
Not from the information I have at this time. Existing service feeds a motel, this is an added feature.

For that matter, won't there be some existing 3-ph feeder rearranging required?

I have to ass-u-me that a feeder will be brought from the motel to the RV park area.

My biggest concern is what is the Line to neutral voltage after a BB is installed for the two pedistals. One leg will remain at 120V, the second appears to be the total of the voltage increase L-L. 12v boost would be 132v or 24v boost would be 144v. Yes/No??

Another would be: Is a 1ph 120/240v service or feeder required for the 2 50A 125/250 4 wire pedistals?

If it is, BB would not be allowed because we only changed the voltage, not the distribution system, regardless of the answer to L-N voltages.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
There are a bunch of different ways to connect 'buck-boost' transformers, especially if you generalize the concept to include any non-isolated arrangement of transformers in a three phase system.

If you have a 208/120V supply and you need 240V _single phase_, then the simplest arrangement uses one single phase transformer, with 208:32V ratio; the primary is connected to the 208V supply, and the secondary is added in series with the 208V supply. In this case, one of the 240V terminals is common with one of your your input 208V terminals, and thus remains at 120V relative to neutral. In this case, the _other_ terminal will be at 149V relative to neutral.

A somewhat more complex arrangement uses _two_ single phase transformers, with 120:18V ratio, to boost each phase to 138V relative to neutral. The phase to phase voltage would then be 240V, balanced relative to neutral, but the line to neutral voltage is now 138V.

If you are willing to go with larger transformers and non-standard connections, you can design non-isolated transformer arrangements that will give you true 120/240V single phase...but really, why bother? Stick with a standard single phase isolation transformer and save the headaches. If there is higher voltage available (480/277V, for example), then use the higher voltage as your primary and save a bit on the voltage drop.

-Jon
 

micromind

Senior Member
I highly recommend that you do not boost voltage to 240. There's no way I know of to boost the line to line voltage without also boosting the line to neutral voltage when using two legs of a wye. Most, if not all, loads in an RV with a 50 amp plug will be line to neutral, thus it's far more important to maintain the line to neutral voltage at 120.

One possible issue with using a 120/208 system is customer metering. If each site is individually metered, you'll need to add a 5th jaw to the meter base, and get meters labeled 120V 3 wire.
 
I will agree with most of them and majorty of the RV park i did see most used the 120v load very rare if i did see the 240v load at all.

the only time i did see the 240 v load is big arse RV camper or semi perment RV and mini cabins.

i did take a look on one of few RV's about the 50 amp connector with 120/240 volt 4 wire connectors and i read the spec's amost all of them are 120 volt loads there.

very common with multi A/C that wired for 120 v and thier breaker box they have pretty goofy way set up the connections there [ i am not getting into that part for now ]

Merci, Marc
 
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