208 to 240 - Boat Yard

[Npstewart]

I know ive seen this question on here a few times over the years but cant seem to find it.

I need to power around about 8+ boat charging pedestals for a boat yard. The client is insistent on using 240v instead of 208 because of the refrigerators and air conditioners. Im sure most can withstand the 208 but he is sure some of the motors would burn up.

Service voltage is a standard 208/3. Im not finished with the loads yet but im assuming it will be around 50 kVA.

What is the best way to go about going from 208 -> 240. The load is to high to even consider single buck boost transformer and the Square D product selector doesent let me use a primary voltage of 208 with a secondary voltage of 240.

The last time I did something like this I had 480/3 available.

Thanks!

 

[Smart $]

Pedestals are 120/240V 1? 3W?

 

[augie47]

I appears to me the wording of 555.19(A)(3) would preclude supplying 240v marina receptacles from a 208v supply.
With a neutral as part of the system, buck-n-boost is ruled out also.
I stand to be corrected but IMO, a 208 to 240/120 transformer is the answer.

 

[Besoeker]

I know ive seen this question on here a few times over the years but cant seem to find it.

I need to power around about 8+ boat charging pedestals for a boat yard. The client is insistent on using 240v instead of 208 because of the refrigerators and air conditioners. Im sure most can withstand the 208 but he is sure some of the motors would burn up.

Service voltage is a standard 208/3. Im not finished with the loads yet but im assuming it will be around 50 kVA.

What is the best way to go about going from 208 -> 240. The load is to high to even consider single buck boost transformer and the Square D product selector doesent let me use a primary voltage of 208 with a secondary voltage of 240.

The last time I did something like this I had 480/3 available.

Thanks!

Just out of interest, why do you think a 50kVA load is too high for a buck boost arrangement?
Bear in mind that this transformer arrangement has to be rated for the voltage ratio rather than the full load kVA.

 

[Npstewart]

Pedestals are 120/240V 1? 3W?

Yes, all pedestals are single phase with a varying arrangement of 30A/120v, 50A/240v.

 

[Npstewart]

Just out of interest, why do you think a 50kVA load is too high for a buck boost arrangement?
Bear in mind that this transformer arrangement has to be rated for the voltage ratio rather than the full load kVA.

I could be wrong but when I went on the Schneider website and used their calculator, as soon as I exceed 12 kVA, it gave me an error. Anything below or equal to 12 kVA gave me a nice little wiring diagram.

http://www.buckboostcalculator.com/

 

[Besoeker]

I could be wrong but when I went on the Schneider website and used their calculator, as soon as I exceed 12 kVA, it gave me an error. Anything below or equal to 12 kVA gave me a nice little wiring diagram.

http://www.buckboostcalculator.com/

I tried the calculator and got this:

Transformer: 2S46F
Actual KVA: 51.10
Frequency: 60HZ
Number Required: 3

It gave three single phase units with a diagram.

 

[iceworm]

... I need to power around about 8+ boat charging pedestals for a boat yard. The client is insistent on using 240v instead of 208 because of the refrigerators and air conditioners. Im sure most can withstand the 208 but he is sure some of the motors would burn up. ...

I'd say your client is dead on. auggie picked it right out. Your client likely read the informational note.

... Service voltage is a standard 208/3. Im not finished with the loads yet but im assuming it will be around 50 kVA. ...

Yes, all pedestals are single phase with a varying arrangement of 30A/120v, 50A/240v.

So, you need 50kva of 120/240 single phase, and the service is 208/120, 3phase?

If so you have a bit of a mess.

No, you can't use buck/boost. Using one 208/32V, 200A transformer gets you 50kva at 240V but there is no 120V neutral. Using three 120/19V transformers gets you 240V, but the phase to neutral is 139V. Not going to work.

As augie said, the answer is single phase, 208 primary, 120/240V sec. If you wish to load all three phases fairly evenly, it will take three transformers, say 15kva each.

ice

 

[Npstewart]

I'd say your client is dead on. auggie picked it right out. Your client likely read the informational note.

it will take three transformers, say 15kva each.

ice

Ive done this arrangement about 5 years ago but cant remember exactly how it was done. I dont expect you have a wiring diagram of this handy?

 

[iceworm]

Ive done this arrangement about 5 years ago but cant remember exactly how it was done. I dont expect you have a wiring diagram of this handy?

3ph, 208V to 3 each 1ph, 240V. I don't ever recall ever being asked to do this. Still, how tough can it be.

Try this. Your 3ph 208V panel will have 3 each, 2pole, 90A CBs feeding the three 208/240V transformers.

Each 240V xfm will feed its own 100A panel. If you use 100A conductors from the xfm, the 100A CB in the 240V panel will be fine.

See if the attachment makes any sense.


ice

 

[winnie]

'Buck-boost' transformer configurations are also called 'autotransformer' configurations. The key feature is that the output circuit is electrically connected to the input circuit, and the KVA carried by the transformer itself is only a portion of the load.

You can use an autotransformer arrangement to derive 120/240 single phase from a 120V supply, and the transformer will only carry half the KVA of a balanced load. The approach is to use a single phase 120V:120V transformer, with the neutral common to both primary and secondary. This will supply a 50KVA 240V load using a 25KVA transformer.

This sort of autotransformer arrangement would put all of the load on a single phase, probably not desirable.

Using a 50KVA 208V to 120/240V single phase transformer will be the simplest approach, and will put the loading on 2 of the 3 supply phases.

You might be able to find a three phase transformer with a 'double delta' secondary, which would permit you to provide a single phase output with the load spread over all three supply phases

You could use a set of 3 120V: 120V transformers, with 1/3 of the load on each.

-Jon

 

[Npstewart]

3ph, 208V to 3 each 1ph, 240V. I don't ever recall ever being asked to do this. Still, how tough can it be.

Try this. Your 3ph 208V panel will have 3 each, 2pole, 90A CBs feeding the three 208/240V transformers.

Each 240V xfm will feed its own 100A panel. If you use 100A conductors from the xfm, the 100A CB in the 240V panel will be fine.

See if the attachment makes any sense.


ice

Definitely makes sense, I really appreciate you drawing the diagram! Most likely this is what will have to be done unless the below will work.

I havent thought this through cause I just thought of it while I was typing so it may not work but basically I need two voltages; 120 & 240 for the pedestals. I cant get the 240 from the 208 source without using the single phase transformers, but I already have the 120v from my 208 source.

I am already using a step up transformer to feed a bunch of overhead lighting so I am going from 208 to 277/480. I dont see why it wouldnt be possible to feed a transformer from my 277/480v lighting panel to get the 240, and then use the 120v I already have to feed the 120v receptacles.

So here is my single line riser :


208/3 (utility) ---> 480/3 (step up)---->240/3 (step down) ---> 240 receptacles in pedestals

and for the 120v required:
208/3 (utility) ---> 120v receptacles in pedestals

 

[augie47]

What Code cycle is enforced on the job ?

 

[Npstewart]

What Code cycle is enforced on the job ?

NEC 2008


Sent from my iPhone using Tapatalk

 

[iceworm]

Definitely makes sense, I really appreciate you drawing the diagram! Most likely this is what will have to be done unless the below will work.

... I need two voltages; 120 & 240 for the pedestals. I cant get the 240 from the 208 source without using the single phase transformers, but I already have the 120v from my 208 source.

I am already using a step up transformer to feed a bunch of overhead lighting so I am going from 208 to 277/480. I dont see why it wouldnt be possible to feed a transformer from my 277/480v lighting panel to get the 240, and then use the 120v I already have to feed the 120v receptacles.

So here is my single line riser:

208/3 (utility) ---> 480/3 (step up)---->240/3 (step down) ---> 240 receptacles in pedestals

and for the 120v required:
208/3 (utility) ---> 120v receptacles in pedestals

That could work. But I'm not seeing the driver to do it this way - you will know that better than I would.

You will need to do something about grounding the the 240V, 3ph. Were you thinking of using 240/120 high leg? If so, and you want to keep the system evenly loaded, then the 240V receptacles will not have a neutral. I don't recall much from my sailboat/marina days, however, I remember the 240V receptacles were 120/240 - similar to an RV park, The boats only had one shore power connection - either 120/240 or 120. The 240V receptacles had to supply the 120 loads as well. But that was thirty years ago - things change.

Or you could go with corner grounded in which case you can even load the phases, but there is no neutral for the 240V receptacles.

And there may not be any money is doing the step-up followed by a step-down:
The 480/277 xfm will have to be bigger so they can supply the 240V receptacles. But that is balanced by the 240V xfm are maybe smaller - but maybe not, depends on if they have to supply 120 along with the 240V.

And you are installing the same number of transformers using either one.

Welcome to the world of being the Engineer-of-Record. You got a bunch of design decisions to make:
Balanced loading
120/240 receptacles or 240 only receptacles
Transformer sizing

Figure the loads, draw it out, do the take-offs. You don't need a plotter. 8.5 x 11s and a pencil is fine. One of the solutions will shine.

ice

 

[Npstewart]

Were you thinking of using 240/120 high leg?

Wasnt planning on it, dont have any use for it cause I have the 208 already.

I remember the 240V receptacles were 120/240 - similar to an RV park

I think you're right which would be reason enough to go the other route.


This is too much thinking for a Sunday. I miss the weekends where I could just sit on the couch and watch Meet the Press in my underwear while eating my fruit loops.

 

[augie47]

...........................................
This is too much thinking for a Sunday. I miss the weekends where I could just sit on the couch and watch Meet the Press in my underwear while eating my fruit loops.

 

[iceworm]

.... I miss the weekends where I could just sit on the couch and watch Meet the Press in my underwear while eating my fruit loops.

Okay - Now I'm going to have to go bleach my eyeballs.

The worm turneth and begins to dig --

 

[Smart $]

...I remember the 240V receptacles were 120/240 - similar to an RV park...

...I think you're right which would be reason enough to go the other route...

Do your pedestals have integral ocpd before the receptacles?

If yes, you might consider a smaller 1? 208 PRI - 120/240 SEC xfmr for and at each pedestal. Run circuits as 3? out of 208/120 panel. You'd have to price it out for economical comparison, but you wouldn't have to put in three 120/240V panels.

 

[iceworm]

... you might consider a smaller 1? 208 PRI - 120/240 SEC xfmr for and at each pedestal. Run circuits as 3? out of 208/120 panel. You'd have to price it out for economical comparison, but you wouldn't have to put in three 120/240V panels.

Good idea. Maybe a 5kva or 7.5kva xfm per pedistal. Even if you ran a 1ph, 2w, 208V for each, that's not too bad.

Definitely consider this

ice