design question

[quinn77]

I have a total connected load of 140 amps at 240 volt single phase. this gives me a load of 34 kva. the source is 400 feet away from proposed panel, however city manager told GC to go as cheap as possible so AC contractor submits bid with single phase equipment. source voltage is 480/277 3 phase. im coming up with a design that seems more cost effective to install single phase 45 kva 480/240 volt txfmr with #2 feeders than 400 feet of 4/0 feeders. what would you guys do?

 

[defears]

Sounds fine.

Except for the source not being able to handle it, and any future loads, why not do it?

Maybe install a 480 to 240/120 transformer. Just in case. And easier bonding.

 

[augie47]

Based on your load, the numbers look good, however, you mentioned AC contractor. If any of the loads are significant size motors or compressors you might be undersized. You need to protect the #2 at it's ampacity and that might be pushing you on transformer inrush. Any chance you can obtain engineering assistance ?

 

[quinn77]

Based on your load, the numbers look good, however, you mentioned AC contractor. If any of the loads are significant size motors or compressors you might be undersized. You need to protect the #2 at it's ampacity and that might be pushing you on transformer inrush. Any chance you can obtain engineering assistance ?

well, here in texas us EC's are allowed to design up to 100,000 in electrical. so, my design experience is growing, but still limited. the source is an 800 amp 480/277 volt MDP with only a small load on it now. (2) 225 amp shore power circuits, and a 45 kva transformer for the building loads. building is a giant boat storage facility with future provisions for a crane to load boats. city hasnt submitted info on crane as requested nor have they provided electrical drawings on building. so, as usual, im responsible for this design. the AC will consist of 15 kw 1 phase heat, 3/4 horse blower, and 5 ton cond. unit. my concern was inrush of cond. unit with 45 kva transformer at 400 ft with #2. am i right in this thinking? should i upsize my txfmr?

 

[quinn77]

ps augie

ps augie

arn't i allowed to size my primary OCP device at 125% of 45 kva for inrush?
45,000/480 * 1.25 = 117a ( next size up ) = 125amp. wich then would require #1 feeders.

 

[kwired]

Tell the GC what it cost to go three phase vs single phase. And about unbalanced loading on existing facility and potential impact of that.

The HVAC guy apparently thinks that as long as his portion costs less then everyhing is in perfect harmony, when in fact he may be passing on more burden to others.

If the GC is going to allow his subs to design the project then he will have a bigger mess than if only one design team does it. If the subs can team up and do it together then that may be ok.

I have several times ran into HVAC guys that just install equipment someplace then you get a call to hook it up and find that it sure would have been nice to be notified at decision time of what equipment should have been installed. Sometimes the single phase equipment did not save any money after all is said and done. Several small units can be balanced across phases but larger units can't always be done as easily.

 

[quinn77]

Tell the GC what it cost to go three phase vs single phase. And about unbalanced loading on existing facility and potential impact of that.

The HVAC guy apparently thinks that as long as his portion costs less then everyhing is in perfect harmony, when in fact he may be passing on more burden to others.

If the GC is going to allow his subs to design the project then he will have a bigger mess than if only one design team does it. If the subs can team up and do it together then that may be ok.

I have several times ran into HVAC guys that just install equipment someplace then you get a call to hook it up and find that it sure would have been nice to be notified at decision time of what equipment should have been installed. Sometimes the single phase equipment did not save any money after all is said and done. Several small units can be balanced across phases but larger units can't always be done as easily.

agreed.