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Joined: Dec 2001
Posts: 440
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The specs call for 500 amps @ 240 volt, single phase for job trailers. The Utility is probably not going to provide that type of service. They will probably want to give us a three phase 480 volt set up. Part of the problem is that the utility company's transformers are over a 1,500' away. I wouldn't want to bring 500 amps @ 240 volts that distance, even if they would provide it. I'm thinking about bringing 150 amps @ 480, 3 phase from the utility to the job trailers, and feeding (3) single phase 45 KVA 480/240-120 volt transformers. Transformer 1 across phases A&B, transformer 2 across phases B&C, and transformer 3 across phases A&C. I will be setting fusible disconnects on the primary side of each transformer. The biggest problem that I have concerns the wording of the spec. It says "a 500 amp, 240 volt power source". The word "a" as we all know means one, and will they consider the (3) transformers as not meeting the specs. I'm going to place a call to the contractor tomorrow, and we should be able to settle the problem, but I thought that I would "throw" the situation out here, and see the discussion. Any input would be obliged.

Temporarily Insane,
Doc


The Watt Doctor
Altura Cogen
Channelview, TX
Joined: Apr 2002
Posts: 2,527
B
Moderator
45kVA is a standard 3ø dry-type rating; 50kVA is the next standard 1ø dry-type rating. Arts. 450 and 240 (+250) 220-calcs are probably the primary code concerns. Voltage drop allowances and available transformer-tap combinations? Xfmr grounding electrodes AND 480V eqpt grounds are needed. Voltage drop allowances? Motor starting/air conditioning?

208Y/120 may be tempting, cheaper but not if the trailers are labeled for 120/240 service.

I’d ask for a schedule of recpetacle loads to level the playing field. Any comm/fire alarm raceways as site improvements?

45kVA is a standard 3ø dry-type rating; 50kVA is the next standard 1ø dry-type rating. Arts. 450 and 240 (+250) 220-calcs are probably the primary code concerns. Voltage drop allowances and available transformer-tap combinations? Xfmr grounding electrodes AND 480V eqpt grounds are needed. Voltage drop allowances? Motor starting/air conditioning?

208Y/120 may be tempting, cheaper but not if the trailers are labeled for 120/240 service.

I’d ask for a schedule of recpetacle loads to level the playing field. Any comm/fire alarm raceways as site improvements?

Who made the "500A" determination?

Insanity if fine -- just don't drool.

Joined: Dec 2001
Posts: 440
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Bj,
You'll have to forgive me for saying 45 KVA. I should have said 50 KVA. It's a 3 phase world out there, and I'm used to living in it (thus 45, 75, 112.5, etc). The Project Specifications call for 500 amps @ 240 volts. The big question is, how strict are they going to be in their interpretation of their own specs.
208 volt, 3 phase is not an option, though I wish it was. Voltage drop is also an issue that I will be taking into consideration. Grounding, transformer taps, etc, etc, etc will all be considered.
Bj, thanks for the input, and if the rest of this job goes as the first few weeks have, drooling will be part of my daily routine.

Drool Rag in Hand,
Doc


The Watt Doctor
Altura Cogen
Channelview, TX
Joined: Apr 2002
Posts: 2,527
B
Moderator
Cryptic/thin specs are always fun.

I like to think for me, strategically placed drool may be useful on a hot day.

Joined: Oct 2000
Posts: 5,389
S
Member
Doc,
how many job trailers you got there?? are they all just lights and space heaters??

Joined: Mar 2002
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Ron Offline
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In using one 500A service you would have to distribute to seperate trailers. In the method you "prescribe", it seems that you could provide the same type of sidtribution, just from 3 transformers.
I can't see any benifit of having one 500A over 3 seperate 200A services.
Whos drawing does this requirement show on (architectual or electrical), it might lead to whether it was a stab in the dark or a calculated/particular requirement.


Ron
Joined: Apr 2002
Posts: 914
E
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You can probably convince the contractor to let you make a change. But what ever you do make sure you get a change order in writing exactly as your contract requires, otherwise they will use it against you at a later date when a problem arrises. I once worked on a project where the super would take photos of things like a pile of debris and then later try to say you just left it and try to back charge for clean up. We started taking our own photos with a date stamp after we finished in any area.

Joined: Jan 2002
Posts: 34
M
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Yeah Doc, I agree w/ everybody else in finding out what you're loads will be. Imbalanced loads on single phase spells trouble...

Joined: Dec 2001
Posts: 440
Likes: 1
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Thanks to all for the replies. I had a meeting with Houston Lighting & Power today. We have come up with a plan, but it is not "set in stone" yet. I will also add that (3) job trailers is now (4), and (1) service is now (2), and the additional service has (3) trailers on it..........man I'm out of breath. Does that make sense. So, how do I explain this, so that all the readership can get a mental picture of what will take place. Let me state this first. This project is located at Intercontenital Airport in Houston. These job trailers (at present there are 4 or 5 general contractors on the site with multiple trailers each) are located near the southern most runway of the airport. Now, I said to myself, "Self....(I actually said that)...what does that have to do with anything?..... Located near a runway..who cares..." Well.......the FAA cares, and if they weren't involved, I wouldn't be posting this message because the power company would have brought primary down the backside of all these job trailers, and we would have dropped straight off the utility poles to the trailers, but they couldn't because of FAA clearance requirements, money, surveys, money, paper work, money, red tape, and did I mention money?.....out of breath again...whew! That doesn't even bring up the fact that I'm dealing with a 60' roadway easement (which has been encroached by one of the GC's fencing for their "laydown yard"), a pipeline "right of way", and a cotton pickin' aerial telephone line that's in the way. Is the "hair ball" big enough yet? This work is for the contractor that has contracted the general that I work for.....yeap what a tangled web we weave. Did I let y'all know that if this temp. power is not complete by June 30th that there are LD's (liquidated damages) that my GC may be subject to. The best part, and this is killer, I'm going on vacation next week. So, when I get back this project will probably be full blown........I don't want to say it.
I will keep you posted.

Bj, pass the box of drool rags, please.....I don't think I can hold it any longer.... [Linked Image]

Temporarily Insane,
Doctor Drool


The Watt Doctor
Altura Cogen
Channelview, TX
Joined: Nov 2000
Posts: 2,236
Member
The plot thickens!

[Linked Image]

Hoo boy, the FAA... Really makes the NEC look tame.

Good Luck Watt Doc.


-Virgil
Residential/Commercial Inspector
5 Star Inspections
Member IAEI
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