480 delta; neutrals & grounds....

[fsck]

I have two proposals offered to carry 480 Delta/200A service about 1000 ft. to a 208/120v destination.

One uses a 480:4160v 112.5 KVA transformer at the meter, 3*#6MV-105 cable, and a 4160:208/120Y transformer at the house.

The other approach uses 3*250 MCM cable to carry 480, & one 480:208/120Y transformer at the house.

The difference in costs is less than I'd have guessed, and the added loss of the 480 approach is also less onerous than I'd have thought.

Now, since the 480 is delta, we don't have a neutral to run. Also, the planner indicates local codes will not require a ground conductor between the transformers; just separate grounds at each location. That's less obvious to me....

Now suppose we run some 5 HP 3ph pumps right off the 480v at [z], not via the 208-3p transformer output. Does that somehow change any of the cabling requirements? The planner thinks so but I'm confused as to how.

 

[rbulsara]

Yes, using 480V motors will save the cost of intervening transformer, if you choose not to go 4160 route.

You have to compare the costs, but for such small load (and for residential, as it sounds like), there is a value in not involving medium voltage cables/ transformer from maintenance and serviceability by point of view over life of the installation. I would be inclined to use 480V and properly sized conductors for the voltage drop. You can also play with transformer taps and keep the cable from over-sizing, if this is the only load fed.

Rafiq Bulsara

http://www.srengineersct.com

 

[fsck]

Sigh; I forgot to include the graphic....

The 4160 aspects will be invisible to the owner; two pad transformers with buried MV cable between. As for maintenance, how much maintenance do the utilities do on their many thousands of treelawn transformers in neighborhoods? I can't recall ever seeing any PM work on such.

The 480 approach has twice the loss of the 4160, but is also ~0.75x the cost.

The variables are in future-proofing: some day, he wants to build a guest house 2.5x as far away from the shed; do we:

a) Go 4160 to house now, extend it from the house to guest house. [Plan the MV cable size for that, upsize the step-up xfmr then.]

b) Go 4160, plan separate leg to guest house [leave duct to house now...], and add 2nd/upsize stepup xfmr then.

c) Go 480 for the house; plan 4160 to guest house whenever.

But I'm still thinking grounds and neutrals. I'd think the two ends' grounds should be interconnected, if not for code reasons, just to avoid differential that any electronics would object to. [The shed will hold pump controls, sat TV gear, a RF link for Internet access, etc.] Perhaps I'll insist we do all that via fiber to avoid that problem.

 

[jraef]

I think you may be missing something. You cannot just feed the 4160V from transformer to transformer, you need switchgear with short circuit protection for the conductors in between. I don't see that on your graphic, that will greatly increase the cost.


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[rbulsara]

Was just my opinion. You are in charge of your project.

Rafiq Bulsara

http://www.srengineersct.com

 

[fsck]

A) I simplified things on my sketch; the proposed plan given to me has a 480/200A breaker on the feed to the transformer, and 40 amp MV fuses mounted at/in the transformer enclosure itself. But nothing visible/obvious to the owner.

B) I found this <

http://www.eng-tips.com/viewthread.cfm?qid=59602>

thread and am reviewing it re: neutrals. We would have only the step-down xfmr & 3-ph motors on the 480; no single leg loads.

 

[fsck]

"Was just my opinion. You are in charge of your project."

Less so than you might think. My tasking on the power stuff is to advise the owner, question assumptions the local designer made, and make suggestions. But eventually, the local designer and whatever PE will sign his plans will prevail.

And I came here seeking opinions, for which I thank you. This is not an area I have your expertise and experience in.

I think it likely we'll use the 480 route, despite the lengths/losses.

 

[alehman]

One other thought that may not be worth considering - run 480V 1-phase to a transformer to develop 240/120 1-phase at the house. Obviously you'd need to get single-phase well pumps or a converter. Would save you one wire, at the cost of more losses and possibly a converter and bigger transformer.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[racobb]

If there is already a designer and a PE to sign off on the installation, then I must ask just what is your job here?

Rafiq said that you were in charge of the project, but sounds like not!

Alan

 

[fsck]

But we're buying all the pumps {bigger than say 200W} in 3ph. That's why we're going 3ph to begin with. The remaining question is: how many on 480, vs 208?

It looks like there are four pumps of 5HP or more [thus 480 candidates], and 2-3 in the 1-4 range. All four 5HP's are near the house/pool but not actually inside the house, rather in the pool mech room. [I don't know if Code calls that "house" or not...]

What I'd like is somewhere to read up on "4 wire delta" esp. is that 4th wire a neutral, or a ground or..? Is it grounded at both ends, should it be bare in the connecting ditch, etc...

 

[fsck]

"If there is already a designer and a PE..."

As I said, this is ~incidental to my work on other things there [comms/entertainment/GSHP], but in general I've been told to look over the electrician's plans, ask questions, look for alternatives, say "why?" as needed, and similar.

I've only chatted with the electrician by email, & he seems to know his stuff, esp. re: local issues with the utility and code officials. But I've outlined a solution for an issue he thought unsolvable, for example.

 

[rbulsara]

fsck:

It sounds like your heart is at the right place but you may get yourself in trouble by getting involved in something out of your comfort zone.

You could be as candid with the Owner (or whoever is asking you to "look over" the electrician's plans), as you are here. Tell him that while you are good at what you do, power is not your expertise and let the electrician and the PE do their work. Or hire another "engineer" for a second opinion.

Rafiq Bulsara

http://www.srengineersct.com

 

[fsck]

I'm perfectly comfortable doing so, and see the good, not just the bad you seem to portend. I think most projects benefit from outside review and some contrary opinions. One trouble with trade people (and some engineers, to be frank..) is they say "but we've always done it that way...." [Or their thinking drifts to the same answers time and again.]

I have a long-standing relationship with the owner; who based on our past, trusts me to ask questions the electrician won't, and explore other answers than the initial one.

He knows that I'm not a power engineer. So does the electrician. I fully acknowledge that there are many details I won't know chapter and verse, at least initially. Along the way I expect to learn new things, as I have on past projects for him (& others). There's lots of give and take before anything is finalized.

 

[waross]

If you are running delta you may use ground fault detection.
If you use high impedance grounding you may use ground fault detection.
Pumps;

If you have a 480V 200A service you may have a four wire delta system. The fourth wire is the grounded circuit conductor, and the neutral for 240:480V circuits.
This is imaterial if you use a 4160V line.
If your pumps are 5Hp+ you are a candidate for single phase 120:240V.
I have seen 125Hp motors DOL on 120:208V circuits so no need for 480V
Overall
I would price out the various options, 480V 3 Ph,
4160V 3 phase and 4160V single phase for the project as it may develop with the addition of more distant loads in the future. You may find that the price for a future 480V system with voltage drops within code limits steers you towards 4160V even though the first stage installation may be more expensive.

Bill
--------------------
"Why not the best?"
Jimmy Carter