Author Topic: Reallly long panel feeder - how to connect to panel and feeder breaker?  (Read 1207 times)

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Offline danjenkins

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I'm working a project involving great distances from a main switchboard or power panel to some 250A 480/3 panels. The champion is an 850-foot run. In order to keep voltage drop to less than 3 percent, I calculated the need for 4 sets of 350 MCM per phase at an 80 percent load on the panel (200A) for this longest run.

My question is, I haven't found a 250A panel that can accept 4 x 350 MCM per phase at the main lugs, nor a switchboard breaker that can do so either. Do we need to spot a wiring box at each end to reduce the feeder size to "normal" (1 x 250 MCM) at each end?

Sorry for the elementary question - I've just never run into these distances before. I've always been able to space my mains as needed to reduce feeder distances, but not in this cavernous building.

Thanks for your help!
 

Offline tpowell1830

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First of all, don't do this run. Get the power company to put in a separate 480/3 transformer closer to your power recipients. It will be much cheaper than running that line. The panel will then be manageable. If the first advice can't work, then get a transformer that will raise the voltage to 880 VAC. You will need to engineer your run more carefully at 880 volts, but it is doable. This will cut your current in half and then place another stepdown transformer at your recipients. Half the current is manageable with smaller wire. This will probably be more costly than getting the power company to install the transformer on a pole (or ground, however it is done in your area).

You are really asking for trouble making that long of a run, either way.

Hope this helps...
« Last Edit: May 21, 2018, 12:23:40 am by tpowell1830 »
PEACE===>T
 
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Online rstofer

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You don't have your country flag enabled but, assuming, US, things get a little more complex when you move about 600V.

Talk to the utility, they may set a transformer or they may not.  In general, they won't serve a single building with more than one service but they might if the existing service is some low voltage like 120/208 or 120/240 and the new service would be at 480V.

So, how come you have such a long run at low voltage?  Your proposed solution is not unworkable and, yes, you would install terminal blocks in a box above the panel to bring the number of cables down to one.  I certainly hope you don't have any large motors to start at the far end.  The flicker is going to be an issue.  You might research 'flicker calculation'.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.516.6521&rep=rep1&type=pdf

 

Offline tpowell1830

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SKIP

Talk to the utility, they may set a transformer or they may not.  In general, they won't serve a single building with more than one service but they might if the existing service is some low voltage like 120/208 or 120/240 and the new service would be at 480V.

SKIP

If you are in the US, the power company will absolutely run another 480/3 service. In some areas, perhaps, where rstofer is, maybe not. But the power companies definitely do it in my area. You should check with them for your area, as it is, IMO, the most reasonable solution.
PEACE===>T
 

Offline IanB

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I suspect big consumers can buy their power at the MV distribution voltage and then supply their own LV step-down transformers for the end user requirements. How big a consumer do you have to be for this to be practical or economical?
I'm not an EE--what am I doing here?
 

Offline Red Squirrel

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First of all, don't do this run. Get the power company to put in a separate 480/3 transformer closer to your power recipients. It will be much cheaper than running that line. The panel will then be manageable. If the first advice can't work, then get a transformer that will raise the voltage to 880 VAC. You will need to engineer your run more carefully at 880 volts, but it is doable. This will cut your current in half and then place another stepdown transformer at your recipients. Half the current is manageable with smaller wire. This will probably be more costly than getting the power company to install the transformer on a pole (or ground, however it is done in your area).

You are really asking for trouble making that long of a run, either way.

Hope this helps...

The issue is this would essentially require a second hydro service, then you're paying all the fixed fees etc for that service too.  Will be much more expensive in the long run.  It will involve having two hydro bills instead of one.  Most of the cost of the bill is the fixed fees and not the usage.

One option might be to source out two big step up/down transformers and do your own HV transmission line.  Though if this is 3 phase it gets more complicated.  If 3 phase is not a requirement you could do a single HV line with a centre tapped transformer at the other end to get a standard split phase service.
 

Online Gregg

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There are a couple of possible solutions.  A lot depends on the space available in the 250 amp panel. 
First suggestion is to find some power terminal blocks and mount them in a separate box ahead of the panel.  Make sure you get them rated for the available short circuit current.  This can also be used at the source end.  Here is an example:
https://ab.rockwellautomation.com/Terminal-Blocks/Power-Terminal-Blocks
Another solution is to mount the main disconnect in an oversized separate box.  IEM used to make me anything I wanted as long as it was reasonable and within code requirements.  The prices were reasonable as well and the big plus was that I could talk directly to the engineering staff and not just silly sales people.
http://www.iemfg.com/
A company like IEM could make you a custom oversize complete panel board to use any of the major breaker company‚Äôs main and branch circuit breakers with added bus extensions to the main breaker input and lugs for 4x350mcm cables.  Remember, labor of installation can eat up any savings on purchasing switchgear.
Enclosed is a picture of a custom size 480 volt panel that IEM made Nema3R in a wet location.  I needed it when I had to convert a long run of 208V to 480V for added equipment.
 

Offline richard.cs

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In the UK it's mainly highway supplies that have this problem, we can end up with several miles of enormous cable to run two streetlights and an illuminated sign. It's usually terminated in sensible size both ends and stepped up/down in size at an underground joint within 10m or so of the endpoints. Your approach sounds plausible and you would just reduce the size at the ends as you've said, but it is on the limit of what's economically sensible. As other's have said it would be worth considering a second supply or stepping up to a higher voltage, when working out the relative costs of the different approaches don't forget to include the cost of the line losses, they will be significant if it's continuously loaded to that level. I make it nearly around 6 kW of cable losses based on your 3% Volt drop.

A rule of thumb that works reasonably well is that when you get to around a metre per Volt you should consider distributing at a higher voltage, you're certainly approaching that point. If the supply is to a specific piece of equipment can you relax the 3% requirement a bit? Although that increases the line losses further.

 

Offline danjenkins

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There are a couple of possible solutions.  A lot depends on the space available in the 250 amp panel. 
First suggestion is to find some power terminal blocks and mount them in a separate box ahead of the panel.  Make sure you get them rated for the available short circuit current.  This can also be used at the source end.

This is the direction that we are currently heading. We plan to group each set of four conductors onto a pair of commercially-available stud terminals with a link bar across them, and take something more manageable out to the load and source. We are looking at Weidmuller's WFF series. https://www.clrwtr.com/UserFiles/CT/Documents/Weidmuller/Weidmuller-Screw-Connection-Stud-Terminals.pdf
 

Online rstofer

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I suspect big consumers can buy their power at the MV distribution voltage and then supply their own LV step-down transformers for the end user requirements. How big a consumer do you have to be for this to be practical or economical?

See Sheet 11 of PG&E Electric Rule No. 2

https://www.pge.com/tariffs/tm2/pdf/ELEC_RULES_2.pdf

For 12kV, the minimum load is 1000 kVA.  It's 500 kVA for 2400V or 4160V

Fire departments want a single service entrance point - one place where they can cut all the power to the building.  This is the reason that utilities don't want to provide two service entrances at 480 and 208V.  They figure they will supply the 480 and the customer can create their own 208V.  The fire department only has to deal with disconnecting the 480 service.

 

Offline T3sl4co1l

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I once toured a facility that was metered on MV (and they seemed proud of it, owning their own (on site) poles, switchgear and transformers, hence why I remember this).  Unsure what service it was (maybe 12kV?), or how much they used average/peak, but they did have several MVA of capacity, at least -- which was enough to test the biggest equipment they made, at full power, which is nice.

Tim
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Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline jmelson

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One option might be to source out two big step up/down transformers and do your own HV transmission line.  Though if this is 3 phase it gets more complicated.  If 3 phase is not a requirement you could do a single HV line with a centre tapped transformer at the other end to get a standard split phase service.
If this is a new installation, how about having the power company supply you with 4160 service, and you get two 4160/480 transformers, one at each building?  (I'm assuming the 850 feet separation implies two buildings.)

We have 4160 distribution at our university, they have a master 4160 breaker panel, and then send the 4160 underground to each cluster of buildings.  They have pad-mount transformers at each cluster, feeding whatever the building needs, either 480, 240 delta or 120/208 Wye power.

With 2 buildings, this may or may not make economic sense, but with more than 2, it really starts to be better.

Shipping 480 V 850 feet could lead to real issues with harmonic distortion and as others mentioned, severe dips when large loads are started.

Jon
 

Online rstofer

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The thing is, we don't know diddly about WHY there is such a long feeder.  We don't know if this is new construction, how many buildings on site, are the served areas on the same piece of property, which utility company (tariffs vary), utility capability, other info on any existing electrical system.  And so on.  We don't know diddly...

All we know is that it is a really long way to another location and the OP intends to serve it at low voltage and accommodate the voltage drop by paralleling cables.  We do know that flicker will be a problem if there are large starting currents.

But, not knowing diddly, I'm at a loss to make any other suggestion.
 


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