Long self storage building design help

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Strombea

Strombea

Senior Member
Location
Prescott, Arizona, USA
We are designing and installing 2 lights and 1 receptacle in each unit in a 80 unit building that is 600 ft long. The customer wants each unit on its own 20 amp breaker and they won’t budge. my original design is that the units within 100 ft get #12 MC, the units from 100 ft to 250 ft get #10 MC but per my math, after 250 ft I should be at #8 and towards the end of the 600ft I should be using #6. So I had designed a 240v to 480v step up TX @ 50kva that served a 480v 100 amp panel, each home run(unit) would then get its own step down 3kva TX mounted in the unit, fed from the 2 pole 480v, this allowed me to still use #10

FYI I’m using 15 amps at end of run and 5% max VD , no continuous loads.

there is No where to add a sub panel in the building as they want all units for rent income and there is only 24” between roll up doors on the face, so I can’t do a sub panel anywhere it seems. The step down tx are around 300$ each @ 40 units is around $12,000 extra to not have to use #8 and #6 wire


any suggestions???
 
Find out why the customer wants a separate 20A circuit for each unit; that will inform your design.

Do they simply want to be able to separately turn off power to each unit, or do they really want to be able to deliver lots of power to all units simultaneously, or do they not want to be bothered when to adjacent units are doing something at the same time and trip a breaker?

Do they need the breaker panel to be in a 'controlled location' and is that location fixed?

Do they have existing installations that they have problems with that they are trying to resolve?

You obviously figure that the actual power consumption is far below 1.8kVA per unit, and I bet an article 220 calc would come in below 25kVA.

-Jon
 
I wired 3 storage buildings for RVs. Each unit was 50' deep and up to 14' wide. One building was around 300' long. They also wanted a 20A circuit for each unit. I put a subpanel half way down. I was going to put the sub outside in back so the unit could be rented. Customer decided to just put it inside and they would build a small closet around it. Saved a lot on wire by adding the sub, and no transformer involved.
 
Little Bill said:
I wired 3 storage buildings for RVs. Each unit was 50' deep and up to 14' wide. One building was around 300' long. They also wanted a 20A circuit for each unit. I put a subpanel half way down. I was going to put the sub outside in back so the unit could be rented. Customer decided to just put it inside and they would build a small closet around it. Saved a lot on wire by adding the sub, and no transformer involved.
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Yes and this makes sense and I did this on the last job but the customer is adamant that they can rent that unit out and if someone's breaker trips because of a battery charger or a little space heater that they can go reset the breaker without having to call someone to unlock their unit but it seems like I will just tell them if they don't let me use the unit halfway down there may be a $20,000 extra cost in transformers and step downs
 
Tap rules can be your friend.

You don't need transformers. You just need thicker wire.

The problem is that running a separate thick wire to each unit would be crazy over-building, because most of the time most unit won't be drawing anything near the capacity of a single 20A circuit. If each unit is drawing 1.8kW on a continuous basis (15A at 120V) then your customer will have bigger problems than the cost of wire! (Or your customer is getting so much in rent that the cost of wire is peanuts.)

So run 2 or 3 50A 240V circuits. Size these for voltage drop with loads distributed along their length.

If because of voltage drop your 50A circuit needs 100A wire, then put a 100A breaker on it.

At each unit tap off this circuit with to a small local panel with a single breaker in it.

If someone's breaker trips, it is in their unit, no need to call anyone. Plus no one can play with the breakers for other people's units. If one of the main feeder circuits trips, then there is a problem and management should probably know about it anyway.

-Jon
 
I agree with most of the responses above.
Don't use transformers.
Set the main panel in the center of one side of the building.
Set subpanels halfway from the ends of the building to the main panel. Subpanels can fit between 24" doors. I'm thinking four subs total, two on each side of building. I didn't do the voltage drop math. It might be cheaper to put in additional subpanels if many of the individual branches have to be upsized.
I would not set an individual panel in each unit. That would drive up the expense too much.

I would run EMT between panels. MC out of each subpanel to units. Running 3C MC MWBC's to two units might save money. I didn't do the math.
 
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It just occurred to me that running MWBC's would require tying two unit breakers together which is probably not what they wanted.

Also, EMT risers out of each subpanel to a group of j-boxes or a trough will give you more KO's to connect the MC cables. Just watch your conductor fill.
 
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zbang said:
24" is more than enough for a 8 or 12 space panel.
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winnie said:
Tap rules can be your friend.

You don't need transformers. You just need thicker wire.

The problem is that running a separate thick wire to each unit would be crazy over-building, because most of the time most unit won't be drawing anything near the capacity of a single 20A circuit. If each unit is drawing 1.8kW on a continuous basis (15A at 120V) then your customer will have bigger problems than the cost of wire! (Or your customer is getting so much in rent that the cost of wire is peanuts.)

So run 2 or 3 50A 240V circuits. Size these for voltage drop with loads distributed along their length.

If because of voltage drop your 50A circuit needs 100A wire, then put a 100A breaker on it.

At each unit tap off this circuit with to a small local panel with a single breaker in it.

If someone's breaker trips, it is in their unit, no need to call anyone. Plus no one can play with the breakers for other people's units. If one of the main feeder circuits trips, then there is a problem and management should probably know about it anyway.

-Jon
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This is what I agree with.

Heck, run a 70-amp circuit and put 20 units on it

Tap to a small 4-space single phase panel in each unit
 
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