FEEDER CALCULATIONS & MOST EFFECIENT INSTALL

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mstrlucky74

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NJ
Here is the scenario:

We need to run a temporary feeders for a tower crane. The total vertical distance 800' but the first run will be to the 5th floor then jumped every 6 floors So essentially a feeder will be run to a box on the 5th floor for the crane then when the crane is moved up a few floors the feeder will have to be spliced and extended up a couple of floors. The load will be 500a fed from a 480v. Engineers/PM calculated thhn Al and it would be two sets of (3)750 +n+g to be run in EMT. I am curious about a few things. Is this the best way to do this?

The calculation of 750mcm is correct(based on VD) but that is based on the 800' run. In the beginning stages the run will be shorter obviously. Can you start will smaller wire then increase the size as run increases? How about running (3) sets of a smaller outdoor rated cable assembly instead of conduit? I say (3) sets because I don't think there is an So or SEU cable assembly that comes in 750mcm. Any thoughts? Thanks a lot
 
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GoldDigger

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mstrlucky74 said:
The calculation of 750mcm is correct(based on VD) but that is based on the 800' run. In the beginning stages the run will be shorter obviously. Can you start will smaller wire then increase the size as run increases? How about running (3) sets of a smaller outdoor rated cable assembly instead of conduit? I say (3) sets because I don't think there is an So or SEU cable assembly that comes in 750mcm. Any thoughts? Thanks a lot
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You can start out with smaller wire, but if you do that leaves even less voltage drop for the rest of the run so you have to use even bigger wire. The end result is that you use a lot more pounds of wire by the time you get to the top than if you keep the same wire size throughout.

Simple example to drive home the point:
I want to go 120 miles in two hours, with time out for lunch halfway. I can either go 60 mph for the first 60 miles, have lunch and then go 60 mph to the end. Or I can decide to enjoy the scenery and save gas by going only 45 mph for the first half. But that leaves me only 40 minutes to go the last 60 miles so I end up having to go 90 mph. Net result: more fuel used and very frazzled nerves.
If I only go 30 mph for the first half, I have zero minutes to go the second half. OOPS.
 
iwire

iwire

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Staff member
Location
Massachusetts
If the job has a tower crane it is a big job and with big jobs comes very large costs for temp power. I would not spend a ton of time trying to nickle and dime this.

It strikes me that I might run a feeder from the source to the crane base in conduit and at the crane base switch to portable power cable with cam locs for the vertical portion. You can rent portable power cable but if it is long term it might be cheaper to buy it and at the end of the job sell it or scrap it.
 
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