Feeder Size

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frogg

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We have a 120/208v 3 phase 4w 250-amp distribution panel we are installing. It is 800 feet from the electrical room. Using the WireGuide99 calculator, it shows that we need to install parrallel runs of #350mcm per phase. If this is right, then we need to run 2-3" conduit runs with all three phases, nuetral and ground in each conduit, right?

Thx
Frogg
 
Re: Feeder Size

I haven't calculated the wire size, but the first thing I would change is the conduit size.

The cost difference between 3 and 4 inch would be made up in the cable pull, and lack of damage to the cable.

[ July 04, 2003, 05:28 PM: Message edited by: bennie ]
 
Re: Feeder Size

Frogg if this is a feeder to a distibution panel, you will want to keep the voltage drop to 3% or less at full load current, otherwise with the additional voltage drop (2%)encountered on the branch circuits will get pretty high (greater than 5%). There is nothing in the code that limits voltage drop to 5% percent of less, but it is a good design goal.

2-350 MCM per phase is not going to get you 3% at 250-amps with 800 feet, more like 4-400MCM per phase to get 3% at 800 feet.
If you run multiple conduits, you will have to run at least one of each phases, neutral and EGC in each conduit.

The distance is killing you. Dereck

[ July 04, 2003, 08:01 PM: Message edited by: dereckbc ]
 
Re: Feeder Size

I always oversize my conduit, so the 4" is a given. This is not my project, but an installation that has been engineered by my customers engineering firm, and is being installed by the general contractor's electrical contractor. I am the electrical contractor that is doing subsequent work in the new facility directly for the customer, not the general.

I noticed that they were installing these feeders in this manner, and I am questioning their method.

I for one would have engineered this completely different. First of all, you don't haul low voltage (120/208) 800 feet through a warehouse into an area that has no existing power. You take high voltage and then step it down once you reach your destination. I would have taken a 400 amp 480 volt circuit out there, then installed a transformer to get 120/208 225-amp PLUS about 300 amps of 480 left for lighting and forklift chargers. I also would have designed it pulling in at least three or four parallel runs.

They have installed one (1) 3" line with 400 mcm for this distribution panel. I said to myself that this can not be, and yet they have approval from the city of LA to complete it as specified.

I am going to have to fix this after they have occupancy... Plus pull in the neccessary 480 volt for 14 forklift chargers. I just wanted to get confirmation that I wasn't crazy, and that the city of LA must have missed something, or the contractors are not reading the prints right.

As for voltage drop, I always do my customers and myself the favor of never using more than 3%. Plus I oversize the wire. We have numerous issues of low voltage from the power company because of heavy usage here in Southern California, so the more precautions one takes the happier the customers. Computer equipment is very touching once you get down to a 3 to 5% loss.

Thanks for the input.

Frogg
 
Re: Feeder Size

The first number necessary for calculating the voltage drop is the calculated load. This 250 amp panel may only be loaded to 180 or so.
 
Re: Feeder Size

In reality, knowing that this is a new construction, and total loads are not yet known, we have to assume they may at some time in the future utilize the panel to its fullest capacity. Not to what the calculated current loads are. That would be very residential type thinking, but in the commercial/ industrial world where every panel is loaded to its max, we must assume that they will eventually load this 250 amp panel to approximately 200 amps.

I'll guarantee you that knowing this customer, it won't be long before they would use the full 200 amps. This is again why you don't pull 120/208 that far. You pull a 480 volt 400 mp or even 480 200-amp, then install the 208 250-volt for now loads, and have the capability to add another sub panel in the future when they need more power... which they will.

If you're going to spend the money on the labor and conduit and wire... all of which costs the same whether its 250 amps at 208 or 250 amps at 480. the cost for materials and labor are the same. So pull the 480... and have extra power for expansion.

For what they are installing, if they were to land it in the 480 volt switch gear instead of the 208 gear, they'd end up with twice the power capacity with the same costs.

Plus... If all they ever needed at any time now or in the future was only the 250 amps at 208, it still would cost less to pull in a 125-amp 480-volt run in smaller conduit... smaller wire ... easier pull, then put in a transformer at the destination. You just don't drag 208v 800feet through the rafters. That's why the power company doesn't give everybody 120/208, because it's more economical to pull the smaller wires for the 480, and then install the transformer at the destination.
 
Re: Feeder Size

frogg,

Go ahead and work up your estimate for the improved job and have it on standby for when they ask you to come back and do it right. :roll:
 
Re: Feeder Size

On the brighter side, you can use the single 3" conduit and the 400 mcm cables as the primary feeder to the 480-120/208 transformer that they are sure to want some day.
 
Re: Feeder Size

I hope someone made sure the lugs on each end of the feeder will accomodate parallel 350's. May not be a standard for 250-amp circuit breakers or panels but I do not know for sure.
 
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