conductor sizing

[estim8r]

We have schematic/narrative design that includes feeders for pairs of
300A, 200,000 CFM fans exhausting a transit tunnel.
I'm not an engineer.

Ugly's Electrical References (book) suggests that 3/350kcmil conductors might be adequate to feed one fan, and further that these will fit in ONE 4" RGS conduit.

The engineer reviewing the estimate came back with THREE 4" conduits and
an unspecified quantity of 600 kcmil conductors, PER FAN.
It appears the feeder run is 2,200 feet so quite a lot of derating applies.

Controls wiring and conduit is separately accounted for.

How does one get from one, to three, conduits?
Is there a derating for grouped cables that leads to pulling one single conductor per conduit? Even in that case, the conduit size would be far smaller.... the Transit Agency uses an EPR insulated cable that is nothing unusual sofar as diameter.

 

[Buck Parrish]

How many fans at what volt?
Check this out if you have not all ready.

http://www.csgnetwork.com/voltagedropcalc.html

 

[charlie b]

This is not an issue of "derating." Distance does not cause a change to the ampacity of a conductor. Instead, we are dealing with voltage drop considerations. With that long a run, a set of 350 conductors will give you a dvoltage drop of 9%. That will not give the fans enough voltage for them to perform acceptably. A set of 600's would allow for a higher voltage level at the end of the run.

I have no idea why you would need three conduits. A set of 600's can fit into a 4" RGS conduit. I can only hope that the engineer is not thinking of putting the two phase A conductors (one per fan) on one conduit, the two phase B conductors in a second conduit, and the two phase C conductors in the third conduit. That would be not only wasteful, it would also be illegal.

Welcome to the forum.

 

[charlie b]

I may have misread the question, in that I missed this part:

. . . THREE 4" conduits . . . PER FAN.

One possible explanation is that the engineer expects to use three parallel conductors for each phase of each fan. That is,

• In the first conduit, you have phases A, B, and C, to serve fan 1.
• In the second conduit, you have a second set of phases A, B, and C, to serve fan 1.
• In the third conduit, you have a third set of phases A, B, and C, to serve fan 1.
• At the beginning of the run, and again at the end of the run, you connect all three phase A conductors to gether, and you connect all three phase B conductors to gether, and you connect all three phase C conductors to gether, with the result that the overall voltage drop gets much smaller.
• You do the same with the other three conduits serving the other fan.

This is an acceptable practice. But if you are going to use three parallel sets on conductors, you certainly do not need each conductor to be 600 MCM. That is overkill in the extreme.

You need to get a clarification from this engineer.

 

[iMuse97]

What C. Beck said: If you do parallel runs, you must have a conductor of each phase in each conduit. What about running medium voltage to the fan site and then using a transformer to step down to the fan voltage? You'd save on a lot of copper, if a transformer is to be a part of the project anyway.

 

[infinity]

What is the fan voltage? I did a quick calculation and the parallel 600's might not be largely over sized for 3% voltage drop.

 

[LarryFine]

Esty, welcome to the forum!

What about running medium voltage to the fan site and then using a transformer to step down to the fan voltage? You'd save on a lot of copper, if a transformer is to be a part of the project anyway.

Absolutely!

 

[petersonra]

This is not an issue of "derating." Distance does not cause a change to the ampacity of a conductor. Instead, we are dealing with voltage drop considerations. With that long a run, a set of 350 conductors will give you a dvoltage drop of 9%. That will not give the fans enough voltage for them to perform acceptably. A set of 600's would allow for a higher voltage level at the end of the run.

A feed this big might have its own transformer. I might be inclined to use the smaller wire and feed it from a tap with a little more voltage, if I was concerned about up front cost.

 

[benaround]

We have schematic/narrative design that includes feeders for pairs of
300A, 200,000 CFM fans exhausting a transit tunnel.
I'm not an engineer.

Ugly's Electrical References (book) suggests that 3/350kcmil conductors might be adequate to feed one fan, and further that these will fit in ONE 4" RGS conduit.

The engineer reviewing the estimate came back with THREE 4" conduits and
an unspecified quantity of 600 kcmil conductors, PER FAN.
It appears the feeder run is 2,200 feet so quite a lot of derating applies.

Controls wiring and conduit is separately accounted for.

How does one get from one, to three, conduits?
Is there a derating for grouped cables that leads to pulling one single conductor per conduit? Even in that case, the conduit size would be far smaller.... the Transit Agency uses an EPR insulated cable that is nothing unusual sofar as diameter.

You state that there are 'pairs' of fans@ 300a each. Continous load 125%=375a ampacity

needed for each motor. 600MCM=420a, (2) 4" C needed for each pair of fans (1) spare.

 

[Cold Fusion]

A feed this big might have its own transformer. I might be inclined to use the smaller wire and feed it from a tap with a little more voltage, if I was concerned about up front cost.

How would you deal with the voltage drop on start up?

cf

 

[iwire]

The project is for a transit tunnel, I would not really be looking to hard to save every nickel and dime. Give them what they ask for and charge them for it. This type of project is not a cheap one ............ no reson the ECs can't get a slice of the pie.

 

[Cold Fusion]

... What about running medium voltage to the fan site and then using a transformer to step down to the fan voltage? You'd save on a lot of copper, if a transformer is to be a part of the project anyway.

Or maybe ditch the xfm, and just install 4160V motors. 300fla at 480V sounds like 250hp motors.

cf

 

[diamonddean]

I can only hope that the engineer is not thinking of putting the two phase A conductors (one per fan) on one conduit, the two phase B conductors in a second conduit, and the two phase C conductors in the third conduit. That would be not only wasteful, it would also be illegal.

Welcome to the forum.

NEC allows non-abc installation in non-magnetic conduit.

 

[benaround]

NEC allows non-abc installation in non-magnetic conduit.

Isolated Phase installations are allowed for parallel conductors, not for seperate motor

feeders.