Paralleled Installation

[dnem]

The question now becomes, is it more cost effective to pull two new 350MCM's and just abandon the unused conductors in the existing conduit, or should I eat the labor to remove the two abandoned conductors to reuse them in the new conduit run?

Before you do something too radical, listen to what people here are telling you.

First we need you to tell us:
Is the load larger or smaller than 560amps ?

 

[HOUSTONWEHAVEAPROBLEM]

Rated Load

Rated Load

I think about 5 or 6 posts ago I explained to everyone that all calculations should be based on a rated load of 600amps. Not 560amps. Have you tried listening?

 

[dnem]

I think about 5 or 6 posts ago I explained to everyone that all calculations should be based on a rated load of 600amps. Not 560amps. Have you tried listening?

Yes, you did say that in post #17

But in post #7, you were telling us 550amps would do it.
"I thought about trying (3)-#4/0 per DC leg, which once derated for the number of current-carrying conductors would be good for around 550 amps, and this would fall within the jurisdiction of the existing 600amp breaker."

Don,
The actual load varies as this feeder is an emergency back-feed to a breaker panel supporting several large DC cranes. I thought about trying (3)-#4/0 per DC leg, which once derated for the number of current-carrying conductors would be good for around 550 amps, and this would fall within the jurisdiction of the existing 600amp breaker. However, I was really hoping to find a way to utilize the existing 350MCM's too. Parallel 350MCM's only get me about 500 amps of current carrying capacity.

And thru this whole thread you've been using the wrong math so I'm not sure that you know where you stand and what you need.

There's plenty of information in this thread.
Pick an option and go with it.

 

[iwire]

I think about 5 or 6 posts ago I explained to everyone that all calculations should be based on a rated load of 600amps. Not 560amps. Have you tried listening?

The people that are responding to you are trying to help, they do not work for you.

 

[don_resqcapt19]

I think about 5 or 6 posts ago I explained to everyone that all calculations should be based on a rated load of 600amps. Not 560amps. Have you tried listening?

You said that you needed 600 amps, but then you suggests an arrangment of 4/0s that you said gives you 550 amps. Your existing installation gives you 560 amps. Given that in general you only load a breaker to 80% of its rating, your load should not exceed 560 amps.

 

[hardworkingstiff]

Have you tried listening?

Have you tried thinking about what has been posted?

 

[ramsy]

If the max load is only 560A, (not continuous) nothing needs to change.
I'm showing the existing two #350's per leg (polarity) operating a non-continuous 560A load with a 68c temp. rise., and 4.3% voltage drop over 500 feet.

If the load must be changed to 600A, perhaps the most efficient option is not likely without the competence to install the two extra cables in the same conduit --without damage--.

 

[don_resqcapt19]

Don,
The actual load varies as this feeder is an emergency back-feed to a breaker panel supporting several large DC cranes. I thought about trying (3)-#4/0 per DC leg, which once derated for the number of current-carrying conductors would be good for around 550 amps, and this would fall within the jurisdiction of the existing 600amp breaker. However, I was really hoping to find a way to utilize the existing 350MCM's too. Parallel 350MCM's only get me about 500 amps of current carrying capacity.

In both the case of the 4/0s and the 350s you are not doing the derating correctly. The ampacity of the three sets of 4/0s is 624 ampd and ampacity of the existing two sets of 350s is 560 amps. You say the ampacity of the three sets of 4/0s is only 550 amps and that is because you are starting the derating at the 75?C column and not the 90?C column.

 

[LarryFine]

If the load must be changed to 600A, perhaps the most efficient option is not likely without the competence to install the two extra cables in the same conduit --without damage--.

Um . . . does that mean you think it's a good idea or a bad one?

I ask because it's sorta what I suggested way back in post #9.

 

[ramsy]

If the max load is only 560A, (not continuous) nothing needs to change.
I'm showing the existing two #350's per leg (polarity) operating a non-continuous 560A load with a 68c temp. rise., and 4.3% voltage drop over 500 feet.

Correction: 560A could burn up four 350's. I forgot to derate, "four" CCC's. Operating a non-continuous 560A load is a 90c temp. rise (when four CCC's share same conduit)., and 4.6% voltage drop over 500 feet.

 

[ramsy]

Um . . . does that mean you think it's a good idea or a bad one? ..back in post #9.

I thought your idea was the most cost-efficient solution. It may be a tight pull in 3" EMT, but three #350's per polarity fits if the required EGC can be a bare #350 cu, or perhaps with GFPE less than #350 --such as 3/0 THxN--.

The only problems may be the tight pull, and at the full 600A a continuous temp rise of 78c (equipment listings), with six CCC's, at 3.15% VD. Even the non-continuous temp rise is 61c at a full 600A.

 

[LarryFine]

I thought your idea was the most cost-efficient solution.

Thank you! That's what I thought was the goal when I read this:

I'm looking for practical and economical rolled into one nice code-compliant solution!

The conduit is already run, and pulling 500' twice (once out, once in) can't be harder than running 500' feet of EMT, and then pulling 500', can it? No need to even blow or suck a mouse. (Sorry)

Added: Why can not the EMT suffice as the EGC?

 

[ramsy]

The conduit is already run, and pulling 500' ..can't be harder than running 500' feet of EMT.

Ya, and if its tight use that huge crane to pull it. If the crane rips the conduit out of the ground, so what, he wanted bigger pipe anyway. Just put a shovel on the end of that crane and dig a bigger trench.

Why can not the EMT suffice as the EGC?

Because its more fun to use the crane.

 

[LarryFine]

Ya, and if its tight use that huge crane to pull it. If the crane rips the conduit out of the ground, so what, he wanted bigger pipe anyway. Just put a shovel on the end of that crane and dig a bigger trench.

"ARRGH, ARRGH, ARRGH!"​

Because its more fun to use the crane.

Absolutely!

 

[ramsy]

Ahh Pictures.. Give me a few days to find a picture of a crane ripping something out of the ground.

 

[Smart $]

Ahh Pictures.. Give me a few days to find a picture of a crane ripping something out of the ground.

Ummm... while we all like to see such (i.e. when not at our expense), I assume the mention of EMT at the very beginning means the run is NOT underground. Additionally, all the temp. rises you stated would put THHN conductors over the 90?C rating if the ambient temp is 30?C.

 

[ramsy]

Ummm... while we all like to see such (i.e. when not at our expense), I assume the mention of EMT at the very beginning means the run is NOT underground.

Yes, my assumptions may be more synical, but we both are guilty of assuming.

2005 NFPA 70, 358.10 Uses Permitted
(B) Corrosion Protection Ferrous or nonferrous EMT, elbows, couplings, and fittings
shall be permitted to be installed in concrete, in direct contact with the earth, or in areas
subject to severe corrosive influences where protected by corrosion protection and judged
suitable for the condition.

According to the 2004 UL General Information for Electrical Equipment Directory
(White Book), category FJMX, galvanized steel electrical metallic tubing (EMT)
installed in concrete, on grade or above, generally requires no supplementary corrosion
protection.

Additionally, all the temp. rises you stated would put THHN conductors over the 90?C rating if the ambient temp is 30?C.

Yes, my grammer is bad, but my math is the same as last year. We looked up IEEE formulas on temp rise a few years back. You liked my results then, and I'm using the same formulas on my spreadsheet now. The label on my spreadsheet actual reads "+Temp Rise." My ambient is adjustable, currently set at 30c.

I believe it is correct to doubt all statements from anyone who hides or disguises their ignorance or intentions without proper recant and ownership of error. If my calcs are wrong please help me fix it.

IMHO People who won't admit mistakes are unreliable, misserable to work with, and do present exploitable character flaws, which the public domain can declare open season within the known limits of physics.

Any help on images of cranes toppeling over would be greatly appreciated.

 

[innov8]

http://www.maxumowners.org/Images/damaged_boats/10wheeler.gif

Will this do?

 

[ramsy]

Will this do?

Thats perfect. Now the boom aligns perfectly with direction of wire pull. Just retract boom and run main hoist. Or in crains new possition, roll down the cab window, poke head out and watch for the hand signs.

 

[ramsy]

please delete --broken links--