Maximum Allowable Sidewall Pressure

[iwire]

Can someone help me find the maximum allowable sidewall pressure for 500 and 600 kcmil copper XHHX-2?

I have asked an engineer to allow me to reduce some conduit sizes and he will allow it if I can show some evidence that the sidewall pressure will not damage the insulation.

Now I firmly believe that the size of the raceway will not change the pulling tension assuming the raceway is at least as large as the NEC requires.

But my believing that is not the same as being able to demonstrate it to the engineer.

For those that are interested the run in question is four 600 and one 250 copper XHHW-2.

The prints show 4" EMT and I want to run 3" EMT.

Thanks Bob

 

[Nick]

Re: Maximum Allowable Sidewall Pressure

Bob,
I calc your fill at just under 44%. Sounds like you need at least 3-1/2" to meet the 40% fill requirement. As far as sidewall pressure you are correct the size of the conduit is not going to have an effect. (provided it is not overfilled)It is mainly a function of the radius of the bends and the total pulling tension up to ther bend. Most cable manufacturers call out about 300lbs of sidewall pressure max. (sidewall pressure and pulling tension are two different forces)If you calc the run and you exceed this at the last 90 all you have to do is increase the radius of the 90 in question. Polywater has a great program for this. You can also get a much better explanation and a calculator over at electrician.com

 

[iwire]

Re: Maximum Allowable Sidewall Pressure

Thanks Nick.

Originally posted by Nick:
Bob,
I calc your fill at just under 44%. Sounds like you need at least 3-1/2" to meet the 40% fill requirement.

I think you may be figuring on RMC or PVC, with EMT I believe I come in under 40%.

But I certainly could be wrong.

Originally posted by Nick:
As far as sidewall pressure you are correct the size of the conduit is not going to have an effect. (provided it is not overfilled)It is mainly a function of the radius of the bends and the total pulling tension up to their bend.

That was my understanding as well.

Originally posted by Nick:
Most cable manufacturers call out about 300lbs of sidewall pressure max.

That was what I could not find, I looked at a couple of wire makers sites and could not find that.

Originally posted by Nick:
(sidewall pressure and pulling tension are two different forces)If you calc the run and you exceed this at the last 90 all you have to do is increase the radius of the 90 in question.

Understood.

Originally posted by Nick:
You can also get a much better explanation and a calculator over at electrician.com

Yes, I am relying on that site for what I know.

 

[iwire]

Re: Maximum Allowable Sidewall Pressure

Sorry, it is not four 600s it is four 500s.

Now I see why Nick came up with 44% fill.

 

[iwire]

Re: Maximum Allowable Sidewall Pressure

By the way, I want to reduce the raceway size to save space, I am not trying to be cheep.

The feeder is 1,600 amps and is specified as five 4" EMTs each with four 500s and a 250.

I have to run the five sets above an existing suspended ceiling that is already crowded.

Any reduction in total size will be a help.

 

[bob]

Re: Maximum Allowable Sidewall Pressure

Bob
You did not give the distance so I used 300 ft.
The SW pressure exceeded the 300 lbs. Why are you pulling the #250? EGC?

 

[iwire]

Re: Maximum Allowable Sidewall Pressure

Thanks Bob, I really do appreciate the help.

Originally posted by bob:
Bob
You did not give the distance so I used 300 ft.
The SW pressure exceeded the 300 lbs. Why are you pulling the #250? EGC?

The distance between the longest two pull points is about 125' with two 90s.

It sounds like the run may need large radius sweeps regardless of the EMT size.

As for the 250 the wire sizes are dictated by the engineer.

Keep in mind it is a 1,600 amp feeder using 1,900 amps of ungrounded conductor.

Thanks again, the more info I get the better.

 

[don_resqcapt19]

Re: Maximum Allowable Sidewall Pressure

The information at Newton's site is based on the Southwire Power Cable Handbook. I have copy of that at work that one of their reps gave me about 15 years ago. I see that they now sell it for $150 a copy.
Don

 

[pierre]

Re: Maximum Allowable Sidewall Pressure

"Keep in mind it is a 1,600 amp feeder using 1,900 amps of ungrounded conductor."

Bob, what does this exactly mean?

 

[iwire]

Re: Maximum Allowable Sidewall Pressure

Originally posted by pierre:
"Keep in mind it is a 1,600 amp feeder using 1,900 amps of ungrounded conductor."

Bob, what does this exactly mean?

Just that the feeder has OCP of 1,600 amps, but the engineer has selected conductors with 1,900 amps of capacity.

The 'other' Bob had asked why a 250 EGC.

I imagine the engineer has up sized the EGC to comply with 250.122(B).

 

[iwire]

Re: Maximum Allowable Sidewall Pressure

Originally posted by don_resqcapt19:
I see that they now sell it for $150 a copy.
Don

No wonder I could not find a free download at Southwire.

That really upsets me.

Go to Advance or Bussman and you can get all kinds of free info to aid in using their products.

Is Southwire not making enough selling copper building wire? :roll:

 

[electricmanscott]

Re: Maximum Allowable Sidewall Pressure

Originally posted by iwire:
By the way, I want to reduce the raceway size to save space, I am not trying to be cheep.

The feeder is 1,600 amps and is specified as five 4" EMTs each with four 500s and a 250.

I have to run the five sets above an existing suspended ceiling that is already crowded.

Any reduction in total size will be a help.

I'll just be over here with my 14/2 romex.

 

[peter d]

Re: Maximum Allowable Sidewall Pressure

Originally posted by electricmanscott:
I'll just be over here with my 14/2 romex.

 

[bob]

Re: Maximum Allowable Sidewall Pressure

Bob
For a 125 ft the SW pressure is 125#. I thought the circuit was a service. Thats why I asked about the #250. IF you can use the EMT as the EGC you can get by with a much smaller conductor.

[ January 30, 2006, 08:49 AM: Message edited by: bob ]

 

[peteo]

Re: Maximum Allowable Sidewall Pressure

Originally posted by iwire:
Any reduction in total size will be a help.

Not trying to change the subject, and maybe I'm reading my 2002 book wrong. What a great question for studying, in any case! What about the ladder tray I've seen around here.

392.11(b)(4) Where single conductors are installed in a triangular or square configuration... then the text and exhibit 392.3 state explicitly that using 310.20 ampacity without any derating is fine... Using 310.20 directly, without any length derating, has 3 X 580 = 1740.

If I'm not in outer space it looks as if you could get 3 inch rail 18 inches wide, just add more width and sets of cable if need be. Don't know whether that would help fit it or not. Nice to look at new things.