MI Cable installation, previous installations and general info

Status
Not open for further replies.

sbaz1024

Member
Location
US
Hello all!

For the first time I am gonna be working with MI cable. The #2 MI cable will be part of a riser that feeds a booster pump for a fire system. The MI cable will run from a pullbox in a basement mechanical/electrical room through a long route to the second floor with plenty of bends and obstructions. From the second floor it changes ober to thhn in a pullbox and from there goes to the roof in conduit enclosed in 2 hour concrete structure.

Our plan is to use a gangload of pulleys at most bend locations and a pulling machine to pull.

We were thinking of supporting PVC elbows at appropriate locations where it would be more feasible than pulleys and pull the MI through the elbow and cut the elbow out after or leave it as a sleeve if poasible. Sound like a good idea?

Would love anybodys input on problems that may arise during the installation or pull of an MI run. Or any suggestions, tips, important things to know about working with MI.

Also if youd like share info about previous installations you were invloved in MI related.

To conclude, I am curious about anything related to MI cable to help me go into this as smooth as possible.

Thanks for reading!

Sent from my SM-N900P using Tapatalk
 

iwire

Moderator
Staff member
Location
Massachusetts
I really can't see how you could support a PVC sweep firm enough to help.


Other than that I will mention that MI cable sheath 'work hardens' as you bend it around so you want to try not to bend it too much.

The best way to install it is have others do it. ;)
 

iwire

Moderator
Staff member
Location
Massachusetts
Oh, do you know that you will need a non-ferrous metal plate to enter the enclosures at each end?

MICableLahey009.jpg
 

peter d

Senior Member
Location
New England
I really can't see how you could support a PVC sweep firm enough to help.


Other than that I will mention that MI cable sheath 'work hardens' as you bend it around so you want to try not to bend it too much.

The best way to install it is have others do it. ;)

There is still a lot I have not done in the electrical trade and I can go the rest of my career having never touched MI cable and I'm quite alright with that. :cool:
 

sbaz1024

Member
Location
US
I really can't see how you could support a PVC sweep firm enough to help.


Other than that I will mention that MI cable sheath 'work hardens' as you bend it around so you want to try not to bend it too much.

The best way to install it is have others do it. ;)
Was thinking of using kindorf in 3 positions of the PVC elbow. One in the bend of the elbow, and 1 towards each outlet of the elbows with 2 part straps to the kindorf. With the kindorf anchored to a concrete surface using double expansions

Sent from my SM-N900P using Tapatalk
 

sbaz1024

Member
Location
US
Not sure how to edit posts on the tapatalk app sorry :(

Also about the numerous bends i am pretty stuck this cable route is a total nightmare, a lot of bends are required.

Sent from my SM-N900P using Tapatalk
 

sbaz1024

Member
Location
US
Oh, do you know that you will need a non-ferrous metal plate to enter the enclosures at each end?

MICableLahey009.jpg
Yes we have the plates speficially for the MI bolted to the pullboxes where we changover to thhn. Thanks for the input though! Ive heard its a common for people to make individual KOs into the enclosure.

Sent from my SM-N900P using Tapatalk
 

tkb

Senior Member
Location
MA
Are your pull boxes in a 2hr rated closet or are they a weak link in your system?
 

iwire

Moderator
Staff member
Location
Massachusetts
Also about the numerous bends i am pretty stuck this cable route is a total nightmare, a lot of bends are required.

I understand you have to go around bends but keep the radius large as you can.

The last time I had to work with it was running through a very complicated route in a finised building. We used a ton of guys.

But the leading end of it, by the time it had been bent so many times to get through tight spots had become almost un-bendable.

All I am saying is keep the bending as little as possible and standard PVCs 90s would not. At the least use a 4" large radius or even a 6" large radius.

Running MI is less like running a wire than it is like running a solid bar of metal.
 

iwire

Moderator
Staff member
Location
Massachusetts
What's the rationale behind this requirement?

MI cable is typically, but not always single conductor so if you enter line A, B, C & N through separate holes in ferrous metal it will cause inductive heating.

In the pictures I posted it is single conductor cable, the copper jacket is used as the EGC.
 

gadfly56

Senior Member
Location
New Jersey
Occupation
Professional Engineer, Fire & Life Safety
What's the rationale behind this requirement?

Each "cable" is actually a single conductor. In AC with Romex for example the grounded and ungrounded currents cancel the magnetic field. Not so here. You'd get inductive heating of the steel.
 

sbaz1024

Member
Location
US
I understand you have to go around bends but keep the radius large as you can.

The last time I had to work with it was running through a very complicated route in a finised building. We used a ton of guys.

But the leading end of it, by the time it had been bent so many times to get through tight spots had become almost un-bendable.

All I am saying is keep the bending as little as possible and standard PVCs 90s would not. At the least use a 4" large radius or even a 6" large radius.

Running MI is less like running a wire than it is like running a solid bar of metal.
Just curious, the armor on the MI im using is copper, shouldnt it get softer and more bendable as it gets bent around every corner? That your bringing up using 4"+ elbows is making me nervous about the 12" pulleys we are using and at some points the cable makes a 90 around the pulley. :/

Sent from my SM-N900P using Tapatalk
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
When you start with soft copper rather than a hard alloy or heat treatment formulation it actually gets harder (and stiffer) the .more you bend it.
The copper atoms move into a more regular crystalline structure that resists bending.
 

iwire

Moderator
Staff member
Location
Massachusetts
Just curious, the armor on the MI im using is copper, shouldnt it get softer and more bendable as it gets bent around every corner?

No, back in the first post I mentioned it I included a link to info all about it.

But it stiffens up and when you are working with a stiff product to begin with it really became hard to work with and the finished installation had a lot of waves that I would have like to make straight.


As far as the pulleys if you have to make a 90 you may want to use two or three pulleys working together to make a large radius turn.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
The copper atoms move into a more regular crystalline structure that resists bending.
I thought it was the opposite--that when annealed, the lattice is quite regular, and that some of the initial flexibility stems from creating dislocations in the lattice. These dislocations inhibit the creation of further dislocations, reducing the flexibility.

Cheers, Wayne
 

sbaz1024

Member
Location
US
Just an update for anyone who is curious. We pulled the ground and one of the phases one by one. We used (2) 3" pvc 90,s as pull sleeves, a bunch of pulleys for the small bends and where we had the MI going over kindorf racks we sleeved 2" emt over the kindorf racks with 3/4" over the threaded rod thats supporting the kindorf to create rollers. We had 6 men at different locations of the pull all pushing along to feed it. Man in the front pulled the rope attached to the nose of the MI. It actually went way smoother than expected! Communication was a big part. I was in the middle in the first floor ceiling talking to the basement crew through my cell phone i had on speaker. The couple men on the second floor and I were able to hear each other by yelling out. After we finish pulling all wires we will go back and tie them all together. The supply house who sold us the cable said every 3 feet the MI needs to be tied together.

Sent from my SM-N900P using Tapatalk
 

JFletcher

Senior Member
Location
Williamsburg, VA
Just an update for anyone who is curious. We pulled the ground and one of the phases one by one. We used (2) 3" pvc 90,s as pull sleeves, a bunch of pulleys for the small bends and where we had the MI going over kindorf racks we sleeved 2" emt over the kindorf racks with 3/4" over the threaded rod thats supporting the kindorf to create rollers. We had 6 men at different locations of the pull all pushing along to feed it. Man in the front pulled the rope attached to the nose of the MI. It actually went way smoother than expected! Communication was a big part. I was in the middle in the first floor ceiling talking to the basement crew through my cell phone i had on speaker. The couple men on the second floor and I were able to hear each other by yelling out. After we finish pulling all wires we will go back and tie them all together. The supply house who sold us the cable said every 3 feet the MI needs to be tied together.

Sent from my SM-N900P using Tapatalk

ty for the update. Speaking of which, unless it's in the 11 or 14 NEC, I do not see a code section in Article 332 regarding the part of your reply I placed in bold, just supported every 6' (332.30) and grouped together (332.31)
 
Status
Not open for further replies.
Top