Replacing MCC Feeder Cables

[richanton]

Just interested in some opinions on this topic. My building is 40 years old. We are replacing the original motor control centers. The original cables used are older fabric insulated cables run underground in the original bldg slab. I want to replace the feeder cables for reliability, since I am putting a new MCC in. Question is, what are the odds that I can pull out the old cable and put new cable in. Right now, I am thinking it's a bad idea and I should put new overhead conduit and cable in. The run length is probably about 150-200 feet.

 

[stephenw22]

Unless there's an accessible pull rope that was left behind in the conduit, you won't know if the conduit is plugged until you try to pull out the old cable.

On our site, if an underground conduit is more than 10-15 years old, we automatically assume that it's plugged.

I'd go overhead, no question.

 

[RAF7]

I have run into this on two projects so far this year. I even wrote a "white paper" to address the issue. How do I / can I attach a file??

The short answer is forget the existing UG raceway!

 

[saladhawks]

If the utility I work for was to assume that every underground conduit more than 10-15 years old was plugged, all construction work would pretty much stop. We have underground infrastructure that is 50+ years old and at least try to pull out all unused cable. If a cable is unable to be pulled out, then we abandon it in place.

 

[resqcapt19]

It is common to remove cables from underground conduits a lot older and longer than that. Some times you need special equipment to get the job done. A couple of times, where I could do it, I made up some adapters and connected a fire hose to the conduit to flush it out before I tried to remove the conductors. This was in an underground system where a there was a lot of mud that had worked its way into the raceway over the years.
There is also a chemical product that is said to help release the bond between the raceway and the cable, but I have never tried it.

http://www.polywater.com/cablfree.html

Don

 

[mpparent]

What about putting a camera down there to assess the condition of the pipe?

Mike

Mike

 

[EEJaime]

You can never tell what you are going to find, but if time, your installation proceedure and your loads will allow it, I would take this on a case by case basis. Isolate one of the MCC's and see what condition you find the conduit in. If you can afford to wait until you know wether or not you can use the existing conduit, to start your new conduit install, that will let you maximize the bang for your buck. If you absolutely need to do this in one step, that may be important enough to justify the expense of new overhead feeds.

We often have to assume the worst, especially if the work is to be publically bid work. Then the only way one is going to be 100% certain is to run overhead. If you have the luxury of being able to have an MCC down while the feeder is removed and/or replaced, I would take advantage of that.

 

[rbulsara]

Only way to know is to try it. Then use camera. So for budgeting purposes figure new conduits.

Metal conduits in direct contact with wet or corrosive earth will not last for 40 years. There may be standing water ( I have found that in one case) or the metal might be simply corroded away. On the other hand a dry earth location or conduits encased in concrete would preserve the conduits.

 

[jraef]

Although I thoroughly agree with the sentiments of "check it out and see", I also know that in the case of MCC retrofits there is often a "go live" situation wherein the process must be off line for as little time as possible because of lost production revenue. If that is the case, I would not bother trying to use the existing underground duct, especially if it was galvanized. I agree with rbulsara's comment on the >40 year life span. My experience is that if there was some minor physical damage to the galvanizing at the original installation, it may have rusted through even if in concrete. So even if you do manage to pull the original cable out intact and feed a pull rope through with it, you will not know if there is a jagged edge from rusted conduit left behind until you pull the new cable, energize it and have it short to ground on you. Although the camera idea is great, you may not be able to afford the time; not just the delay from pulling the cables while scoping the duct, but in not having already run the overhead option and having it ready to connect.

This comment comes from my personal experience, gained at considerable expense.

 

[ramski]

I totally agree with jraef, the new cables might be damaged during the pulling in.

How about this one:
If the capacity of the existing cables are sufficient for the new MCC, try to test the insulation resistance level of that cables, if its still intact why bother to change it?

 

[richanton]

I think the consensus is I'm probably better running overhead, especially if being down for an extended period is an issue, which in most cases here, it is.

I could leave the cable in, on the basis that it still works now, megger it to get a base reading, and then megger it yearly to confirm that it has not degraded, but that just adds another maintenance task which adds ongoing yearly cost, especially if you use $100/hr electricians.

Well, thanks for all the opinions.

 

[BJC]

One option is to trench in the basment slab and put in new conduit. I have seen it done in hospitals where there was so much stuff in the basement overhead that it was a viable option.
The particular job I worked on had most of the pipe in the slab, with the ends as surface runs to facillitate a quick cable connection.
The part in the slab when quick, saw cut, dig lay pipe and cover. It may go faster than an all surface run.
Some place don't work so well, you could strike water or pipes that were never "as-built"