Installed a new service, 3 parallel runs of 500. Discovered 30 years ago they ran all their phases in its own rigid conduit. 1st pipe was A phase with a netrual, 2nd pipe was B pages with a netrual and last pipe was c phase with a netrual. I know thats a big no no,I seen theres an exception its ok with it was pvc not rigid. Also seen if you cabinet was open at the bottom its,something to do with the magnetic loop. Only thing we could think of doing at the time was to make our new mdp open at the bottom. Looking for some input on this topic. I have never dealt with something like this before. Thank you.
MDP switch over, isolated phases
- Thread starter Tudmutt
- Start date
- Location
- New Jersey
- Occupation
- Journeyman Electrician (retired)
So you're looking for a fix of the original installation with the phases and neutrals in separate metal raceways?
zbang
Senior Member
- Location
- Roughly 5346 miles from Earls Court
Are these a PoCo service lateral? Remember that the PoCo has different rules, and IIRC they're allowed to do that.
- Location
- Illinois
- Occupation
- retired electrician
Having an open bottom does not change the issue with having the phases in separate ferrous metal conduits. Since the system has not failed, I assume that the actual current does not exceed 400 or 500 amps.
C
Correct, be plan would be to rearrange them in the transformer but power company says it cant be done.
- Location
- Illinois
- Occupation
- retired electrician
They might be correct. The conductors may be too short to re-arrange them. Typically when an isophase installation is made the conduits are directly below the transformer terminations and run straight up to the terminations resulting in no extra wire to move conductors around in the transformer.
If this has been in service for some time, and there is no evidence of heat damage, and if you are not adding load, I would really think about leaving it alone.
Its a 1000amp service
Roger that, im freaking out cause I know its a no no. And where im at there's no inspections.
- Location
- Illinois
- Occupation
- retired electrician
The rating of the service has little to do with the actual load. Assuming this has been in service for sometime, and there is no evidence of heat damage it is likely that the actual load current is below 500 amps. If there is no additional loads being added and if there is no evidence of heat damage, I probably would not change it to be code compliant.
Thank you for the piece of mind. I appreciate it
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
The reason that these installations are prohibited is because you are accidentally creating a transformer, where the conduits are both the magnetic core (ferrous) and a shorted secondary. When you have such an installation you can have many amps flowing on the conduit, heating it up and letting the magic smoke out.
In the installation you describe (with 1 neutral per phase), you can have a separate problem: induced circulating current or current imbalance on the neutral conductors.
Since these are 30 years old without a problem, you have evidence that features of the installation have prevented problems.
The open bottom of the cabinet breaks the 'shorted secondary' created by the conduit, so you won't have circulating current in the conduit.
It may be that the conduit runs are actually PVC, with RGS stub-ups. This greatly reduces any problems.
Finally the previous use may have been very well balanced, so that there wasn't much load on the neutral.
I'd suggest 1) informing the customer that you've identified an issue, 2) measuring the current on each individual neutral conductor when the service is under load to see if there is an overload issue on the neutrals, and 3) measuring conduit for continuity when the service is not energized.
-Jonathan
I
as the nec is concerned this is still a violation though. So technically the customer should pay to have it corrected?
hillbilly1
Senior Member
- Location
- North Georgia mountains
- Occupation
- Owner/electrical contractor
Many years ago I was doing a job walk on a Costco, and opened the gear for the engineers. One of them immediately spotted the phases ran that way. Steel was stubbed up under the gear, but we didn’t know if it was ran in pvc after that, or steel all the way to the transformer. The store was probably at least 20 or more years old. The job scope got much bigger, as the engineers wanted that corrected.
Great lol. Im 100% certain its rigid from my mdp to the transformer. The old mdp was bottom open.
herding_cats
Senior Member
- Location
- Kansas
- Occupation
- Mechanical Engineer
Yup. Single wire transformer rule.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
It is an NEC violation.
If the customer wants it corrected then they should pay for that.
But since it has been working fine the customer may not want to correct the issue. I don't think you can force them to correct the issue, you can only protect yourself if the issue isn't corrected and causes a problem later.
The bottom open isn't particularly evidence that the run is 100% rigid, but if you know that for other reasons then make sure the customer understands the problem.
It was working fine with the old mdp which was open ended. The new mdp isn't ,so this could create a problem.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Ahh. That wasn't clear from your original post. I'd assumed that both old and new were open on the bottom.
-Jonathan
My fault, yeah so its a different setup,which is why im concerned. Would 300.20 (B) help me with my situation?
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
A slot would help for isolated conductors, but if you attach metal conduits to holes with slots you defeat the slots.
If you can replace a steel panel with something insulating, like a fiberglass panel, that would be the equivalent of an open bottom.
If you can replace a steel panel with something insulating, like a fiberglass panel, that would be the equivalent of an open bottom.