amperage supply to renovated NYC coop apartment 1

A 1950's coop apartment on a high floor in NY city currently is fed by only 50 amps (110-120V). Renovations will install circuit-breakers in place of glass fuses. Because of offset bends in the original conduit pipe from the basement to the apartment it is not feasible to pull "new" (larger diameter) wire from the basement feed. The cost of running "new" conduit with new wire all the way up from the basement feed in the buiilding is prohibitive ($$$$$). One suggestion has been to increase the basement feed fuse from 50 to 60 amps to slightly protect a possible small overload in the apartment from tripping one circuit. A second suggestion (although no costs were proposed) was to install two transformers, one at the basement feed and the second just inside the apartment. Could someone explain the beneftis/liabilities of these suggestions, especially the latter? Are there any other options that would allow for a 10-15 amp increase in the apartment's amperage above the current 50 amps? Thanks very much.

 

[dpc]

With a pair of transformers, The circuit running from basement to the apartment could be 480 V instead of 120 V. (Is the existing really 120 V, or 120/240 V).

If the voltage is doubled, the same current in the feeder transmits twice as much power. So you would have the equivalent of a 100 A feeder.

This would require new, larger feeder to the transformer down in the basement, and probably a new panel upstairs.

Also, the transformers will make audible noise at 120 Hz.

 

[peebee]

Here's a liability of both suggestions: you're dealing with wiring that has reached or exceeded it's useful lifespan. I suspect you have cloth-covered wiring... open up a couple junction boxes and see how much insulation is still hanging on to those poor old wires after all these years. Overcurrents, in particular, really do a number on cloth wiring. And what's more, it might well have been rated for 300V operation, not the 480 you're considering.

Yet despite this, you are seriously considering subjecting this wiring to a continuous overvoltage or overcurrent situation?

Maybe with all the money you save out of the electrical budget you can install some sprinklers to put out the electrical fires and keep your building from burning down.

 

[dpc]

Good points, peebee.

 

Thanks for your considered thoughts, peebee & dpc.

In fact, the riser wiring in this particular (Emery-Roth designed) 1956 hi-rise building I'm referring to seems to have sturdy, non-decayed (not old frayed cloth) insulation -- at least as far as I can see from opening the old fuse panel box in the apartment and inspecting the main feed.

I remember when I was a kid (around 1958) I worked summers as an electrician's helper on a number of NYC buildings (commercial as well as new apartment "project" complexes). We bent a lot of half- and three-quarter inch conduit and pulled a lot of #10 and #12 wire. In my memory the wire insulation was not merely woven cloth or asbestsos, but some sort of plastic. I hope I'm not mis-remembering what the wire codes were back then. In any case, I'm trying to avoid incurring $2500 per floor (x 21) in labor & materials to run new conduit with new wire up from the basement. That would break the bank!

 

[dpc]

6952C,

We were concerned about the wiring being much older than that. A 500 V megger test of the wiring might be a good idea. If you're thinking my 480 V suggestion, peebee is right - you need to verify if the insulation is rated 600 V and not just 300 V.

 

Thanks, dpc. How would I verify that the insulation is rated 600V? Not being an EE (nor very smart about this stuff), what is a "500 V megger"?

 

[dpc]

"Megger" is a brand-name for insulation resistance tester. You insulate the conductor from ground then apply specific dc voltage between the conductor and ground. If insulation is good, the current stabilizes at a very low level, indicating a very high resistance to ground.

If you can find any marking on the cable or conductors, you may be able to find a voltage rating. If not, I'd have a local electrician take a look at it for you. Someone with some grey hair will probably be able to tell you what it is.

If this a "home improvement" type project, you should probably look for another discussion group, since this is supposed to be limited to engineering discussions and this may get "red-flagged".

I really would recommend getting an electrician with some applicable experience to check it out for you. I know they seem expensive, but it could save you a lot of headaches down the road, not to mention fires.