1 Phase 3 Wire feeder

[DHkorn]

Long time reader, I love this forum. I never found anything else like it. Congrats to all.

I have a voltage drop question.

I was recently called to a very old high rise building on a voltage drop question. It seems that originally an EM lighting feeder was installed 120/240v 1?3W and revised at some point to 120/208v 1?3W. The feeder is 3#8 awg. cu. RHW to the 6th floor loaded to about 30 amps. The branch is #14 cu. RHW loaded to about 12 amps extended up 5 floors and down 5 floors feeding a single compact flourescent outlet at each floor and two exit signs.

The complaint is that this circuit drops to 90 volts or so while the 120/208 volt service is nominal.

Is there an engineering reason why a 1?3W feeder fed from a 3?4W source would drop more volts than normal? I came up with about 9.4% but we're seeing about 25%.

Thanks in advance.

 

[mdshunk]

Loose connection, bad termination, or corroded conductor (AL?) are among the possibilities. Other than that, are you sure that the load you think is on this circuit is ALL the load on this circuit?

 

[quogueelectric]

voltage drop

voltage drop

I see it as a wire size problem. You first of all should not have 14 wire for such a long run in a commercial building. I do realize it is preixisting so thus leaves you with a few options. Since voltage drop is a function of ampereage across a conductor which has a specific resistance you should first check to see if the emergency lighting has multiple voltage taps. You could then go to a higher voltage thus cutting your amperage and voltage drop in half anyway. In an older building you must be careful that the em lighting ckt has not been hacked into for a receptacle in an office somewhere if you are going to bump up the voltage. See first if the circuit is being fed from both phases and not loaded all on one side this will effectively do the same thing by reducing amperage. Only other option is to increase wire size and hire lots of electricians to rewire entire building. Can you estimate the total footage of the wire runs??

 

[Snorks]

Voltage drop is a function of total resistance or impedance. There are many factors contributing to this including harmonics, power factor, ambient temperature, dielectric, raceway type, distance, etc.

You'll need to do further checking on the circuit to really know what is happening.

 

[DHkorn]

Thanks.

Thanks.

I told the engineer to turn if off when he came in the next day and leave it off for a day. I bet a secretary will call complaining her coffee maker or copy machine wasn't working.
I'm not sweating it. Someone changed all the fixtures to CF and I think it was the engineer. Those old RHW splices dont take well to reworking. You're probably right.
Unless something stops working they won't let us keep working on it.
I just wanted to rule out 1? branch issue with the feeder.

 

[mdshunk]

I told the engineer to turn if off when he came in the next day and leave it off for a day. I bet a secretary will call complaining her coffee maker or copy machine wasn't working..

It is sort of unorthadox, but that's probably what I'd have done too. If you know the loads and distances, and your actual voltage drop is many times your calculated voltage drop, there's only a copule of things that can cause that. A bad connection in a commercial building is a possibility, but that's pretty low on my list in a that type of occupancy. I'd bet something more along the lines of extra load that's not really supposed to be on that circuit.

I also agree with the person who commented on the 14 gauge. I think #14 has no real reason on a commercial job, let alone roped up and down 5 floors. It is what it is, though, and this is what you have to work with.

 

[LarryFine]

The branch is #14 cu. RHW loaded to about 12 amps extended up 5 floors and down 5 floors feeding a single compact flourescent outlet at each floor and two exit signs.

Any chance you can split the circuit at the top of the 5 floors and re-feed the far half from the bottom with a new circuit? That would halve the load on the existing circuit.