second guessing wire size due to voltage drop...

Status
Not open for further replies.
090207-0747 EST

emahler:

iwire is pointing you in the correct direction.

Without being very accurate if I pick on 1% voltage drop at 400 V and 4 A this is 4 V and 4 A or 1 ohm loop resistance. Ignoring temperature #8 wire has 0.628 ohms per 1000 ft and 20 deg C. 1300 ft of #8 is 0.82 ohms. This just provides a rough background.

If you scale this to 2.8% voltage drop and 480 V the maximum loop resistance is 1*2.8*480/400 = 3.36 ohms. Now you could consider #14 wire. I really doubt that a greater voltage drop is not acceptable, and therefore I would not bother going to #12 to accommodate the rise in copper resistance from temperature. But these are factors you need to analyze.

Do you really need to limit yourself to 2.8% drop? Is the load steady? Even if the load is not steady with what voltage range can it work?

If the load is steady then use a little higher voltage than 480 at the source end to compensate for the voltage drop. Maybe you can do this even if the load is highly variable if there is no problem with over voltage on the equipment. You might also combine this with a Sola constant voltage transformer at the destination.

Do not just look for some equation to give you an answer. Analyze the problem, the limits, apply logic, and make judgements.

Verify that I did not make a mistake in the calculations above.

If I need a 1 HP motor because of the torque requirement, but my duty cycle is 1%, then what size motor should I use?

.

.
 
gar said:
Analyze the problem, the limits, apply logic, and make judgements.
Click to expand...

We sure agree there, and that applies to all types of problems.

We should try not to get stuck trying to make the same answer work for every problem. :smile:
 
this is for a RFID system being installed in a distribution center...the load is a cooling fan (constant) and 2 switches to control the data equipment.

we are putting in a 16kw ups to protect the entire system...

voltage drop is fairly critical, due to the cost of the electronics we are powering...and the run cycle is high...like all the time:D

so, what we need is a good, clean circuit from 650' away....

this all came up about 2pm yesterday...the entire project is a cluster...the SOW is non-existent...and there are way too many chiefs and not nearly enough answers...add to this the levels of companies involved...it's ugly...but for us chaos is equaling cash...

i'll think this through over the weekend, but i think bob is onto something...

thanks
 
one question regarding using transformers....

will they work if powered by the batteries and inverter? is there a specific transformer designed for this type of installation?

never come across this situation before.

thanks
 
I think that you will be better off sizing the conductors for the voltage drop as you first thought of and call it a day. It will work.
 
emahler said:
one question regarding using transformers....

will they work if powered by the batteries and inverter? is there a specific transformer designed for this type of installation?
Click to expand...


Not an issue that I know of assuming the UPS puts out 60 Hz. Gar could answer this much better then I can.
 
090307-1019 EST

emahler:

Electronic circuits do not necassarily mean a sensitivity to input voltage. This is very much a function of design.

If you are really concerned about voltage stability and reduction of certain types of transients, then a Sola constant voltage transformer at the destination is an outstanding and very reliable solution. I believe many of these are standard with nominal 120, 240, and 480 inputs. Output is 120 V +/-1%. See http://www.sola-hevi-duty.com/products/powerconditioning/cvs.htm
The transient and noise filtering characteristics of the Sola may be more important than its voltage regulation. This is not a cheap solution, but may have great value.

If your UPS supply is a sine wave output at all times, then there is no problem with a transformer load unless the UPS was poorly designed. If it is a very rough simulated sine wave, then you would need to talk to both the UPS and transformer manufacturers to determine compatability.

Big heavy wires over a long distance are not a satisfactory solution to voltage drop. Edison understood this in 1879. Low resistance filaments were not a practical solution for an electrical system with parallel loads and thus the development of the carbon filament lamp, and 110 V for distribution. Obviously this was still a limited distance method, but it allowed the industry to get started.

.
 
emahler said:
one question regarding using transformers....

will they work if powered by the batteries and inverter? is there a specific transformer designed for this type of installation?
Click to expand...
I would think you'd want a UPS at the load end of the line, after the transformers.
 
emahler said:
no generator...
Click to expand...

Then without a doubt in my mind Larry's suggestion is the way to go, at least from the customers cost view point. It never struck me for a minute anyone would carry a 120 VAC circuit 650' across a building for virtually no reason.
 
iwire said:
Then without a doubt in my mind Larry's suggestion is the way to go, at least from the customers cost view point. It never struck me for a minute anyone would carry a 120 VAC circuit 650' across a building for virtually no reason.
Click to expand...

you may be correct...not my call...i was told to run a circuit 650'...
 
i had to do this once before for a access control/security system about 750 feet from the building at the parking lot gate we needed 3 or 4 circuits (about 30 amps all together)at the gate cabinet and it ended up being cheaper and a lot easier using 2 transformers and mounting a WP panel on the cabinet
 
Status
Not open for further replies.
Top