Maximum Allowed Voltage Drop

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Josephus

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I work for a broadcaster in FM radio, and these folks are brilliant electronics engineers. They understand what is "Possible" but have no interest in working within NEC code.
Does anyone have a code reference indicating the maximum allowable voltage drop for branch circuits?
These guys want to run 480v through a 14-2 UF for 2500 feet and they are counting on the voltage drop bringing the output down to something close to usable.
I'm not asking for voltage drop calculators, I have used those and it can work in theory.

What I am looking for is a code reference to cite them, and refuse the proposal. That way I can offer a proper installation according to code.
 
Voltage drop is a design issue unless your location has adopted 'energy codes' which mandate a maximum voltage drop.

But what the heck do you mean by "they are counting on the voltage drop bringing the output down to something close to usable". Is 480V too high for the load and they are hoping that the resistance of the wire will drop the voltage to appropriate values, or is the load tolerate of wide voltage swings so they are comfortable that it can handle a large voltage drop?

If they think that this circuit can feed a 20A 240V load, then they are looking at a world of hurt. When the load is drawing 20A, then the voltage at the end of the circuit would be about 240V...but when the load drops the voltage will increase; so when _off_ the load needs to be able to tolerate 480V, and needs to be able to tolerate huge supply voltage swings. Such an installation would probably violate the code that requires equipment to be installed per manufacturer's instructions. On the other hand if they are feeding some sort of lighting system designed for 480V but tolerant of much lower voltages then it is probably just fine. (Or at least legal...)

So what sort of load is this circuit supposed to feed?

-Jon
 
Josephus said:
I work for a broadcaster in FM radio, and these folks are brilliant electronics engineers. They understand what is "Possible" but have no interest in working within NEC code.
Does anyone have a code reference indicating the maximum allowable voltage drop for branch circuits?
These guys want to run 480v through a 14-2 UF for 2500 feet and they are counting on the voltage drop bringing the output down to something close to usable.
I'm not asking for voltage drop calculators, I have used those and it can work in theory.

What I am looking for is a code reference to cite them, and refuse the proposal. That way I can offer a proper installation according to code.
Click to expand...

there isn't any section in the code for "linear transfomers"
that i'm aware of.

radiant underground cabling is normally used for ice and
snow removal, and doesn't run on 480.....

so, the connected load is going to drop the voltage to the
required level?

sorta like "ether starting spray will blow up the engine if used
continuously without load, but what if we use the starting spray
going up a hill? will the load balance the starting spray?"

underground burial... can't catch fire. well not much.

hell, let them do it. you gotta promise to bring back pictures.
 
What's being powered at the far end? If it has a switching power supply with a wide-enough input range, it might actually work.
I calculated 195 volts of voltage drop at 15 amperes. (2.9 kW) You might remind them that once installed, they'll be paying for that voltage drop in perpetuity.
 
drcampbell said:
What's being powered at the far end? If it has a switching power supply with a wide-enough input range, it might actually work.
I calculated 195 volts of voltage drop at 15 amperes. (2.9 kW) You might remind them that once installed, they'll be paying for that voltage drop in perpetuity.
Click to expand...

only when the equipment is operating.

so, the sooner they burn it up, the more money they will save!

i did look in the code for this.... if you look under the fine print notes,
there is a section that begins "hold my beer.... watch this..." that speaks
clearly to this application.
 
Interesting idea, not really practical or most likely safe, but intriguing. Design is somewhat simple.

Source:480V, series circuit.

2 loads, conductors and equipment. Voltage needs to drop 360V from conductors impedance to 120V at equipment. Mr Kirchhoff likes 0V when we total.

Amperage is same through both loads.

So, one one would need to model the conductors as a fixed resistance and the equipment would need to be also.

Something to play with as more as of an classroom exercise then a real world application.

A little more complicated then 12V battery and two resistors.
 
Fulthrotl said:
only when the equipment is operating.

so, the sooner they burn it up, the more money they will save!

i did look in the code for this.... if you look under the fine print notes,
there is a section that begins "hold my beer.... watch this..." that speaks
clearly to this application.
Click to expand...

Hold your own darn beer, I gotta go find a calculator.

The geek in me has to run these numbers.:D
 
jumper said:
Interesting idea, not really practical or most likely safe, but intriguing. Design is somewhat simple.

Source:480V, series circuit.

2 loads, conductors and equipment. Voltage needs to drop 360V from conductors impedance to 120V at equipment. Mr Kirchhoff likes 0V when we total.

Amperage is same through both loads.

So, one one would need to model the conductors as a fixed resistance and the equipment would need to be also.

Something to play with as more as of an classroom exercise then a real world application.

A little more complicated then 12V battery and two resistors.
Click to expand...

That sounds fine as long as the temperature doesn't change. The conductors won't be 'fixed resistors' as they change in temperature.
 
K8MHZ said:
That sounds fine as long as the temperature doesn't change. The conductors won't be 'fixed resistors' as they change in temperature.
Click to expand...

Yep. Coefficients.

Plus equipment would have be a fixed load or one would have to calculate the various VDs as that would change. Conductors and equipment would have be rated for operating in whatever ranges were possible. Could work in theory but......

All in all, this idea ain’t gonna work.:)
 
Josephus said:
I work for a broadcaster in FM radio, and these folks are brilliant electronics engineers. They understand what is "Possible" but have no interest in working within NEC code.
Does anyone have a code reference indicating the maximum allowable voltage drop for branch circuits?
These guys want to run 480v through a 14-2 UF for 2500 feet and they are counting on the voltage drop bringing the output down to something close to usable.
I'm not asking for voltage drop calculators, I have used those and it can work in theory.

What I am looking for is a code reference to cite them, and refuse the proposal. That way I can offer a proper installation according to code.
Click to expand...

They don't sound all that brilliant to me. What they are proposing is a highly variable voltage divider network with the voltage at the other end of the conductors completely dependent on the resistance of the load. As they turn stuff off and the voltage will be all over the map. Ditch this hare-brained scheme. Get a transformer and run conductors sized for the load.
 
ggunn said:
They don't sound all that brilliant to me. What they are proposing is a highly variable voltage divider network with the voltage at the other end of the conductors completely dependent on the resistance of the load. As they turn stuff off and the voltage will be all over the map. Ditch this hare-brained scheme. Get a transformer and run conductors sized for the load.
Click to expand...

Well, you’re no fun....:D

8340ec00ab2fd3e7d5d1d1708bf02a4b.jpg
 
Welcome to the forum!


jumper said:
Interesting idea, not really practical or most likely safe, but intriguing. Design is somewhat simple.

Source:480V, series circuit.

2 loads, conductors and equipment. Voltage needs to drop 360V from conductors impedance to 120V at equipment. Mr Kirchhoff likes 0V when we total.

Amperage is same through both loads.

So, one one would need to model the conductors as a fixed resistance and the equipment would need to be also.

Something to play with as more as of an classroom exercise then a real world application.

A little more complicated then 12V battery and two resistors.
Click to expand...

480V in the classroom hahahaha! Heck, I survived shop class, I guess millennials can survive playing with 480V!

I suppose the OP could rig a purely resistive load with his intended load to minimize voltage swings at the load... rig being the key word there.

I dont have a code section to dissuade the OP and his brilliant engineers from the planned course of action, tho i do have to wonder why anyone would pay to trench and backfill nearly half a mile of earth ($$$$) to lay just $600 worth of unprotected, small wire. Have fun WHEN it faults underground, if it doesnt smoke the intended load first.
 
JFletcher said:
480V in the classroom hahahaha! Heck, I survived shop class, I guess millennials can survive playing with 480V!
Click to expand...

Under proper supervision of course, but all electrical class rooms (college level) should have 480V. We did.

If one is going to teach applied technology in electrical installations to students, one uses what is going to be routinely encountered in the field IMO.
 
JFletcher said:
I dont have a code section to dissuade the OP and his brilliant engineers from the planned course of action,
tho i do have to wonder why anyone would pay to trench and backfill nearly half a mile of earth ($$$$)
to lay just $600 worth of unprotected, small wire. Have fun WHEN it faults underground, if it doesnt smoke
the intended load first.
Click to expand...

shhh.... i wanna see what happens.... you're gonna spoil it.

in days of yore, doing trade shows.... we rented 30 kva buck and boost
transformers to people with odd voltages... and a "brilliant engineer"
for a medical equipment manufacturer was going to save money.

he had this weird open delta drawn, one transformer boosting and one bucking,
to end up with this magical voltage needed..... the electrical engineering
equivalent of a speedball.

so we gave him a cord cap with a tail on it, so he could hook it up himself.
single phase 208 with a 30 amp scoop size was what the cord from the overhead
supplied.

it was impressive. with no connected load, it pulled 25 amps of 208, and
unicorns leapt into the air, and the magic smoke fled.

we never got close enough to measure the developed voltage.

after the smell dissipated, we rented him a 30 kva buck boost. it worked fine.
 
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