General Electrical Question - 240 vs' 480 Volt Systems

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oldbay444

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I'm working on the NAED EPEC program and there is a question I am not 100% sure I have the correct answer for.

Hoping you can lend some of your expertise.

The question reads: "If operating equipment at higher voltages allows the use of smaller conductors, why do we commonly use 120 or 240 volts, or even 12 volts? Why don't we operate everything at, say 480 volts?"

My inital thought is that safety is the main issue. Higher voltage systems tend to have more violent ground faults, and of course the explosive nature of a 480 phase to phase fault.

Additionally, I imagine there is a play here with GFCI and the threshold 30MA being reached faster in a 480V system??? Not sure here.

Anything else I should consider?

Thanks in advance!

Scott
 
A lot of these things just are. Many people believe that 24 and 48V systems evolved from the use of lead acid batteries, as they are commonly multiples of 12 Volts.

Why would anyone would use 480V when 550V would be arguably more efficient.

Why do we consider 600V to be the upper limit in so called low voltage systems while IEC stuff considers 690V to be that limit?
 
oldbay444 said:
I imagine there is a play here with GFCI and the threshold 30MA being reached faster in a 480V system???
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FYI...GFCIs for personal protection trip around 6 ma

GFP for equipment protection can be around 30 ma
 
You may be working as an electrical distributor or similar. Take a look at your 480v gear, its twice the size and cost of 120/240. As the voltage goes up the amperage goes down. Also 480 volt is far more dangerous for shock and arc fault hazards. 480v arcs may not burn clear where a 120 does not have enough energy to sustain the arc.
120/240 is safer and costs less, but as the connected load goes up, you have more concerns with voltage drop, and larger conductors. A large building may have a 480v service with step down transformers to 120/240. Also doubling the voltage cuts the amperage by 1/4.
 
I suspect the reason the nominal voltage was initially set at "110" lies in the limitations of the early technologies used to effect working electrical systems.

The conductor wasn't the problem.

Durable insulation took a while to develop. Mean time between failure in the early circuits could be increased by lower voltage stress on insulation.

Also, I remember reading in one of my early 1900s texts that a practical and economical "230" volt incandescant lamp was yet to be developed. This was printed some decades after nominal distribution voltages were agreed to. The implication to me was that "110" volt supported the early incandescant lamps.
 
Another thought.

Edison's early empire was built on DC generators. It wasn't until Nikola Tesla developed the AC motor, several decades later, that the use of transformers made long distance power distribution practical. Edison's DC generator had to produce utilization voltage, that is, the generator worked only at 120/240 volts. As a result, the current densities were enormous, as the connected load grew, and the distribution distance was limited to just a few city blocks. The early engineering limitations made it practical to imagine building generator stations every few blocks, rather than using higher voltages.
 
oldbay444 said:
"If operating equipment at higher voltages allows the use of smaller conductors, why do we commonly use 120 or 240 volts, or even 12 volts? Why don't we operate everything at, say 480 volts?"
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It comes down to cost. The voltage rating of a conductor or a motor or any other electrical component is based on the ability of its insulation system to prevent current from leaking (e.g., from the wire within the insulation to the metal case of the motor). A component designed to operate at 480 volts has to have a thicker layer of insulation (or a better type of insulation), to prevent leakage current, that a component designed to operate at 120 volts. That has an impact on the size of the component and its cost.

So I guess that the real reason we don't use 480 volts for everything is that we don't have to.
 
Pierre C Belarge said:
Tradition
Costs
Safety
Comfort level

Technology will most likely change all of this in the future.
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Heh!

And the installed base of "old technology" will be a juggernaut heading in its old, and out-dated direction, that will resist change.
 
Oh sorry, I was thinking of I(squared)R = P. To keep the same power, if the resistance is the same, the current is squared for a change in voltage.
 
Thanks to all for the feedback!

Thanks to all for the feedback!

Thanks everyone for your insight; much appreciated.

Please enjoy the rest of your weekend.

Sincerely,

Scott
 
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