In the US, distribution voltage is usually 480V. However, most industrial equipment is only rated for 460V. Why is this the case?
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asked May 5, 2015 at 20:18
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1\$\begingroup\$ Is it usually 480V? \$\endgroup\$– SamuelCommented May 5, 2015 at 20:27
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1\$\begingroup\$ Yes. From what I have seen. The supply panels usually have a 480V rating sticker on them. I once read equipment is rated at 460V because the voltage loss between the distribution transformer and the equipment is taken into account. However I can no longer find any literature to support this. \$\endgroup\$– SivaDotRenderCommented May 5, 2015 at 20:31
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1\$\begingroup\$ Have a look here - Mains electricity. Section on Voltage levels. \$\endgroup\$– DzardaCommented May 5, 2015 at 23:03
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2 Answers
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The feeder wires that deliver power have a small resistance/reactance. When a current flows through them a percentage of the source voltage is lost to the wires. So the voltage at load will be less. 120V at panel, but 115V at loads. If the voltage is higher than 115V, it is just better efficiency.
Electric codes set maximum loss to feeders. It's how we size wires for installations.
In your case, 480V at source, 460V required by load (nominal ratings) and a maximum of 20V lost to the wires.
It's just Ohm's law applied to a series circuit with a small resistance (wires) and a large resistance (load).
answered May 7, 2015 at 2:10
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\$\begingroup\$ I think this might be a more accurate answer \$\endgroup\$ Commented May 7, 2015 at 15:34
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\$\begingroup\$ Afterall, Ohm's Law is a law. At 120V, 3% maximum voltage drop (for Canada) means 3.6V maximum will be lost to wires (116.4V). We round this down to 115V. Actual voltage will be between 115V and 120V depending on current and distance load is from source. So your equipment is rated for 460V utilization from a 480V source. 3% would give you 14.4V to wires. Actual voltage will be above 460V, but below 480V. The concept of the power company raising voltage to increase consumption is not really viable when you realize what is involved in the power system. \$\endgroup\$ Commented May 7, 2015 at 17:10
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This is exactly the same situation as regular 120 Vac Mains voltage. Although North American AC Mains voltage is often called 110 Vac or 115 Vac or even 117 Vac, the actual voltage as specified by the power utility is 120 Vac.
That's because the power utility tries really hard not to exceed 120 Vac at your power outlets. Doing so causes incandescent lamps to burn out faster and causes resistive loads to consume more current than they would at a lower voltage such as 115 Vac.
The 480 Vac Mains is treated exactly the same way. Maximum voltage is supposed to be 480 Vac but nominal voltage is set to 460 Vac.
answered May 6, 2015 at 2:09