Electrical Inspector

[bandylb18]

Industrial installation, utility receptacles w/three recepts. on 20A - 120V circuit, furthest distance to receptacle approx. 300', feed is #10AWG THHN. Using Art.210.23(A)(1) as standard to calculate voltage drop, i.e. code allows use of 16 amps of a 20A ckt. and figuring 80% of that which is 12.8A shows #10 to be too small a wire due to voltage drop. Is this a valid calculation, assuming 12.8 amps as a load to be anticipated? In an industrial setting, would you use the 180W standard for residential or commercial offices to figure utility receptacle load? The setting is a gas compressor station with five nine thousand horsepower compressors. Utility receptacles would be used by maintenance.
Let me know what you think.

 

[charlie b]

First, you are applying the 80% twice, and that is not necessary. 80% of 20 is 16, so that is the circuit limit.

Secondly, a general thumb rule for 120 volt circuits is that you upsize the wire one size for every 100 feet. You can back that up with a calculation, if you wish, especially if you have a good handle on the amount of current that the load will draw. But knowing nothing else, a 20 amp circuit that is run 300 feet could easily require a #6 wire, to reduce voltage drop to desired levels. I did a quick calculation at 16 amps, and found that a #6 gives a 4% VD.

Finally, I would calculate any general purpose receptacles (i.e., one for which you don't know what will be plugged in or when) at 180VA each. That gives you a load of 4.5 amps for three receptacles. A #10 will give you a 3% VD, and that is acceptable. But I would go with #8 wire anyway, just to improve the VD, and to accomodate the occasional use of higher amp loads at the other end. You can put a junction box close to the end of the run, and transition back to #12 wire, to make it easier to connect to the receptacles themselves.

 

[renosteinke]

Let's look at 'voltage drop' in a broader context.

First off, there's a difference between 'real volts' and 'nominal volts.' That is, you may actually be supplying that 120v receptacle with 124 (or 117) volts. This is important.

Next, you need to know that, for most product categories, NEMA standards expect the product to operate within 10% of the NAMEPLATE rating. It's quite common for the nameplate rating to differ from the nominal voltage.

Then, as you discovered, your load matters. Your percentage of voltage drop will differ depending on whether you're drawing 3 amps or 18 through that wire.

What really matters is not that the voltage drop be a certain percent, but that enough volts arrive to power the load.

For example, a 230v (nameplate) motor on a 240v (nominal) circuit needs 207v to operate properly. As long as you're getting more than 207v, you're good to go. ..... MAYBE.

I say 'maybe,' because you need to allow for starting current in your calculations - rather than using the size of your breaker. If that motor has an FLA of three amps, you need to have that 207v available when the motor is drawing 18 amps at start-up. You would miss this detail if you based your calculations on either the 15 amp breaker or the 7-1/2 amp fuses you have in the motor circuit.

So, you need REAL numbers ... the actual volts delivered to the circuit, AND the actual current draw of the load.