Voltage Drop in Buried Cable

The cable cannot know whether it is buried in the ground or suspended overhead, so the voltage drop for identical cables in these two situations will be the same. However, the cable in the ground does not have the benefit of free air flow around it, and therefore the ratings of the cable will be different in the two cases, the suspended cable being able to operate at a slightly higher maximum current.

Check out British Standard 7671 for further guidance.

Check out British Standard 7671 for further guidance.

Wonderful, But what about the poeple who aren't in the UK. Do they get special dispensation and not have losses?

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The original post gives no indication as to the country applicable to the installation, like so many on CR4. British Standard 7671 gives requirements for use in the UK, and as such is guidance applicable for other countries, which may be what the original poster [OP] wants. The OP is able to choose whether to follow or not.

Given a country, there may be similar codes applicable to that country. As installation circumstances vary from country to country, so do the electrical codes.

Following the reading of BS7671 for illumination, the OP is then in a position to find a similar standard applicable to the country of intallation. BS7671 is therefore a signpost, rather than a solution.

Guest, below, refers to Australian Standards AS/NZS 3000:2007 OR AS/NZS 3008.1.

So there, with knobs on.

1. Use the correct size copper wire. No matter what the amperage, if the size is correct, it will carry the current with little voltage drop.

2. For below-ground installation: Put the wire in pipe. The size of the pipe is also critical. If it is too small, or if you simply bury the wire in the ground, there is not enough free air space around the wires to allow the heat to dissipate. Larger pipe than is required is okay.

Tom

But wouldn't you have the chance of miceers ( rodents) getting inside the pipe or condensation and doing damage.

You could also try Australian Standards AS/NZS 3000:2007 OR AS/NZS 3008.1

Voltage drop will increase marginally with temperature rise as a result of direct burying, check the manufacturers data.

The voltage drop is a consequence of how much current flows through the wire, so increasing the current will increase the voltage drop. The "correct" wire size is basically dependent on the insulation material (rated for a maximum temperature) and how much of the developed heat can be dissipated. So basically a wire with a good insulation (listed for the same ampacity and resistant to higher temperatures), when compared with one (also the same ampacity) designed to resist at a lower max temperature, providing both are made of the same material, will lead to a higher voltage drop at the same current run through it, because the better one, because of its higher allowable temperature is smaller (and subsequent with an increased resistance). So basically there is a trade off situation: less copper - higher voltage drop and vice-versa. Of course, regulations regarding a maximum temperature and allowable voltage drop in a particular application have to be considered.

Now with regard to the pipe, if it's metalic, its heat conductivity is much better than the air, so a smaller pipe in this case would do a better job than a bigger one (wires are in closer proximity with metallic walls). For undeground wires the idea is to eliminate the air for a better thermal conductivity.

In overhead lines it is the movement of the air that is considered, higher air speed, better heat dissipation.

See the "US" "National Electrical Code" 2008 version, Article 310. This article has tables relative to conductors in free air and in raceways above grade and below grade on the temperature differential and current carrying capacity. Conductors in free air will carry more current then those in a raceway system. Also current carrying ability conductors is reliant on the ambient temperature, either below grade conductors in direct contact with earth or in raceway or above grade in raceway or free air and whether the raceway is metallic or non-metallic.

Voltage drop is also dependent the actual current (amperes) based on the length of the conductor. I.E. A 200 amp rated conductor used in a 200 amp service size drawing 150 amps will have will have lower voltage drop then the same conductor at full load over a given conductor length.

You are right about the fact that the US National electric code lists a different ampacity (higher) for conductors in free air as compared to underground ones. But, as mentioned in my previous response, the reason is different: it is assumed that in a free air conductor the created heat is dissipated via air movement (convection) as compared to underground conductors where heat dissipation occurs only via conduction.

The thermal conductivity of different materials [in W/(mK)] is as follows:

Air = 0.025

Wood = 0.04 - 0.4

Concrete = 1.7

Water = 0.6

Soil = 1.5

Steel = 12

CU = 380

The thermal resistivity, which is the inverse of the thermal conductivity, is thus 40 for air and 0.7 for soil [mK/W], which allows heat sources in close contact with soil a higher degree of heat dissipation as compared with air (in the absence of air movement).

So, when selecting an underground conductive conduit, it is better to select a conduit with the lowest possible cross-section in order to allow a closer contact between wires and conduit and eliminate air pockets (that act as a thermal insulator and dissipate heat 40 times lower than soil/conduit.

The NEC deals primary with safety and practical execution aspects, otherwise it would not (for instance) limit conductors in parallel (art.310.4) to #1/O or larger for power supply, without a physically based justification.

Voltage drop for a single conductor is the result of resistance of the conductor and the temperature of the medium around it, not how the conductor is run. The greater the total length of the wire and the hotter the air around the wire, the greater the resistance, thus the greater the voltage drop.

If temperature around the wire were not a factor, then it would not matter how the wire was run, the voltage drop would be the same. But temperature is a factor that can be reasonably predicted based on the equation from NEC Table 8, Conductor Properties, Note 2:

R2 = R1 [1 + α (T2 – 75)] ; Where:

• R1 is the conductor's DC Resistance at 75 deg C,
• α is a constant dependant on the conductor, and
• 75 is the temperature the conductor's ratings are based on (in deg C).

So, as the ambient temperature around the wire increases the resistance of the wire increases, thus the voltage drop becomes greater. If the wire is hung overhead with adequate air around it to transfer most of the heat, then the worst case ambient temperature is slightly higher than the hottest day of the year for the location where the wire is used provided that the conductor is not overloaded.

If the wire is buried in the ground, then the dirt acts as insulation, and the ambient temperature steadily rises over time, so underground wire is often sized a little larger to minimize the amount of heat dissipated by the wire. (If the wire stays cool to the touch, then the wire's resistance won't change - much.)

This gets more complicated if you throw in multiple phases of AC current, because then inductive reactance (XL) becomes a factor.

For 200 ft of wire the voltage drop will be pretty insignificant, because resistance of wire is only a few Ohms per hundred feet. But before you do an installation you need to call your power company. They will have specific requirements for overhead and underground service entrance conductor sizes, and the local provider's standards override the NEC.