The AC current is carried from the surface to the motor using either copper or aluminum cable conductors. For ESP applications, four sizes of conductors have been standardized: #1, #2, #4 and #6 AWG.
AWG stands for “American Wire Gauge”.
Power losses and operating temperature decrease by increasing the conductor size for a given current. Moreover, for a given conductor size increasing current will increase both the power losses and cable operating temperature.
A Cable AWG size is selected based on well clearance, ampacity rating, flexibility and cost. Since the ESP cable is run in hole banded to the production tubing, the maximum allowable cable diameter is determined by wellbore clearance. In the other hand, the minimum conductor size is determined by the required motor current and permissible voltage drop.
Stranded conductor vs. Solid conductor:
There are two type of cable conductor: stranded or solid.
For the same AWG cable, stranding increases conductor diameter. Therefore, stranding is more common in larger conductor size.
Stranding properties give stranded conductors more flexibility compared to solid conductors for the same AWG size. So that, it can be run wellbore with more severe trajectories and doglegs
Solid conductors have the smallest diameter. This is why Motor Lead Extensions (MLE’s) are usually solid.
Conductor selection based on gas migration considerations:
Solid conductors minimize gas migration through cables and minimize H2S deterioration. For stranded conductors, an alternative approach to addressing these problems is “Compacted” stranded conductors filled with gas blocking compound.
Aluminum conductor vs. Copper conductor:
For ESP applications, aluminum is generally less desirable as a conductor because it provides only 61% of the conductivity of an equivalently sized copper conductor. Therefore, a larger diameter wire has to be used to carry the same current as copper. Thus, clearance problems can be caused in the wellbore. In addition, aluminum conductors are more difficult to splice compared to copper conductor.
In the other hand, aluminum is less expensive than copper conductors and it is lighter in weight, thereforealuminum cable manufacturing costs should be less. Also, the main disadvantage of copper conductor is it’s susceptibility to damage by hydrogen sulfide (H2S). This problem can be resolved by using a continuous lead sheath completely covering the insulation.
API Recommended Practice 11S5 (RP 11S) – First edition, February 1, 1993.