Related Products: |
EGMA-004, EGMA-005, EGMA-006, EGMA-007, EGMA-008, EGMA-013, EGMA-015, EGMA-016, EGMA-019 |
Related Documents: |
EZDP 2020 and 2038 |
Type: |
Technical information |
Keywords/Labels: |
Premium Monitoring System, Cable Length |
Summary: |
This article includes recommended length and size of grounding cables, a wire size characteristics table, and common installation information. |
The length and size of the grounding cable are recommended based on the following plant-provided information:
The generator manufacturer’s recommendation for ground wire size
The manufacturer’s recommendation for shaft voltage and ground current alarm limits
The shaft voltages and ground currents as measured historically by the plant
Conduit run distance between the shaft grounding location (Shaft Grounding Assembly) and the Premium Monitoring System installation location
Conduit run distance between the Premium Monitoring System and the turbine case (ground)
A common installation will use 8 AWG wire, which carries 30 A DC at 12 V, assuming a maximum allowable voltage drop of 10% at 30°C (86°F).
30 amps x 60 ft = 1800 Famps (Feet x Amps). As shown in the following table, this puts the required wire size at 8 AWG.
30 A DC is approximately 10x what is commonly accepted as the DC threshold of a normally operating generator, so our wire size recommendation represents a safety factor of essentially 10x.
The 30 A DC minimum we have established also covers a single-phase AC load of 32 V on an 8 AWG wire over the same distance at a capacity of 80 A.
Note that ground current from turbine generators is most commonly not DC. More typically the ground current has a strong AC component. The AC component is commonly not a sine wave. Observed signals may have multiple momentary peaks implying that loads are quite large, but further examination reveals that the actual power load on the wire is remarkably less. For example, a specific generator we have under maintenance is showing a peak-to-peak ground current of 4.22 A. However, the average RMS ground current of that same time period is in actuality 0.12 A. So, if an engineer or technician is applying the typical RMS calculation of 0.35355 x "V PK-PK" to our reported value of 4.22 A, they would incorrectly assume that the RMS voltage is 1.49 when in reality this number in this circumstance is 12.43 times greater than the actual RMS ground current measured during the snapshot.
Table 8. Wire Size Characteristics
Wire Size |
Minimum Acceptable CM Area |
Wire Ampacity Nonengine |
Wire Ampacity Engine |
12 Volts Volt Drop |
24 Volts Volt Drop |
32 Volts Volt Drop |
||||
|---|---|---|---|---|---|---|---|---|---|---|
AWG |
Metric* |
3% Famps |
10% Famps |
3% Famps |
10% Famps |
3% Famps |
10% Famps |
|||
16 |
1 |
2336 |
25.0 |
21.3 |
86 |
288 |
173 |
576 |
230 |
768 |
14 |
2 |
3702 |
35.0 |
29.8 |
138 |
459 |
275 |
918 |
367 |
1223 |
12 |
3 |
5833 |
45.0 |
38.3 |
219 |
729 |
437 |
1458 |
583 |
1944 |
10 |
5 |
9343 |
60.0 |
51.0 |
348 |
1159 |
695 |
2317 |
927 |
3090 |
8 |
8 |
14810 |
80.0 |
68.0 |
553 |
1843 |
1106 |
3686 |
1474 |
4915 |
6 |
13 |
24538 |
120.0 |
102.0 |
879 |
2929 |
1757 |
5858 |
2343 |
7811 |
4 |
19 |
37360 |
160.0 |
136.0 |
1398 |
4659 |
2796 |
9319 |
3727 |
12425 |
2 |
32 |
62450 |
210.0 |
178.5 |
2222 |
7408 |
4445 |
14815 |
5926 |
19754 |
1 |
40 |
77790 |
245.0 |
208.3 |
2803 |
9342 |
5605 |
18684 |
7474 |
24912 |
0 |
50 |
98980 |
285.0 |
242.3 |
3536 |
11788 |
7073 |
23576 |
9430 |
31434 |
00 |
62 |
125100 |
330.0 |
280.5 |
4457 |
14858 |
8915 |
29715 |
11886 |
39620 |
000 |
81 |
158600 |
385.0 |
327.3 |
5619 |
18731 |
11239 |
37462 |
14985 |
49950 |
0000 |
103 |
205500 |
445.0 |
378.3 |
7086 |
23620 |
14172 |
47241 |
18896 |
62988 |
This chart assumes wire with 105°C (221°F) insulation rating and AWG wire sizes.
*Metric wire sizes may be used if of equivalent circular mil area.
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