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Allowable Negative Pressures in Round Spiral Duct
The following table lists the recommended metal gauge and reinforcing for various negative pressures in different diameters of round spiral duct. The negative pressures listed are working pressures with safety factor of 3 included.
MAXIMUM NEGATIVE PRESSURE
| Diameter | 0" - 10" wg | 10" - 20" wg | 20" - 30" wg | 30" - 40" wg |
| 3-7 | 26 ga | 26 ga | 26 ga | 26 ga |
| 8 | 26 ga | 26 ga | 26 ga | 24 ga |
| 9-12 | 24 ga | 24 ga | 22 ga | 22 ga |
| 13-15 | 24 ga | 22 ga | 22 ga | 20 ga |
| 16-18 | 22 ga | 20 ga | 20 ga | 18 ga |
| 19-22 | 22 ga | 18 ga | 18 ga |
w/fl 18 ga or 16 ga |
| 24-26 | 20 ga | 18 ga |
w/fl 18 ga or 16 ga |
w/fl 18 ga 6' c-c or 16 ga 12' c-c |
| 28-34 | 18 ga |
w/fl 18 ga or 16 ga |
w/fl 18 ga 6' c-c or 16 ga 12' c-c |
w/fl 18 ga 4' c-c or 16 ga 12' c-c |
| 36-42 |
w/ fl 18 ga or 16 ga |
w/fl 18 ga 6' c-c or 16 ga 12' c-c |
w/fl 18 ga 4' c-c or 16 ga 12' c-c |
w/fl 16 ga |
| 44-50 |
w/ fl 18 ga 6' c-c or 16 ga |
w/fl 18 ga 4' c-c or 16 ga 12' c-c |
16 ga 12' c-c |
w/fl 16 ga 6' c-c |
| 52-60 |
w/ fl 18 ga 6' c-c or 16 ga 12' c-c |
w/ fl 18 ga 4' c-c or 16 ga 12' c-c |
w/fl 16 ga 6'c-c |
w/fl 16 ga 6' c-c |
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