看一下2464的style page吧，屏蔽是可有可無的。
至于外被用多少P的料，只要能通過758里面要求的物性要求就可以。
| UNDERWRITERS LABORATORIES | APPLIANCE WIRING MATERIAL |
| Subj.758 | Section 2 | Page 2464 | Issued:1968-05-02 |
| Revised:2009-05-26 |
| Style 2464 | Jacketed Cable. |
| Rating | 80 deg C, 300 Vac, Cable flame. |
| Insulation | Labeled or complying with manufacturer's AWM Procedure. |
| Assembly | Two or more singles, twisted pairs of groups of twisted singles twisted together or singles or groups of singles may be laid parallel to form flat, oval or round cable. Lay not specified. Barrier layer and/or fillers optional. Manufacturer shall maintain a complete description of each assembly. May use same or mixed AWG size. |
| Braid | Optional, provided over Jacket. |
| Dia of Cable under jacket | Min average | Minimum at any point |
| 0.700 inch or less | 30 mils | 24 mils |
| 0.701-1.000 inch | 45 mils | 36 mils |
| 1.001-1.500 inch | 60 mils | 48 mils |
| 1.501-2.500 inch | 80 mils | 64 mils |
| 2.501-4.000 inch | 110 mils | 88 mils |
| 4.001 inch and larger | 140 mils | 112 mils |
| Standard | Appliance Wiring Material UL 758. |
| Use | Internal wiring or external interconnection of electronic equipment (such as desk-type calculators, dictating machines, or x-ray equipment). |
你們所說的55P、65P的“P”是什么意思？
