求翻译：4． 暗线敷设必须配阻燃PVC管。插座用SG20管，照明用SG16管。当管线长度超过15m或有两个直角弯时，应增设拉线盒。天棚上的灯具位设拉线盒固定。是什么意思？

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The line 4 must be placing retardant PVC pipe laying. socket used SG SG lighting used 20 tubes, 16. When the pipeline length of more than 15 m or two at right angles, bend, there should be the line box. ceiling lamps are located on the fixed line box.

4. The concealed wiring placing must match is flame-resistant the PVC tube.The plug with the SG20 tube, the illumination uses the SG16 tube.When the pipeline length surpasses 15m or has two quarter bends, should additionally build the back guy box.On the ceiling lamps and lanterns position supposes

4. Must have fire-retardant PVC pipe line laid. SG20 tube for outlet, SG16 tubes used for lighting. When the line is longer than 15m or when there are two orthogonal bending, adding terminal box. Lamp set terminal box fixed on the ceiling.