patrox2000
Member
- Location
- Baltimore, MD
Hello Mr.
I am a recently graduated electrical engineer. Not long ago, my work mates and I were prompted, by our boss, to read chapter 9 of the NEC Handbook, and we found what seems to be a contradiction.
The third paragraph explaining FPN No. 2 reads as follows:
?? At a bend, however, the major internal diameter of the [EMT] raceway may increase due to bending, particularly in tubing, to a diameter slightly larger than .648 in., permitting the middle conductor to be pulled between the outer two conductors. As the conductors exit the bend and the raceway returns to its normal shape with an internal diameter of 0.622 in, the conductors may jam. This can also occur in straight runs where the ratio of the raceway?s internal diameter to the conductor?s outside diameter approaches 3. ??
From reading this paragraph, it looks like having a jam ratio of less than 3 would be desirable, but the next paragraph says:
?To avoid difficult conductor installations and potential conductor insulation damage due to jamming within the conduit or tubing, a jam ratio between 2.8 and 3.2 is recommended.?
FPN No. 2 explicitly says that for 3 conductors, when the jam ratio is between 2.8 and 3.2, jamming may occur but this last paragraph says that ratios between 2.8 and 3.2 are recommended. Is this a contradiction, typo or what? Please reply with your thoughts on this.
I am a recently graduated electrical engineer. Not long ago, my work mates and I were prompted, by our boss, to read chapter 9 of the NEC Handbook, and we found what seems to be a contradiction.
The third paragraph explaining FPN No. 2 reads as follows:
?? At a bend, however, the major internal diameter of the [EMT] raceway may increase due to bending, particularly in tubing, to a diameter slightly larger than .648 in., permitting the middle conductor to be pulled between the outer two conductors. As the conductors exit the bend and the raceway returns to its normal shape with an internal diameter of 0.622 in, the conductors may jam. This can also occur in straight runs where the ratio of the raceway?s internal diameter to the conductor?s outside diameter approaches 3. ??
From reading this paragraph, it looks like having a jam ratio of less than 3 would be desirable, but the next paragraph says:
?To avoid difficult conductor installations and potential conductor insulation damage due to jamming within the conduit or tubing, a jam ratio between 2.8 and 3.2 is recommended.?
FPN No. 2 explicitly says that for 3 conductors, when the jam ratio is between 2.8 and 3.2, jamming may occur but this last paragraph says that ratios between 2.8 and 3.2 are recommended. Is this a contradiction, typo or what? Please reply with your thoughts on this.
