I haven't really done a whole lot with MV cable relating to ampacity so I was curious to know which tables in section 310 were typically used for the ampacity of MV cables.
For MV cables routed in above ground conduit I have typically always used table 310.60(C)(74) (2011) and used the 90deg or 105deg columns depending on which type of cable I had.
For MV cables routed in underground conduit I have typically always used table 310.60(C)(77) (2011) depending on the number of conduits in a duct bank. Typically if it is only one conduit then I use the one circuit configuration for the type of cable that I have.
Would you guys agree that these are the two correct tables to use for thes two situaltions with MV cable in conduit which is both run in the air and underground?
I also notice that 240.101(A) allows for MV cable to be protected by a fuse at 3x the cable ampacity or with the long time setting on a CB set to 6x the cable ampacity. Although the code allows this, is this typically a good practice. I'm used to sizing LV cables at very close to their OCPD as required by code so these new MV requiremens are kind of new to me and I was wondering what was good practice. Is this more of a case where we plot MV cables on a tcc plot and as long as the cable damage curve is above the fuse or relay settings and within size allowed by code we are o.k.? Or are there other factors for being able to protect cables up the maximum values allowed by the NEC.
Also I assume that 240.101(B) simply means that any conductor tapped from a feeder can be protected by the upstream breaker protecting the feeder as long at the tapped conductor falls within the 3x fuse rating of 6x breaker rating for ampacity?
For MV cables routed in above ground conduit I have typically always used table 310.60(C)(74) (2011) and used the 90deg or 105deg columns depending on which type of cable I had.
For MV cables routed in underground conduit I have typically always used table 310.60(C)(77) (2011) depending on the number of conduits in a duct bank. Typically if it is only one conduit then I use the one circuit configuration for the type of cable that I have.
Would you guys agree that these are the two correct tables to use for thes two situaltions with MV cable in conduit which is both run in the air and underground?
I also notice that 240.101(A) allows for MV cable to be protected by a fuse at 3x the cable ampacity or with the long time setting on a CB set to 6x the cable ampacity. Although the code allows this, is this typically a good practice. I'm used to sizing LV cables at very close to their OCPD as required by code so these new MV requiremens are kind of new to me and I was wondering what was good practice. Is this more of a case where we plot MV cables on a tcc plot and as long as the cable damage curve is above the fuse or relay settings and within size allowed by code we are o.k.? Or are there other factors for being able to protect cables up the maximum values allowed by the NEC.
Also I assume that 240.101(B) simply means that any conductor tapped from a feeder can be protected by the upstream breaker protecting the feeder as long at the tapped conductor falls within the 3x fuse rating of 6x breaker rating for ampacity?
