JovialBulge
Member
- Location
- New England
- Occupation
- Automation Dude
In section 5.2 of 508A it states "For calculations involving amperes, calculations resulting in a fraction of less than 0.5 shall be rounded down to the next whole number. Calculations resulting in a fraction of 0.5 or more shall be rounded up to the next whole number." Which seems pretty straightforward, or so I thought.
When a question on my recent MTR re-qualify exam came up about sizing fuses for the primary and secondary sides of a control transformer (Table 42.2) I got turned around for a bit. My math came out to the primary side protection being 7.8125 and the secondary side being 15.625. In my haste I chose the 8a/20a option on the test, which turned out to be incorrect. Now that I look back on my results of what I got wrong it seems as if I should have chosen the 7a/20a option. For the primary side protection that was calculated out to be 7.8125, using section 5.2 I rounded up to 8a which was seemingly incorrect.
Additionally, if you're calculating the sizing of a MCCB for a panel and using:
32.3.1 The size of the overcurrent protection shall not exceed the ampere value determined from (a) and
(b), whichever is larger:
a) The rating of the largest branch circuit protective device in the circuit plus 125 percent of all heater loads plus the full-load currents of all other motors or other loads in the group that could be in operation at the same time;
Say your calculations came out with a number of 22.62, using section 5.2 I initially thought that you'd round that up to 23 then use Section 31.3.8 to get to your next standard size which would be 25. However, now that I re-read that section over, 32.3.1 is giving you the MAXIMUM number allowable so you really should be rounding down to the nearest standard ampere rating of 20. Is that correct?
Last but not least, is it an issue if your largest BCPD, in this case fuses for a VFD, end up being the same amperage rating as the MCCB needs to be based on the calculations from 32.3.1?
When a question on my recent MTR re-qualify exam came up about sizing fuses for the primary and secondary sides of a control transformer (Table 42.2) I got turned around for a bit. My math came out to the primary side protection being 7.8125 and the secondary side being 15.625. In my haste I chose the 8a/20a option on the test, which turned out to be incorrect. Now that I look back on my results of what I got wrong it seems as if I should have chosen the 7a/20a option. For the primary side protection that was calculated out to be 7.8125, using section 5.2 I rounded up to 8a which was seemingly incorrect.
Additionally, if you're calculating the sizing of a MCCB for a panel and using:
32.3.1 The size of the overcurrent protection shall not exceed the ampere value determined from (a) and
(b), whichever is larger:
a) The rating of the largest branch circuit protective device in the circuit plus 125 percent of all heater loads plus the full-load currents of all other motors or other loads in the group that could be in operation at the same time;
Say your calculations came out with a number of 22.62, using section 5.2 I initially thought that you'd round that up to 23 then use Section 31.3.8 to get to your next standard size which would be 25. However, now that I re-read that section over, 32.3.1 is giving you the MAXIMUM number allowable so you really should be rounding down to the nearest standard ampere rating of 20. Is that correct?
Last but not least, is it an issue if your largest BCPD, in this case fuses for a VFD, end up being the same amperage rating as the MCCB needs to be based on the calculations from 32.3.1?