SC/GF Protection on A/C

[belect]

I`m studying to take the masters. I know when you are sizing feeder protection you have to take the next size down on protection if the size is not standard. Also on overload protection you have to take the next size down if you are using fuses and the size is not standard. In Mike Holt`s 2005 NEC Exam Prep Book chapter 11 page 458 he shows an A/C unit with rated load current of 18Amp, max. fuse size on nameplate is 40amp, minimum circuit ampacity is 23 amp.
In the short circuit example part of the problem goes like this:

Protection size=18A x1.75
Protection size=31.5
Next size down. 30A [240.6(A)]

My question is I don`t find where you have to size down? I`m missing something. Can anyone help?

 

[Cavie]

I`m studying to take the masters. I know when you are sizing feeder protection you have to take the next size down on protection if the size is not standard. Also on overload protection you have to take the next size down if you are using fuses and the size is not standard. In Mike Holt`s 2005 NEC Exam Prep Book chapter 11 page 458 he shows an A/C unit with rated load current of 18Amp, max. fuse size on nameplate is 40amp, minimum circuit ampacity is 23 amp.
In the short circuit example part of the problem goes like this:

Protection size=18A x1.75
Protection size=31.5
Next size down. 30A [240.6(A)]

My question is I don`t find where you have to size down? I`m missing something. Can anyone help?

A/C comp. are the exception. You usually upsize per nameplate rating. Comp are protected by internal overload protection.

 

[augie47]

I believe 440.22 would be your Code reference.

 

[Pierre C Belarge]

See 240.4, which will direct you to 240.4(A)-(G).

See 240.4(G), there is also a Table 240.4(G).
The table directs you to Art 440, Parts III and VI.

The sections in 440 provide some relief from the standard overcurrent protection, such as 240.4(D).
The A/C manufacturers do all the math, all one needs to do is read the label and apply 240.4(G) - Art 440, the Parts mentioned above.