430-52 question

[noxx]

Need some help resolving a dispute regarding sizing of the overcurrent device for a single motor load. 430-52 states that the overcurrent device "shall not exceed" the calculated value (in this case for a 200hp 480 motor protected by a dual element fuse) of 420A.

Immediately following this is an exception which states that should the calculated value not be a standard fuse/cb size (it isn't)the next higher standard size "Shall be permitted". In the definitions section, "shall be permitted" = allowed but not required.

So, here is the question, regardless of which is advisable, it seems to me that on an odd size for a single motor load, it is permissible to round either up *or* down, as the exception allows you to round up but does not require it, and rounding down prevents you from exceeding the calculated value, thereby staying legal.

I'm having quite an arguement with someone about this point, who insists that you are "required" to round up from an odd size, but I don't see it. Any help or clarification would be appreciated.

-Noxx

 

[iwire]

Re: 430-52 question

Noxx, I read it the same way you do, "Shall be permitted" means it is optional or permissive.

For back up to this look at NEC 2002 90.5(A) and 90.5(B)

This makes it clear what rules are Mandatory and what rules are Permissive.

But what you mean by this?

[ April 29, 2003, 05:36 AM: Message edited by: iwire ]

 

[jim dungar]

Re: 430-52 question

Rounding down is fine as long as you accept the possiblity that the motor may not be able to be started (this is why we can "oversize" in the first place). Always keep the option of being able to go to the rounded up device instead. In your case rounding down means a 400A fuse holder while rounding up requires a 600A fuse holder.

My suggestions (unless you are using a soft-starter) are to install a 600A device and round up to 450A fuses, use fuse reducers, or leave the ability to eventually replace the 400A holders.

 

[noxx]

Re: 430-52 question

Thanks guys I appreciate the resposne, this one is theory only not an install. In the real world I'd want to round up wherever permitted on a motor in order to allow for starting current, the argument I'm having with another electrician is that I'm "required" to round up, which isn't correct, just wanted some back-up on that.

-Noxx

 

[tom baker]

Re: 430-52 question

For a dual element time delay fuse you could probably size it at about 125% of FLA, per bussman, or 350 amps with FRS type fuses. if you go to their web site you can download the electrical protection handbook which has tables, they also have a software program that will determine the size.
And the ultimate size depends on the motor load and how hard it is to start.