ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

Topic Options
Rate This Topic
#191801 - 01/13/10 06:51 PM Switch gear
rknikko Offline
Member
Registered: 02/02/04
Posts: 41
Loc: NY
I see that in some cases where a 400A switch is fused at 400A and 600A switch is fused at 600A. For a 30 apartments service switch, can a 800A switch fused at 800A or does it require to be fused at lower amperage? Any code restriction shown in the code book? Please help.
Top
Work Gear for Electricians and the Trades
#191804 - 01/13/10 08:43 PM Re: Switch gear [Re: rknikko]
Tesla Offline
Member
Registered: 06/16/04
Posts: 1273
Loc: Sacramento, CA
Perform a load calculation per the NEC and size the Service accordingly.

Obviously, 800A fuses will be accepted by a 800A switch -- presuming they are the right type.

Your query gives the appearance of someone new to electrical engineering -- as against field wiring of apartments.

As a general rule Service equipment is sized per the Code and fused to the maximum of the frame size. Further, the typical Poco will only run a limited selection of wire sizes for their drops or laterals. You'll want to know what those values are, too.

After the MAIN, typically a fused bolt-switch, each dwelling will have a disconnecting means. These days each dwelling will be normally individually metered. If so, the Poco will have very definite preferences as to locations and meter heights and working clearances.

I do not recommend attempting any design work of this nature unless it is overseen by an experienced engineer or electrical contractor active with your Poco. In this market, a newbie is going to get into trouble.
_________________________
Tesla
Top
#191805 - 01/13/10 09:34 PM Re: Switch gear [Re: Tesla]
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9038
Loc: Estero,Fl,usa
I would say you have to select the fuse that protects the wire you are using.

You can also have a situation where 400a switchgear is wired with an ampacity of 250a (or some lower number) and as long as the appropriate fuse is used, that is legal. As far as I am concerned this may be a problem because, if that fuse blows, you know the "handyman" will put a 300 in it or whatever fits. I questioned this in a proposal (to the 99 I think, maybe 2002) just to get their take on it and they said, basically, we only inspect to what is installed at the time we inspect and you can't do anything about unqualified people coming along later and overfusing. I suppose that was not the same CMP that came up with the type S requirement wink

_________________________
Greg Fretwell
Top
#191807 - 01/14/10 06:55 AM Re: Switch gear [Re: gfretwell]
renosteinke Offline
Cat Servant
Member
Registered: 01/22/05
Posts: 5316
Loc: Blue Collar Country
Please not that "800-amps" is something of a 'magic number' as far as the code is concerned. For example, fuse sizes change at this point, and you can no longer 'round up.'

In short, you can't put a 799-amp fuse in 800-amp gear - at least, not without some sort of adapter.

I don't encounter this threshold very often - so jump in if I'm confused!
Top
#191808 - 01/14/10 10:25 AM Re: Switch gear [Re: renosteinke]
gfretwell Offline


Member
Registered: 07/20/04
Posts: 9038
Loc: Estero,Fl,usa
Reno, 240.4(B) says "Devices Rated 800 Amperes or Less." so the break is at 801 amps. (similar to language referring to 601 volts). I agree the difference is largely semantic since plus/minus 1 volt or 1 amp is well within the tolerance of the devices but they have to draw the line somewhere.
I do agree with Tesla that you need engineering guidance when you get up into this big stuff.
_________________________
Greg Fretwell
Top
#191814 - 01/14/10 11:56 AM Re: Switch gear [Re: gfretwell]
renosteinke Offline
Cat Servant
Member
Registered: 01/22/05
Posts: 5316
Loc: Blue Collar Country
Thanks, Greg, for the correction! I had a feeling I was not quite correct.

(Look it up? I should have, but my access to my materials is severelt hampered these day, as I move about in the job search. With luck, that quest will be ending soon).
Top

Member Spotlight
Member Since: 02/23/13
Posts: 204
New in the Gallery:
SE cable question
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box


Who's Online
0 registered (), 59 Guests and 9 Spiders online.
Key: Admin, Global Mod, Mod
 
New in the Gallery:
SE cable question
 
Top Posters (30 Days)
Admin 51
HotLine1 43
gfretwell 17
Trumpy 16
Ruben Rocha 13
 
Newest Members
clee512, Jdscott2005, FAIZAN, Regitest2, sureshazhagai

ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals