Nec 408.36(a)

[erickench]

I had stated this problem in a previous thread about a year and a half before so I'm now bringing it up again. The above NEC section in the 2008 NEC states:

Panelboards equipped with snap switches rated at 30 amperes or less shall have overcurrent protection of 200 amperes or less.

Question: How can a switch rated at 30A be protected by a 200A OCPD?

 

[infinity]

Sounds like you're not familiar with this type of panel. Here's a photo of one:

 

[iwire]

Question: How can a switch rated at 30A be protected by a 200A OCPD?

In general the NEC requires switches be used within their ratings not necessarily protected at their ratings.


For example ...

A standard 15 amp wall switch is often used on 20 amp circuits.

404.14 Rating and Use of Snap Switches. Snap switches
shall be used within their ratings and as indicated in
404.14(A) through (E).

 

[infinity]

Panelboards equipped with snap switches rated at 30 amperes or less shall have overcurrent protection of 200 amperes or less.

If you look at the photo in my previous post you might be able to see that this is a 70 amp panel. That means that the bus feeding each individual cartridge fuse is rated for 70 amps. Each fuse gap is limited to 30 amps due to its configuration and each fuse is in series with the snap switch. This panelboard have been equipped with up to 200 amp bus and protected at 200 amps.

 

[erickench]

Yes but if a snap switch was in series with a 200A OCPD wouldn't that destroy a 30A switch? I mean I understand the configuration in the photo. The fuse gap is limited to 30A and therefore the switch is protected. But how is this rule written and why? Wouldn't it be safer to say that a 30A snap switch can be used in branch circuits in a panel rated at no more than 200A?

 

[infinity]

My guess is that some form of this code standard was written when these panels were actually used in great quantities which translates to about 50 years ago. There could have been some other compelling reason to write the standard in it's present form back then.

 

[jetlag]

Yes but if a snap switch was in series with a 200A OCPD wouldn't that destroy a 30A switch? I mean I understand the configuration in the photo. The fuse gap is limited to 30A and therefore the switch is protected. But how is this rule written and why? Wouldn't it be safer to say that a 30A snap switch can be used in branch circuits in a panel rated at no more than 200A?

I see your point the code doesnt say a 30 amp fuse has to be present and in series . I guess they assume we know panels with snap switches are made that way . Do they make a snap switch panel with out fuses ?

 

[erickench]

Glad you guys agree. I'm making a proposal.

 

[infinity]

I see your point the code doesnt say a 30 amp fuse has to be present and in series . I guess they assume we know panels with snap switches are made that way . Do they make a snap switch panel with out fuses ?

Remember that these are fuse panels so it's likely that part of their listing required the switch to be as large as the biggest fuse that the gap could take. A snap switch without the fuse would make no sense.

 

[jim dungar]

Bussmann makes a similar fused panel, they push as a selective coordination solution.

 

[LarryFine]

Panelboards equipped with snap switches rated at 30 amperes or less shall have overcurrent protection of 200 amperes or less.

Question: How can a switch rated at 30A be protected by a 200A OCPD?

To me, that means the panel's OCP cannot exceed 200a. There can be lower OCP between the main and the switch.

 

[jetlag]

Remember that these are fuse panels so it's likely that part of their listing required the switch to be as large as the biggest fuse that the gap could take. A snap switch without the fuse would make no sense.

I agree , I was just trying to think why the code doesnt mention the 30 amp fuses in series, they only mention the the 200 . And I guess that is the question he is asking :grin: