100% Rated Breaker to Add Load?

[wwhitney]

Almost all bolted pressure switches are 100% rated by design, but enclosure manufacturers may not provide enough ventilation.

To expand on this, looks like to use a fuse at a continuous load of 100% of its rating, you need to use a UL 977 (Fused Power Circuit Devices) switch. That standard says "1.2 Fused power-circuit devices . . are considered to be either bolted pressure contact switches or high-pressure butt-type contact switches." Oddly, it doesn't discuss 100% rating that I could see on a quick perusal, but it also doesn't have the UL 98 language imposing an 80% continuous load limit across the board.

The standard also specifies that fused power circuit devices have provisions for mounting fuses larger than 600A. So I guess if you want to use a fuse of 600A or smaller with a continuous load of 100% of the fuse rating, you're out of luck?

Cheers, Wayne

 

[tortuga]

Oddly, it doesn't discuss 100% rating

Agreed there does not appear to be a special additional UL listing or rating of the 'assembly' (in the NEC's words) of a 100% rated knife switch or a bolted pressure switch with a 100% rated fuse. Although I see now how the NEC wording can be construed to imply that, as that would be the case for breakers.

So I guess if you want to use a fuse of 600A or smaller with a continuous load of 100% of the fuse rating, you're out of luck?

Use a UL 363 knife switch, they are rated for continuous load.

 

[wwhitney]

Use a UL 363 knife switch, they are rated for continuous load.

Can you point me to a fused product listed under UL 363 that is labeled as 100% rated for continuous loads as an assembly?

I looked through Eaton's catalog of "Safety Switches," and there was no mention of UL 363. The General Duty and Heavy Duty switches all seemed to be listed to UL 98. So as per post #40, they would all be limited to 80% of the fuse rating for continuous loads.

Cheers, Wayne

 

[tortuga]

Here's one LOL

Antique Industrial Knife Switch - Great Condition - Large - 200a Throw Switch | eBay

<p dir="ltr" style="margin-top:0; margin-bottom:0;">This is a 200a 2 pole, single throw, fused knife switch. The conductors are high purity copper.</p> <br> <p dir="ltr" style="margin-top:0; margin-bottom:0;">Brand: Trumbull</p> <br> <p dir="ltr" style="margin-top:0; margin-bottom:0;">Era...

www.ebay.com

 

[MJRobinson]

All that said, I'm looking for a fast resource for a 1200A 100% rated, or a 1600A for 1200A fuses in short order? Anyone have any recommendations?

 

[petersonra]

I don't know what you mean by "fast". You can probably get a 1600 amp breaker in two or three weeks from someone. You would have to buy an enclosure to put it in but that is not a big deal.

The devil is in the details. How many poles? Voltage? Aic rating?

 

[infinity]

All that said, I'm looking for a fast resource for a 1200A 100% rated, or a 1600A for 1200A fuses in short order? Anyone have any recommendations?

I'm still lost as to why you need to do this for a service. Can you explain? Your service is 1200 amps and you want to use 1200 amp fuses? What have you gained? What is the calculated load of this service?

 

[petersonra]

Probably can order it today, can be in your hands tomorrow if you pay the freight.

 

[Jraef]

A breaker rated for 100% requires a whole lot more ventilation than does an 80% rated one. It would be extremely unlikely that you could upgrade an existing breaker without modifying/replacing its enclosure.

I’m encountering that exact situation right now. The gear is Arc Resistant and as it turns out, the ventilation requirements to make the Main 100% rated are precluded in the Arc Resistant design. So it’s a no-go.

 

[wwhitney]

I’m encountering that exact situation right now. The gear is Arc Resistant and as it turns out, the ventilation requirements to make the Main 100% rated are precluded in the Arc Resistant design. So it’s a no-go.

Aren't there other types of breakers that manufacturers can make, like electronic trip or hydraulic magnetic, that are less sensitive to ambient temperature than thermal magnetic, and so could pass the 100% rating test without additional ventilation? Wondering.

Cheers, Wayne

 

[Jraef]

Aren't there other types of breakers that manufacturers can make, like electronic trip or hydraulic magnetic, that are less sensitive to ambient temperature than thermal magnetic, and so could pass the 100% rating test without additional ventilation? Wondering.

Cheers, Wayne

This is electronic trip.
It's not about the trips, it's about the temperature rise of the breaker.

 

[wwhitney]

It's not about the trips, it's about the temperature rise of the breaker.

OK, so I took a look at UL 489 on molded case circuit breakers. My take away is that the breaker terminal temperature rise is limited to 50C. First you test it in free air at 100% rated current. If it doesn't pass that test because the terminal temperature rise is over 50C, you have the option to instead test it in the smallest intended enclosure at 80% rated current, and pass that test. [This latter test is how the breaker is used in practice, but I guess the assumption is that the heat dissipation rate inside an enclosure will never be worse than (80/100)² = 64% of the heat dissipation rate in free air?] And if you want to 100% rate the breaker, you need to pass the test at 100% rated current in the smallest enclosure intended for 100% use.

With electronic trip, you can set a trip point wherever you want, as long as it's less than the frame size? And the heat generation will not change based on trip setting? If so, it seems to me that with any, say, 1200A frame breaker, you could set the trip point to 960A or lower and use it as a 100% rated breaker. Is that right?

Of course, if you have existing gear, or a limit on the size of the gear and thus the resulting frame size, that doesn't help you.

Cheers, Wayne