Came across a 800 amp "frame" 480/277volt 3 phase switchboard today. The main circuit breaker has an adjustable digitrip installed with 400, 500, 600, 800amp settings. The digi trip was set at 600amp and the parallel cables feeding it are 350mcm with 90 degree lugs (350amp/cable total 700amp) . Should the cables have been rated for 800amp and or a "non-adjustable" digitrip rated at 600amp been installed?
800amp 480/277volt 3 phase switchboard
- Thread starter Kob
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jumper
Senior Member
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- 3 Hr 2 Min from Winged Horses
240.6
(B) Adjustable-Trip Circuit Breakers. The rating of
adjustable-trip circuit breakers having external means for
adjusting the current setting (long-time pickup setting), not
meeting the requirements of 240.6(C), shall be the maximum
setting possible.
(C) Restricted Access Adjustable-Trip Circuit Breakers.
A circuit breaker(s) that has restricted access to the
adjusting means shall be permitted to have an ampere rating(
s) that is equal to the adjusted current setting (long-time
pickup setting). Restricted access shall be defined as located
behind one of the following:
(1) Removable and sealable covers over the adjusting
means
(2) Bolted equipment enclosure doors
(3) Locked doors accessible only to qualified personnel
(B) Adjustable-Trip Circuit Breakers. The rating of
adjustable-trip circuit breakers having external means for
adjusting the current setting (long-time pickup setting), not
meeting the requirements of 240.6(C), shall be the maximum
setting possible.
(C) Restricted Access Adjustable-Trip Circuit Breakers.
A circuit breaker(s) that has restricted access to the
adjusting means shall be permitted to have an ampere rating(
s) that is equal to the adjusted current setting (long-time
pickup setting). Restricted access shall be defined as located
behind one of the following:
(1) Removable and sealable covers over the adjusting
means
(2) Bolted equipment enclosure doors
(3) Locked doors accessible only to qualified personnel
Its unlikely the breaker assembly is 90° so the 90° lugs would not alter the 110.14(C) rules on termination. Assuming 75° rating and no de-rating of the conductors you would appear to have a 600 amp rated breaker supplied by 620 amp of conductors(310 x 2)
Provided the conditions jumper noted are in place, the job, IMO, would be compliant.
Provided the conditions jumper noted are in place, the job, IMO, would be compliant.
Just one comment. Since the breakers has adjustable long time whic means that anybody could walk up and change the breaker's longtime setting would't it be required either sized the cable to 800a or provide a means to prevent the breasker's setting to be changed but locking it in place or allowing only authorized persons to access it.
Also, I am intregued that the terminals are rated 90degC as. I wasn't aware that there were any. It must be a C-H MC or MD3800. Even if it was a 100% rated breaker it would have -75degC rated terminals where 90degC rated cable sized per 75degC column is used. I am not aware of any breaker where 90degC rated cable is sized per the 90degC column ond terminated on a breaker which then would require 90degzC tasted terrminasls..
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Which can be as simple as a 'wire tie'.
- Location
- Massachusetts
How is that any different than using fuses? Anybody can replace them with higher rated ones up to the limit of the frame size.
highlegdelta
Member
- Location
- US
Had an inspector make me write in Sharpie on a fuse block "150A Max fuse" to prevent anyone from putting a bigger fuse in. The guy who replaces a 150A fuse with a 200A fuse without investigating the situation isn't going to be stopped by a message written in Sharpie. But hey, whatever you have to do.
Help me out a bit.
Doesn't the subjest 800a breaker have an adjustable trip?
Interesting that you are not familiar with NEC art 240-6(B) adjustable-trip breakers.
The rating of an adjustable trip breaker having EXTERNAL MEANS for adjusting the current setting (long-time pickup setting), not meeting the requirements of 240-6(C), SHALL BE THE MAXIMUM SETTING POSSIBLE.
Unless it can comply with 240-6(C).
And, what do fuses have to do with it?
- Location
- Massachusetts
Yes it does.
I am familure with it, here are the ways to comply with it.
2011 NEC 240.6(C)
Very easy to comply.
I was using fuses as an example.
You seem to be concerned someone could raise the OCP of conductors by adjusting the breaker and of course that is true, just as someone can replace fuses with higher rated ones.
My guess that is my it has been included in the NEC.
Like fused one could go as far as replaced the trip unit off an interchangeable trip breakerr or just replacing the breakerr all together, yes more trouble than changing fuses but it can be done.
My guess is that this is why there should be qualified and knowledgeable people.
But walking up and tweeking the adjustment on a breaker is so enticing and very simple to do.
kwired
Electron manager
- Location
- NE Nebraska
At least someone has to actually bring a replacement fuse, or go through further steps to change a adjustment that is not readily accessible.
If that person is not really that qualified they may not even realize the non readily accessible adjustment even exists.
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Replacing fuses is generally not something you would consider working hot, and in any case requires at least a momentary interruption.
As mentioned, tweaking a trip setting, OTOH, appears to be something you could do more casually if you have access.
As mentioned, tweaking a trip setting, OTOH, appears to be something you could do more casually if you have access.
- Location
- Massachusetts
So you guys envision someone just randomly walking around turning breakers up?:huh:
I don't see it, no one cares about breakers or fuses until they are tripping or blowing which means they would have to have a replacement fuse. So they may use whatever fuse they have on hand or choose to buy a larger fuse just as easily as they might decide to crank up breaker setting.
kwired
Electron manager
- Location
- NE Nebraska
True, but from my experiences, owners or not so electrically qualified maintenance people are less likely to have real quick access to 600 or 800 amp fuses, or even 400 in many cases and someone qualified will be called upon. For 200 amp or less and particularly 250 volt fuses - those are available at most any local hardware store, and of course if it has a higher number on it and will fit - that is what gets used.
I also find these sort of people to be the ones likely to crank up an adjustable motor overload setting when it trips frequently. Which is why I still like melting alloy type overloads better then any type with easily adjustable dials - unless someone that knows better will be watching over them, then the electronic overloads do provide even better protection. Most are not going to change overload elements - though I have seen that done by non qualified people where the overloads were readily available to them. Then they ask you some time later why that motor burned out.
- Location
- Massachusetts
Alright you win, you are right. Breakers are getting turned up all the time and only proper fuses are used for replacement. :roll:
kwired
Electron manager
- Location
- NE Nebraska
Was just my opinion based on my experiences on the matter. Quite frankly I don't know that I have ever seen an adjustable breaker other then a one with a mag trip setting. My guess is where one would see those the most probably has better chance of qualified person's that regularly maintain it.
I have seen many cases of small frame circuit breakers that should be 15 or 20 amp get replaced with 30 amp breakers so that sort of thing doesn't just apply to fuses either, but you still find the unqualified will less likely mess with something over 250 volts and/or 200 amps.
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