Transformers Secondary protection

Status
Not open for further replies.
dionysius said:
I will show my ignorance here,:? but so be it. What does protecting the panel board really mean???? Does it mean if the bus bars are shorted together they will be protected?? Is not that all it means??? And lets face it that should be a rare occurrence......someone drops a crescent wrench into a live panel.....
Click to expand...

It could be a short and that would be a reason to have an OCPD.

But on the other hand what if someone connects 250 amps of load to an unprotected panel that has a bus rated for 200?
 
iwire said:
It could be a short and that would be a reason to have an OCPD.

But on the other hand what if someone connects 250 amps of load to an unprotected panel that has a bus rated for 200?
Click to expand...

But all of the loads are protected by their own breakers.....so that should not be a scenario???? Just asking.....
 
electrofelon said:
No...I mean yes, it is a scenario because there is no requirement to limit the sum of all the branch breakers to less than or equal to the panel rating.
Click to expand...

So would that be a way for the code guys to protect the panel in a simple way???? Simply limit the sum of all the loads to be less than the panel MB or panel rating....whichever is less???
 
charlie b said:

This ornery old (pretending to be) inspector ain’t giving up that easily. ...

Q.E.D. ;)
Click to expand...
What texie said... as we cannot forget the requirement of 408.36 general statement, "a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard." We [you] have to correlate the primary protection rating with the rating of the panelboard—as if the secondary conductor ampacity—per 240.21(C)(1).
 
dionysius said:
So would that be a way for the code guys to protect the panel in a simple way???? Simply limit the sum of all the loads to be less than the panel MB or panel rating....whichever is less???
Click to expand...

That would work but would be ridiculously inefficient.

A better plan is to just follow the existing code and provide protection for the panelboard.
 
Dexie123 said:
I'm sure this subject has been discussed to death but I still hear a lot of different opinions to this question.

Is there any situation where a transformers secondary conductors can be protected by the primary's ocpd and then terminate in a 42 circuit panel without a main circuit breaker in that panel? I am aware the primary ocpd protects the transformer if it's 125% rated and it can protect the secondary conductors for 10ft or 25feet depending but I believe there are no exceptions where a panel can not do without a main ocpd to protect the panel.

Thanks for your input.
Click to expand...

http://static.schneider-electric.us...on/Medium Voltage Transformers/0110DB0201.pdf
 
Smart $ said:
... as we cannot forget the requirement of 408.36 general statement, "a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard."
Click to expand...
In my hypothetical situation, I have a 40 amp breaker and I have a 100 amp panel. I seem to meet the “rating not greater than” part of this rule. The issue is whether I can do what the code requires, and only what the code requires, and protect everything that is in need of protection. I think I have offered a scenario in which following the code does not provide adequate protection.
Smart $ said:
We [you] have to correlate the primary protection rating with the rating of the panelboard—as if the secondary conductor ampacity—per 240.21(C)(1).
Click to expand...
That article only talks about the protecting the secondary conductors at their ampacity, by using the primary side breaker and taking into account the turns ratio. I find nothing anywhere that says to compare the rating of the panel to the rating of the primary side breaker by taking into account the turns ratio.
 
dionysius said:
So would that be a way for the code guys to protect the panel in a simple way???? Simply limit the sum of all the loads to be less than the panel MB or panel rating....whichever is less???
Click to expand...
408.30 already requires that. The earlier suggestion was that the sum of the breaker ratings can (an often will) exceed the rating of the panel. Consider a 225 amp panel with 42 breakers, each rated at 20 amps. I could load that panel to 840 amps in such a way that none of the branch circuit breakers would trip to protect the panel. The only thing that would protect the panel is either its own main breaker or the upstream overcurrent device.

 
charlie b said:
408.30 already requires that. The earlier suggestion was that the sum of the breaker ratings can (an often will) exceed the rating of the panel. Consider a 225 amp panel with 42 breakers, each rated at 20 amps. I could load that panel to 840 amps in such a way that none of the branch circuit breakers would trip to protect the panel. The only thing that would protect the panel is either its own main breaker or the upstream overcurrent device.

Click to expand...

So, a single phase (line to neutral) 42 pole panel?
 
dionysius said:
What does protecting the panel board really mean????
Click to expand...
If you run more current through the panel's bus bars that the panel is rated to handle, it will overheat. I doubt that the bus bars themselves would melt. But there is a risk of some other part of the enclosure becoming compromised. This could result in a short circuit or a fire.
 
dkidd said:
So, a single phase (line to neutral) 42 pole panel?
Click to expand...
I changed subjects while you were not looking. My hypothetical situation involved a single phase panel. My 840 amp overload description was an answer to the question of why panels need overcurrent protection.

 
charlie b said:
In my hypothetical situation, I have a 40 amp breaker and I have a 100 amp panel. I seem to meet the “rating not greater than” part of this rule. The issue is whether I can do what the code requires, and only what the code requires, and protect everything that is in need of protection. I think I have offered a scenario in which following the code does not provide adequate protection.
That article only talks about the protecting the secondary conductors at their ampacity, by using the primary side breaker and taking into account the turns ratio. I find nothing anywhere that says to compare the rating of the panel to the rating of the primary side breaker by taking into account the turns ratio.
Click to expand...
It's one of those ambiguously implied correlations and you have to infer the intent. So either you see it or you don't. Let me ask you this. Why do you think that article is stipulated if it says nothing specifically about protecting a panelboard? Isn't that enough to stimulate the spidey senses and indicate something is being implied?
 
charlie b said:
I changed subjects while you were not looking. My hypothetical situation involved a single phase panel. My 840 amp overload description was an answer to the question of why panels need overcurrent protection.
Click to expand...

The only way the math works to get 840 amps overload is with a single phase (line to neutral) panel.
 
dkidd said:
The only way the math works to get 840 amps overload is with a single phase (line to neutral) panel.
Click to expand...
You are right. Thank you.

My point remains, however. I can take a 225 amp, 3-phase, 120/208V, Main Lug Only panel with 42 breakers, load each breaker to 20 amps, for a total of 840 amps of single phase load, which equates to a three phase load on the feeder to the panel of 280 amps, thereby overloading the 225 amp panel, and none of the breakers will trip to protect the panel. Therefore, some form of protection, either a main breaker on the panel or an upstream OCPD, is needed.

 
Status
Not open for further replies.
Top