Panel board ratings?

Status
Not open for further replies.
petersonra said:
I think the same ones you would use on a 240V delta system would be appropriate.
Click to expand...
Umm... that depends on whether it has a neutral bus (or option), if the 240Y/138V neutral is used. :happyyes:

All breakers would have to be rated straight 240V, and not 120/240.
 
petersonra said:
I think the same ones you would use on a 240V delta system would be appropriate.
Click to expand...


That actually would be a huge win:D



Heres some job info: Building is fed via 240 delta secondary but is to be changed out soon. I considered changing everything over to 208Y120 but most of the CNC and milling lathes are rated for 240, not sure how well they would handle 208. The other options would be bringing in a center tapped delta or keeping the system ungrounded which from the looks of it is a bad idea.
 
Smart $ said:
Umm... that depends on whether it has a neutral bus (or option), if the 240Y/138V neutral is used. :happyyes:

All breakers would have to be rated straight 240V, and not 120/240.
Click to expand...


No neutral loads that I know of. The loads are 3 phase CNC and drill lathes and a few 240 single phase motors. The existing panels are 240 delta.


By how much would the AIC be reduced on a slash rated breaker just out of curiosity?
 
mbrooke said:
...

Heres some job info: Building is fed via 240 delta secondary but is to be changed out soon. I considered changing everything over to 208Y120 but most of the CNC and milling lathes are rated for 240, not sure how well they would handle 208. The other options would be bringing in a center tapped delta or keeping the system ungrounded which from the looks of it is a bad idea.
Click to expand...
What about 120V loads? I have to assume there are at least a few...
 
Smart $ said:
What about 120V loads? I have to assume there are at least a few...
Click to expand...

Plenty exist, for now the plan is to reuse the existing 240D 208Y120 volt dry units.:p Basically its an ungrounded delta service thats going to a wye. 208Y would've been nice since we could just bypass the transformers but its not worth the hassle of then having to boost the voltage to the CNC machines. :happysad:
 
mbrooke said:
Plenty exist, for now the plan is to reuse the existing 240D 208Y120 volt dry units.:p Basically its an ungrounded delta service thats going to a wye. 208Y would've been nice since we could just bypass the transformers but its not worth the hassle of then having to boost the voltage to the CNC machines. :happysad:
Click to expand...
So the POCO will supply you with a 240Y/138 service???
 
hurk27 said:
A 240/139 volt WYE transformer is not a common animal but not unheard of?
Click to expand...

They make them, some larger manufacturers might have a few in stock, but certainly able to do in a custom order. Commonly used for VFD isolation and when changing a delta to a wye. Much safer than an ungrounded system with standing ground faults with under qualified maintenance:eek:


Most generator manufacturers actually offer it as a standard option on genets (see alternator ratings):

http://www.kohlerpower.com/onlinecatalog/pdf/g5361.pdf
 
mbrooke said:
They make them, some larger manufacturers might have a few in stock, but certainly able to do in a custom order. Commonly used for VFD isolation and when changing a delta to a wye. Much safer than an ungrounded system with standing ground faults with under qualified maintenance:eek:


Most generator manufacturers actually offer it as a standard option on genets (see alternator ratings):

http://www.kohlerpower.com/onlinecatalog/pdf/g5361.pdf
Click to expand...

True I have wired a few gen sets for 240/138.57 which I always round up to 139? but I have yet to wire a 240/139 transformer.

As for a ungrounded system being unsafe, I kind of like them as for one as long as the first fault hasn't been made yet(which the alarm should be sounding if it was) you have one less path to create an arc flash, and anyone who works around them should always treat them as if there will be a voltage between the ungrounded and grounds, so I'm not sure why people are afraid of them, not just you but many are.

I had an inspector who tried to not allow us to install an ungrounded system at a plant that had a critical operation that would be more dangerous if it lost power, he tried to tell me that if a hot faulted to ground (foundry) then all the grounding would have full voltage to Earth and no breakers would trip, I tried to explain to him that that would be true if the secondaries were referenced to Earth at the transformer, but you don't have that in an ungrounded system (at least it better not be) and since there are no reference to Earth on the secondaries then once a phase faults to ground the first time it just turns the system into a corner grounded system and a second fault will trip the breaker, which is the whole idea behind using an ungrounded system in the first place, and this is why it is allowed in the NEC, took me a little bit and some drawings to get him to understand but he came around and signed off on it.

He admitted that he had some misconceptions that he didn't understand until I had made the drawings of the system which he said made it allot clearer.
 
Last edited:
hurk27 said:
True I have wired a few gen sets for 240/138.57 which I always round up to 139? but I have yet to wire a 240/139 transformer.

As for a ungrounded system being unsafe, I kind of like them as for one as long as the first fault hasn't been made yet(which the alarm should be sounding if it was) you have one less path to create an arc flash, and anyone who works around them should always treat them as if there will be a voltage between the ungrounded and grounds, so I'm not sure why people are afraid of them, not just you but many are.

I had an inspector who tried to not allow us to install an ungrounded system at a plant that had a critical operation that would be more dangerous if it lost power, he tried to tell me that if a hot faulted to ground (foundry) then all the grounding would have full voltage to Earth and no breakers would trip, I tried to explain to him that that would be true if the secondaries were referenced to Earth at the transformer, but you don't have that in an ungrounded system (at least it better not be) and since there are no reference to Earth on the secondaries then once a phase faults to ground the first time it just turns the system into a corner grounded system and a second fault will trip the breaker, which is the whole idea behind using an ungrounded system in the first place, and this is why it is allowed in the NEC, took me a little bit and some drawings to get him to understand but he came around and signed off on it.

He admitted that he had some misconceptions that he didn't understand until I had made the drawings of the system which he said made it allot clearer.
Click to expand...



I agree, they are very misunderstood. Even inspectors don't get them. Im not afraid of them though when serviced by qualified personal and have a ground detector. However at this sight service continuity isn't a major issue. Believe it or not the system already had 2 sanding ground faults on the C phase:happysad: No idea how long its been like this, but onsite maintenance had no clue. IIMO the best system for critical operations is a high resistance ground with a continues duty resistor. If the resistor is sized correctly the system can still run with a grounded phase, but any arc faults will not puncture insulation. A win win IMO.:D


Out of curiosity, what was your gen running? Similar scenario?
 
Not sure if I should start another thread but anyone know how much AIC is lost when interrupting 135 instead of 120 volts on a pole?
 
JoeStillman said:
We don't have enough information to answer this one. It's not a simple volts-ratio thing. Do you know the transformer kVA and impedance?
Click to expand...


Don't know the impedance yet, but one is at least 225kva, the other a 150kva dry type. The feed length will play a role as well.


My concern is a lot of the older panels are full of slash rated breakers (they shouldn't be:roll:), on how that will effect the AIC rating.


Just to be clear no 138 volt loads are present. Everything on these panels is 240.
 
mbrooke said:
Don't know the impedance yet, but one is at least 225kva, the other a 150kva dry type. The feed length will play a role as well.


My concern is a lot of the older panels are full of slash rated breakers (they shouldn't be:roll:), on how that will effect the AIC rating.


Just to be clear no 138 volt loads are present. Everything on these panels is 240.
Click to expand...

Without the necessary data, let's assume your transformer percent impedance is 2%. At 240, your full-load amps will be 541 amps. Your AIC will be 541/0.02 = 27 kA (assuming there are no motor current contributions during fault and an infinite source). That's how far I can go here.
 
topgone said:
Without the necessary data, let's assume your transformer percent impedance is 2%. At 240, your full-load amps will be 541 amps. Your AIC will be 541/0.02 = 27 kA (assuming there are no motor current contributions during fault and an infinite source). That's how far I can go here.
Click to expand...

Good input!:)


But that part I know, my question is, if the calculated fault current under all conditions comes out to say 14,000 amps, which will require the use of 22kaic breakers, how far will that rating drop under a phase to ground fault when the poles are interrupting 135 volts instead of 120? I know its not something that's published as a chart but was wondering if someone knew the resources or something to help guide me.


I would guess (again its a guess) that a 20 volt rise would reduce the AIC to 20ka instead of the 22ka rating which would still be acceptable in most of the panels.
 
mbrooke said:
Good input!:)


But that part I know, my question is, if the calculated fault current under all conditions comes out to say 14,000 amps, which will require the use of 22kaic breakers, how far will that rating drop under a phase to ground fault when the poles are interrupting 135 volts instead of 120? I know its not something that's published as a chart but was wondering if someone knew the resources or something to help guide me.


I would guess (again its a guess) that a 20 volt rise would reduce the AIC to 20ka instead of the 22ka rating which would still be acceptable in most of the panels.
Click to expand...
Actual avaliable fault current is an issue with the source and not the breakers. The breakers need to be able to handle the available fault from the source they are connected to. True if the kVA rating of the transformer remains the same and all you did was modify it to so it operated at a sightly higher voltage that the available fault current probably does decrease a little bit. But breakers are not designed to be matched to a specific source so I think this is a little bit of the wrong way of approaching the issue.

Isn't the bigger question whether or not a 120 volt nominal rated breaker able to take 139 volts nominal from an insulation point of view? I know it don't seem like all that big of a voltage change but if they were intended for such a system I could see them being rated for 150 instead of 120. If you factor in the common 10% tolerance levels that 139 volt system could be operating at almost 153 volts on the high end.
 
kwired said:
Actual avaliable fault current is an issue with the source and not the breakers. The breakers need to be able to handle the available fault from the source they are connected to. True if the kVA rating of the transformer remains the same and all you did was modify it to so it operated at a sightly higher voltage that the available fault current probably does decrease a little bit. But breakers are not designed to be matched to a specific source so I think this is a little bit of the wrong way of approaching the issue.

Isn't the bigger question whether or not a 120 volt nominal rated breaker able to take 139 volts nominal from an insulation point of view? I know it don't seem like all that big of a voltage change but if they were intended for such a system I could see them being rated for 150 instead of 120. If you factor in the common 10% tolerance levels that 139 volt system could be operating at almost 153 volts on the high end.
Click to expand...


If by insulation you mean interrupting that's exactly my question.


If it helps, assuming the phase to phase voltage doesn't go over 253 (the max for most of these machines, the ground voltage wont go above 146 volts max. In any case its better than the previous ungrounded delta system since the ground voltage could reach 240 under a fault.
However, the issue is still the breakers being subjected to higher than the lower of the slash rating.


One thing that's confusing is that double pole breakers are available straight rated but only for 10kaic, not 22kaic. Even more interesting is that the straight rated breakers when applied to a corner grounded system are only rated 5ka even though they say 10ka? While the new system will be far away from a corner ground the listings have me confused.


Page 2: http://static.schneider-electric.us/assets/digest/17601.pdf
 
Status
Not open for further replies.
Top