Cable Sizing for Switchgears

I sized a cable for a 2000 A, 480 V Switchgear. I sized the cable to handle loads equal to the rating of the switchgear which is 2000 A. The Electrical Engineering lead came back saying that I need to size it for total loading on the switchgear. Please let me know as to what should i be sizing the cable... the swgr rating or the total load?

Thanks
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
What is the general philosophy for sizing cables for switchgears ???
I don't know the norm for switchgear unless it had a main. If there is a main breaker then you size it to that. You certainly can size it to the calculated load but then any additional work may need a service upgrade. It doesn't matter what is usual it is what the approved plans call for.
 

chris kennedy

Senior Member
Location
Miami Fla.
You mean parallels right, cables or raceways and conductors?

2000 A, 480 V Switchgear. I sized the cable to handle loads equal to the rating of the switchgear which is 2000 A. The Electrical Engineering lead came back saying that I need to size it for total loading on the switchgear. Please let me know as to what should i be sizing the cable... the swgr rating or the total load?

Thanks
Whats on the upstream end of this thing? OCPD or POCO? What is the calculated load on the gear?

As Dennis stated, what did the EE spec?
 
You mean parallels right, cables or raceways and conductors?



Whats on the upstream end of this thing? OCPD or POCO? What is the calculated load on the gear?

As Dennis stated, what did the EE spec?
its actually an emergency switchgear........ fed from another 480 V switchgear / Emergency generator by means of an ATS.​
 

augie47

Moderator
Staff member
Location
Tennessee
Then in this case it appears we are addressing feeders.
Other than a few exceptions and the 240.21 tap rules, once you exceed 800 amps, the feeder should have the ampacity for the load and should be equal to or greater than the size of the overcurrent device at its source.
 

kwired

Electron manager
Location
NE Nebraska
I don't understand the problem here. My understanding is we have 2000 amp gear supplied with 2000 amp conductors. Engineer is saying conductors must be sized according to load.

If the gear is 2000 amps isn't the load going to be 2000 amps or less? So what is the problem with 2000 amp conductors? You possibly spent more on conductors than you needed to but did not create an unsafe condition, and certainly is not worth making it so the conductors are less than 2000 amps at this point.
 

edward

Senior Member
Maybe the total calculated load is less than 2000Amps (lets say 1400Amps) but you have sized the conductors according to the rating of switch gear that is 2000Amps. But the engineer is asking you to size the conductors according to the calculated load vs the rating of the switch gear???

What is it that the engineer is asking exactly? or what you have done exactly?
 
I sized a cable for a 2000 A, 480 V Switchgear. I sized the cable to handle loads equal to the rating of the switchgear which is 2000 A. The Electrical Engineering lead came back saying that I need to size it for total loading on the switchgear. Please let me know as to what should i be sizing the cable... the swgr rating or the total load?

Thanks
It appears that the engineer is aking that you calculate the actual load and size the feeder cable and its OCP accordingly. There are various load calculation rules in the NEC and you can use the applicable one. There are also various practices in the industry as to how to calculate loads, taking load diversity and utilization factors into consideration, but your lead really needs to be more specific what rules does he wish you to follow. As you latter mentioned the switchgear is fed by a generator via an ATS. The size of the ATS and Generator could also be limiting factors unless the lead also implies that you should also size those too according to the expected load. Of course after you sized to the load, you still need to check for voltage drop on the cables and voltage stability on the generator on the running and expected starting/inrush loads.
 

kwired

Electron manager
Location
NE Nebraska
It appears that the engineer is aking that you calculate the actual load and size the feeder cable and its OCP accordingly. There are various load calculation rules in the NEC and you can use the applicable one. There are also various practices in the industry as to how to calculate loads, taking load diversity and utilization factors into consideration, but your lead really needs to be more specific what rules does he wish you to follow. As you latter mentioned the switchgear is fed by a generator via an ATS. The size of the ATS and Generator could also be limiting factors unless the lead also implies that you should also size those too according to the expected load. Of course after you sized to the load, you still need to check for voltage drop on the cables and voltage stability on the generator on the running and expected starting/inrush loads.
If the gear is 2000 amps then I would hope the load is no more than 2000 amps. What is the problem other than more cost of conductors, or the engineer's wanting to be right, to supply it with 2000 amp conductors?

Why did the engineer likely design this, select the 2000 amp gear and not specify what he wanted for supply conductors in the first place? Usually they do tell you what size and type and have about every detail of how deep to bury them, how to arrange the raceways in the trench, etc.

Maybe he did specify a conductor size and the OP somehow missed it. Still, what is it going to hurt other than an available fault current calculation maybe needs re examined. If cost is an issue then maybe the contractor has to absorb whatever was extra, it will likely cost him even more to change it now when what is there really is not a problem.
 
If the gear is 2000 amps then I would hope the load is no more than 2000 amps. What is the problem other than more cost of conductors, or the engineer's wanting to be right, to supply it with 2000 amp conductors?

Why did the engineer likely design this, select the 2000 amp gear and not specify what he wanted for supply conductors in the first place? Usually they do tell you what size and type and have about every detail of how deep to bury them, how to arrange the raceways in the trench, etc.

Maybe he did specify a conductor size and the OP somehow missed it. Still, what is it going to hurt other than an available fault current calculation maybe needs re examined. If cost is an issue then maybe the contractor has to absorb whatever was extra, it will likely cost him even more to change it now when what is there really is not a problem.
The lead engineer would need to explain some of those questions. The OP did NOT post any of the reasoning why his sizing to 2000A was not accepted by the lead.

Sizing the gear for future needs is not unusual. It is easier to add parallel future cable(s) than change the gear.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
Nobody is saying the switch gear conductors have to match the size of the switch gear. We all know that a 200 amp panel may be fed with 100 amp breaker as long as the load is 100 amps or less. No main breaker then the conductors can be sized to the load however the EE wants to be certain that if more load is added then there isn't an issue.

Of course, if this were a service panel with no main you could still potentially load the 6 disconnects beyond the conductors capacity. Again if the EE specs the conductor size then you must do what the signed prints call for. Interesting the OP has never answered this simple question.
 

kwired

Electron manager
Location
NE Nebraska
Nobody is saying the switch gear conductors have to match the size of the switch gear. We all know that a 200 amp panel may be fed with 100 amp breaker as long as the load is 100 amps or less. No main breaker then the conductors can be sized to the load however the EE wants to be certain that if more load is added then there isn't an issue.

Of course, if this were a service panel with no main you could still potentially load the 6 disconnects beyond the conductors capacity. Again if the EE specs the conductor size then you must do what the signed prints call for. Interesting the OP has never answered this simple question.
I'm not saying the conductors need to match the size of the gear either, but fail to see why it is a problem that they do happen to match the gear. Specifications have been met or exceeded.

A person could disconnect a set or two of the parallels I guess but don't see that being worth the effort other than if it would happen to mean too high of available fault current if left connected.
 
I'm not saying the conductors need to match the size of the gear either, but fail to see why it is a problem that they do happen to match the gear. Specifications have been met or exceeded.

A person could disconnect a set or two of the parallels I guess but don't see that being worth the effort other than if it would happen to mean too high of available fault current if left connected.
Th engineer has the responsibility to select equipment for the specific needs identified for the task to be accomplished. Good engineering practice dictates to do that in the most cost effective way. It takes into consideration current and future needs. The feeder size thus maybe variable.
 

kwired

Electron manager
Location
NE Nebraska
Th engineer has the responsibility to select equipment for the specific needs identified for the task to be accomplished. Good engineering practice dictates to do that in the most cost effective way. It takes into consideration current and future needs. The feeder size thus maybe variable.
Again, why is this a problem after it has been done already, other than it may have cost more than it needed to, or possibly raised the available fault current to an unacceptable level?

If the feeder were too small for the load I think we all see a problem with that, but too large without other substantiation makes no sense to make it be changed. Sounds like an engineer that has a case of needing to be right no matter what to me. If I were OP I would want good reason to change this other than "because I say so".
 
Again, why is this a problem after it has been done already, other than it may have cost more than it needed to, or possibly raised the available fault current to an unacceptable level?

If the feeder were too small for the load I think we all see a problem with that, but too large without other substantiation makes no sense to make it be changed. Sounds like an engineer that has a case of needing to be right no matter what to me. If I were OP I would want good reason to change this other than "because I say so".
The engineering lead requested that the engineer size the cable to the load. Cost could be one of the 'good reason' but there may be others.
 
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