Basic Sizing Question

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Alwayslearningelec

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Seem to always get confused on rules of sizing etc. Thank you very much.

Have 3P-100A switch w/ 100A trip...not sure of load amperage.

1. The load the switch is feeding can be any size/amperage?
2. The wire size CAN'T have current carrying capicity LESS than 100A?
3. You size the wire based on switch/fuse size or LOAD amperage?
 
horsegoer said:
Seem to always get confused on rules of sizing etc. Thank you very much.

Have 3P-100A switch w/ 100A trip...not sure of load amperage.

1. The load the switch is feeding can be any size/amperage?
2. The wire size CAN'T have current carrying capicity LESS than 100A?
3. You size the wire based on switch/fuse size or LOAD amperage?
Click to expand...
What is the load, some load types have different rules than others, motors can have breaker that is 2.5 times motor rated current easily with conductor that is only 1.25 times motor current. The motor still needs an additional "overload protector" that will not only protect the motor but also protects the conductor.
 
The basic I follow (with some exceptions such as motors, qas pointed out by kwired))
1. Calculate the load (100% of non-continuous + 125 % of continuous)
2. Select the conductor to supply the load based on the calculation
3. Choose the overcurrent device based on Art240 rules

The conductor must be sized to carry the load, the OCP sized to protect the conductor.
Your switch can supply any calculated load 100 amps or less.
Your switch could supply any size conduictor with an ampacity greater than 90 amps (the next lowest size standard OCP) as longf as the laod does not excced the ampacity of the conductor.
 
kwired said:
What is the load, some load types have different rules than others, motors can have breaker that is 2.5 times motor rated current easily with conductor that is only 1.25 times motor current. The motor still needs an additional "overload protector" that will not only protect the motor but also protects the conductor.
Click to expand...
elevator controller

Sent from my SM-N960U using Tapatalk
 
Have 3P-100A switch w/ 100A trip...not sure of load amperage.
Click to expand...
You say "switch" but then "trip", so is that a circuit breaker then? With a CB, you then also have to know if the load is continuous (defines as 3 hours or more) or not. For continuous loads, you must size the conductors at 125% of the load, then you have to size the breaker to protect those conductors, so you end up with the breaker being rated for 80% of the load. So because of that, breakers in (most) panelboards will be rated that way, i.e. "80% rated", and to get breakers that are listed as "100% rated", they need to be stand alone. Technically, all breakers are tested and listed at 100% of their rated current, but that changes when they are put into a panelboard with other breakers. So not only do you need to understand the nature of the load, you must understand how the breaker is going to be used.

For elevators, I believe (but am not 100% sure) that they are NOT considered to be "continuous" loads, so the conductors and therefore breakers can be used at their full rating. But don't take my word for it...
 
Jraef said:
You say "switch" but then "trip", so is that a circuit breaker then? With a CB, you then also have to know if the load is continuous (defines as 3 hours or more) or not. For continuous loads, you must size the conductors at 125% of the load, then you have to size the breaker to protect those conductors, so you end up with the breaker being rated for 80% of the load. So because of that, breakers in (most) panelboards will be rated that way, i.e. "80% rated", and to get breakers that are listed as "100% rated", they need to be stand alone. Technically, all breakers are tested and listed at 100% of their rated current, but that changes when they are put into a panelboard with other breakers. So not only do you need to understand the nature of the load, you must understand how the breaker is going to be used.

For elevators, I believe (but am not 100% sure) that they are NOT considered to be "continuous" loads, so the conductors and therefore breakers can be used at their full rating. But don't take my word for it...
Click to expand...

Based on submittal( see attached) sure looks like fused switches but then the circuit schedule does list "trip". Isnt trip only for CB as you mentioned?
 

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Looks like fused switches with 100 amp Type R fuses which is pretty standard for elevators.
 
"VBS" is the trade name for Siemens fused switchboard disconnect switches (Short for Visi-Break Switch). So their use of the term "trip" is not really accurate, it must mean the fuse size, because there is no "tripping" of a fuse, they "clear" and must be replaced. It's just poor semantics on the part of whomever made that drawing... 100A switch with 100A fuses.
 
Jraef said:
"VBS" is the trade name for Siemens fused switchboard disconnect switches (Short for Visi-Break Switch). So their use of the term "trip" is not really accurate, it must mean the fuse size, because there is no "tripping" of a fuse, they "clear" and must be replaced. It's just poor semantics on the part of whomever made that drawing... 100A switch with 100A fuses.
Click to expand...
Thanks a lot forr that info. appreciate it.
 
So back to my question. If we know fuse is 100A the feeders( after calculations) current carrying capacity can;t be less than 100A? TY
 
horsegoer said:
So back to my question. If we know fuse is 100A the feeders( after calculations) current carrying capacity can;t be less than 100A? TY
Click to expand...

In this case the 100 amp circuits are branch circuits not feeders. Branch circuit motor loads can have an OCPD larger than the conductor size. If these elevator controllers are VFD's things may be a little different than just a motor load.
 
infinity said:
In this case the 100 amp circuits are branch circuits not feeders. Branch circuit motor loads can have an OCPD larger than the conductor size. If these elevator controllers are VFD's things may be a little different than just a motor load.
Click to expand...

Thanks. If it has a VFD how could it be different?
 
horsegoer said:
Thanks. If it has a VFD how could it be different?
Click to expand...
Different rules. Conductors feeding a VFD are to be sized at 125% of the VFD input current, not the motor FLC as per 430.122. Input current on a VFD is usually less than the output current because of the change in power factor, so the input conductors can sometimes be smaller than the motor feeder conductors under the right circumstances.

But you have to be careful because on some VFDs that were actually designed for IEC motor sizes at 380V and then had their nameplates adapted to use here on 460V HP ratings, it can be the other way around and that can bite you because it is based on whatever the VFD nameplate says is the maximum input current. Several Asian drive mfrs have this problem.
 
Jraef said:
Different rules. Conductors feeding a VFD are to be sized at 125% of the VFD input current, not the motor FLC as per 430.122. Input current on a VFD is usually less than the output current because of the change in power factor, so the input conductors can sometimes be smaller than the motor feeder conductors under the right circumstances.

But you have to be careful because on some VFDs that were actually designed for IEC motor sizes at 380V and then had their nameplates adapted to use here on 460V HP ratings, it can be the other way around and that can bite you because it is based on whatever the VFD nameplate says is the maximum input current. Several Asian drive mfrs have this problem.
Click to expand...
My issue with said rule is when you have a drive that is rated for more than the motor connected to it. Technically you have to size input conductors to drive input, I have had a few cases where an oversized drive just happened to be available and why not use it. Like a 250 hp drive on a 100 hp motor. Drive is never going to draw anywhere close to it's input rating in this instance, if it does chances are it is also going to have a fault code on it in virtually no time and will shut down anyway.
 
infinity said:
In this case the 100 amp circuits are branch circuits not feeders. Branch circuit motor loads can have an OCPD larger than the conductor size. If these elevator controllers are VFD's things may be a little different than just a motor load.
Click to expand...

So you size the conductors to the load and not the switch?
 
Jraef said:
Different rules. Conductors feeding a VFD are to be sized at 125% of the VFD input current, not the motor FLC as per 430.122. Input current on a VFD is usually less than the output current because of the change in power factor, so the input conductors can sometimes be smaller than the motor feeder conductors under the right circumstances.

But you have to be careful because on some VFDs that were actually designed for IEC motor sizes at 380V and then had their nameplates adapted to use here on 460V HP ratings, it can be the other way around and that can bite you because it is based on whatever the VFD nameplate says is the maximum input current. Several Asian drive mfrs have this problem.
Click to expand...

So one needs either the VFD or motor current . Switch size no bearing on conductor sizing?
 
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