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Soft start in group installation


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#1 FLOW_EE

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Posted 22 August 2006 - 05:26 PM

Hello,

I am wondering if solid state soft starts are commonly used in group installations as permitted by NEC article 430.53 ©. What makes me wonder is the wording used in some of the requirements such as "each device (overload or controller) is listed for group installation with maximum rating of fuse, circuit breaker, or both". I have only seen the "listed for group installation" description for the manual motor protectors or the motor starter protectors in various manufacturers' catalogs or websites. Does it mean that you would always have to use these MMPs or MSPs along with solid state soft starts in order to use them in a group installation, even if the soft starts may have internal overload?

Any advice would be appreciated.


Thank you.
FLOW_EE

#2 jraef

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Posted 24 August 2006 - 04:56 AM

Group fusing is all about the "fusing", or really the Short Circuit Protective Device (SCPD) ratings, not the overload protection. It relates to the devices' ability to function in the even of a short circuit given the let-through energy of the next device up-stream. Soft starters are never SCPDs, so they don't fit into the rule, they would be the equivalent of a contactor in that sense. Many people use one soft starter with multiple MMPs downstream, as long as the upstream SCPD follows the sizing rules for the MMPs.
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#3 FLOW_EE

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Posted 24 August 2006 - 09:59 PM

jraef,

Thank you for the information. I didn't know that the use of multiple MMPs downstream from one soft start was commonly done. What about installing multiple soft starts downsream from one branch circuit over current protection, which is what I had in mind when I wrote my initial posting on this topic? I apologize for not being clearer about my question. As you point out, a soft start is like a contactor, and I know that the intallation of multiple contactors under one branch circuit over current protection is commonly done with an MMP for each contactor. If the contactors in this scheme were replaced by soft starts with integral overload, would the MMP still be required for each soft start? Since I have not seen any manufacturer's documentation that specifically states their soft start is listed for use in group installation, I am wondering if this complies with NEC 430.53.


Thank you very much in advance.
FLOW_EE


QUOTE(jraef @ Aug 23 2006, 09:56 PM) View Post

Group fusing is all about the "fusing", or really the Short Circuit Protective Device (SCPD) ratings, not the overload protection. It relates to the devices' ability to function in the even of a short circuit given the let-through energy of the next device up-stream. Soft starters are never SCPDs, so they don't fit into the rule, they would be the equivalent of a contactor in that sense. Many people use one soft starter with multiple MMPs downstream, as long as the upstream SCPD follows the sizing rules for the MMPs.



#4 jraef

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Posted 25 August 2006 - 03:47 AM

I'm in the field and don't have my NEC with me.
There is a series of rule steps in the NEC that can be followed to do that. I have not done it in a long time now, but I know it is possible with careful attention. From what I recall off the top of my head, if you size the conductors going from the breaker to the soft starters for the breaker rating and the breaker short circuit protection trip rating can be adjusted to fall within the range of acceptability for motor winding (430.52 exception C maybe?) and, and, and, etc., you can get around the fact that the soft starter is not a SCPD. An example might be having 1 x 150A thermal-magnetic breaker with short circuit trips adjustable down to 1x Amp rating feeding 3 motors with 40A each on 3 x 40A soft starters. The NEC will allow up to (I believe) 400% FLA on the breaker size, so that's 160A for each motor circuit, and you have a 150A breaker with adjustable mag trips set at 150A, so it meets that requirement. The aggregate current is also under the 150A thermal trip size, and the fact that you have soft starters eliminates the potential problems from inrush current tripping the breaker. The only problem might be in running wire rated for 150A from the breaker to the line side of each soft starter because the lugs of the soft starter may not accept big enough wire. What it boils down to is using that one single SCPD to protect all of the elements in the circuit, meaning the wire and motor windings. The soft starters are already self protected, they don't "count" in that consideration, but they also don't contribute acceptable protection from short circuits for the other downstream elements. I have never done it for more than 3 motors on one breaker though. At some point it just becomes impractical compared to adding fuses or MMPs.
"He's not dead, he's just pinin' for the fjords!"

#5 FLOW_EE

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Posted 25 August 2006 - 06:53 PM

jraef,

Thank you very much for your insight on this question. The fact that you have done it in the past gives me confidence. I will study the NEC 430.53 requirements more.


Thank you very much.
FLOW_EE


QUOTE(jraef @ Aug 24 2006, 08:47 PM) View Post

I'm in the field and don't have my NEC with me.
There is a series of rule steps in the NEC that can be followed to do that. I have not done it in a long time now, but I know it is possible with careful attention. From what I recall off the top of my head, if you size the conductors going from the breaker to the soft starters for the breaker rating and the breaker short circuit protection trip rating can be adjusted to fall within the range of acceptability for motor winding (430.52 exception C maybe?) and, and, and, etc., you can get around the fact that the soft starter is not a SCPD. An example might be having 1 x 150A thermal-magnetic breaker with short circuit trips adjustable down to 1x Amp rating feeding 3 motors with 40A each on 3 x 40A soft starters. The NEC will allow up to (I believe) 400% FLA on the breaker size, so that's 160A for each motor circuit, and you have a 150A breaker with adjustable mag trips set at 150A, so it meets that requirement. The aggregate current is also under the 150A thermal trip size, and the fact that you have soft starters eliminates the potential problems from inrush current tripping the breaker. The only problem might be in running wire rated for 150A from the breaker to the line side of each soft starter because the lugs of the soft starter may not accept big enough wire. What it boils down to is using that one single SCPD to protect all of the elements in the circuit, meaning the wire and motor windings. The soft starters are already self protected, they don't "count" in that consideration, but they also don't contribute acceptable protection from short circuits for the other downstream elements. I have never done it for more than 3 motors on one breaker though. At some point it just becomes impractical compared to adding fuses or MMPs.






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