I'm involved with a project in which we're trying to re-use existing cabling as much as possible. Long story short, there's a cabinet that contains a wye-delta starter for a 200hp motor. The motor requires a tremendous amount of torque to start up. The wye-delta starter timer is set to switch over to delta after 7 minutes. I'm wanting to rip out the wye-delta starter and install a soft start. We've used these soft starts on identical motors before, so I know that using the soft-start, it draws about 575A for 6-7 minutes. However, I found that the old wye-delta is being fed with single 300kcmil cabling. NEC code says 300's are good for a little over 300A. My question is if I re-use this cable with the soft start it will be overloaded for 6-7 minutes by almost 300A. Can cables handle that amount of overload, for that amount of time? I know best-case scenario I re-pull new, but that would require an outage, which is a big ordeal in the plant I work in.
Cable Overload - How Much?
- Thread starter adamscb
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Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Adam...
I concur with your Soft-Start approach!
Re the cable! Wye-Delta starting requires two sets of cable between the starter and the motor, so, 6 conductors should already be installed! Paralleling them into a 2-conductor per phase feeder should take care of the any V-drop problem!
Re starting time... I'm appalled. Please provide detail!
Phil Corso
In terms of this cable size, I'm referring to the line side of the circuit breaker. In order to re-pull from the top of the circuit breaker to the buss plug requires an outage. On the starting time, it takes 6-7 minutes to ramp up to speed while drawing about 275% full load amps. It's just the nature of the beast, this thing is a sharple seperator centrifuge.
This might help
http://sm-industry.ru/titan_img/ecatalog/Appl_07_Thermal_Overload_Protection_en.pdf
this will give the sc withstand for various cable sizes
http://www.cooperindustries.com/con...ductor_Protection_Wire_Cable_Protection_2.pdf
http://sm-industry.ru/titan_img/ecatalog/Appl_07_Thermal_Overload_Protection_en.pdf
this will give the sc withstand for various cable sizes
http://www.cooperindustries.com/con...ductor_Protection_Wire_Cable_Protection_2.pdf
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
If you intend to follow IEEE Std. 242/2001 IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems ch.9.5.2 Overload capacity, and if the cable is in conduit, in air and you limit the emergency operating temperature to 90oC, if the start is from ambient temperature [no operating current prior to emergency, Io=0] then the cable
-for 7 min.- 630 A it could be loaded.
-for 7 min.- 630 A it could be loaded.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
By the way, if you don't have to follow NEC in IEEE 835/1994 pg.14 for a single circuit triplex single-core cables Direct Buried in 120 RHO-more as per NEC[90]-25oC Earth 75oC copper conductor 90oC insulation 300 MCM 100% LF the ampacity is 379 A and applying IEEE 242/2001 above way, for direct buried you'll get 595 A.
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
JFletcher
Senior Member
- Location
- Williamsburg, VA
Waste water plant? If so, for others, the typical max starts for something like this is 1x/day if run batch operation (non-continuously), it may be 1x a month if run continuously. OP mentioned it would require a plant outage to replace, which is either terrible engineering, overload (requiring all centrifuges to run 24/7), or a different animal altogether.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
If we compare this table: Table 9-6—Emergency overload current IE, percent of continuous rating at 40 ˚C ambient temperature with formula from chapter
9.5.2.4 Development of intermediate characteristics and put Io=IN the conclusion is the expression e^(-theta*K) has to be e^(-theta/K) where theta it is the overload duration in hours.
In this case the cable temperature- in underground direct buried [K=6] -will rise only 2.5oC. That means-in my opinion the number of start -up to 10-it is not relevant and it is not important if the cable is hot [was operating before] or not.
By the way, if the cable-single triplexed-of 300 MCM copper is underground
direct buried according to next chapter 9.5.2.5 Direct buried cables the conductor operating temperature needs to be kept at no more than 65 ˚C...
Therefore, for intermediate emergency overload, a maximum conductor temperature of 80 ˚C has been selected as suitable...
In this case as NEC recommends 240 A [as IN] for 60oC and using IEEE 242
formula the permissible current for 7 min will be 1385 A.
9.5.2.4 Development of intermediate characteristics and put Io=IN the conclusion is the expression e^(-theta*K) has to be e^(-theta/K) where theta it is the overload duration in hours.
In this case the cable temperature- in underground direct buried [K=6] -will rise only 2.5oC. That means-in my opinion the number of start -up to 10-it is not relevant and it is not important if the cable is hot [was operating before] or not.
By the way, if the cable-single triplexed-of 300 MCM copper is underground
direct buried according to next chapter 9.5.2.5 Direct buried cables the conductor operating temperature needs to be kept at no more than 65 ˚C...
Therefore, for intermediate emergency overload, a maximum conductor temperature of 80 ˚C has been selected as suitable...
In this case as NEC recommends 240 A [as IN] for 60oC and using IEEE 242
formula the permissible current for 7 min will be 1385 A.
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