Required Feeder Breaker size for 500kW load

[Shahzad]

Hi everyone,

I am working on a project for a client in Chile, where the 3 phase low voltage supply is 380V, 50 Hz, and Medium voltage is 13.2kV. The client is going to install an Electric heater for some industrial process which is almost 300 ft away from MCC room. The load of electric heater given by process engineering team is 500kW and now I have to figure out the power supply. I am wondering what size of voltage supply and breaker size should be used for such pure resistive load? From options point, I see below, but not sure at the moment which one will be cost effective and feasible.

(1) 500kW load, at 380V, 50Hz, the required Full load amps= 760.5Amps, so with 125% factor, I guess I would need 1000A in 2000A Frame, 380V, thermal mag insulated case circuit breaker (ICCB) from low voltage switchgear breaker, correct?

(2) Not sure if I will be able to find spare space in the 380V switchgear that could feed that big a single load. Is it possible and viable to have a breaker breaker in such size will not cause dip on the transformer upon start up?

(3) Would it make sense to feed the 500kW load from MV supply 13.2kV? Though, I will have to buy a step down transformer 13.2kV (Delta) / 380V (star), ad then feed the 500kW heater from the step down transformer?

Would appreciate if any one has come across similar problem in load sizing and has any suggestion?

 

[ron]

Hi everyone,

I am working on a project for a client in Chile, where the 3 phase low voltage supply is 380V, 50 Hz, and Medium voltage is 13.2kV. The client is going to install an Electric heater for some industrial process which is almost 300 ft away from MCC room. The load of electric heater given by process engineering team is 500kW and now I have to figure out the power supply. I am wondering what size of voltage supply and breaker size should be used for such pure resistive load? From options point, I see below, but not sure at the moment which one will be cost effective and feasible.

(1) 500kW load, at 380V, 50Hz, the required Full load amps= 760.5Amps, so with 125% factor, I guess I would need 1000A in 2000A Frame, 380V, thermal mag insulated case circuit breaker (ICCB) from low voltage switchgear breaker, correct?

(2) Not sure if I will be able to find spare space in the 380V switchgear that could feed that big a single load. Is it possible and viable to have a breaker breaker in such size will not cause dip on the transformer upon start up?

(3) Would it make sense to feed the 500kW load from MV supply 13.2kV? Though, I will have to buy a step down transformer 13.2kV (Delta) / 380V (star), ad then feed the 500kW heater from the step down transformer?

Would appreciate if any one has come across similar problem in load sizing and has any suggestion?

This might one of the times it is worth to get a 100% rated circuit breaker since the load is so close to 800A and an 800A frame breaker 100% rated breaker is generally easily available, rather than going to a 1200A or 1600A frame breaker to accommodate a 1.25% for an 80% rating breaker.
The 800A breaker can be a simple molded case breaker.
You can get an 800A frame breaker with a thermal mag breaker trip unit or solid state trip unit, either able to accommodate the inrush of a transformer.
You can feed it at low voltage direct or via a step down xfmr from MV.

 

[Shahzad]

This might one of the times it is worth to get a 100% rated circuit breaker since the load is so close to 800A and an 800A frame breaker 100% rated breaker is generally easily available, rather than going to a 1200A or 1600A frame breaker to accommodate a 1.25% for an 80% rating breaker.
The 800A breaker can be a simple molded case breaker.
You can get an 800A frame breaker with a thermal mag breaker trip unit or solid state trip unit, either able to accommodate the inrush of a transformer.
You can feed it at low voltage direct or via a step down xfmr from MV.

When u say 100% rated 800Amp breaker, what do you mean, could you please clarify? I thought now a days the breaker we see are almost all of them are 100% rated. So, when I ask supplier that I need a molden case 800A breaker, is there any specfic criteria I need to ask him to quote?

 

[ron]

When u say 100% rated 800Amp breaker, what do you mean, could you please clarify? I thought now a days the breaker we see are almost all of them are 100% rated. So, when I ask supplier that I need a molden case 800A breaker, is there any specfic criteria I need to ask him to quote?

Most molded case circuit breakers are 80% rated.
For the circuit breaker to be 100% rated, it has to be tested in an assembly to be 100% rated, so its heat is able to get away and is tested to do so.
But the breaker model numbers are often only one digit apart.
For example a Schneider PGL36080U31A is 80% and a PGL36080CU31A is 100% rated.

 

[Shahzad]

Most molded case circuit breakers are 80% rated.
For the circuit breaker to be 100% rated, it has to be tested in an assembly to be 100% rated, so its heat is able to get away and is tested to do so.
But the breaker model numbers are often only one digit apart.
For example a Schneider PGL36080U31A is 80% and a PGL36080CU31A is 100% rated.

Thanks Ron. On a second thought, I am exploring if I can buy medium voltage breaker 4160V to feed this 500kW load. When I do the math, I see the amps required at 4160V for 500kW 3 phase load would be 70Amps. Should I put standard 600 Amp breaker or I should put 600Amp fuse. My load is 500kW, resistive load, continuous operation.

 

[topgone]

Thanks Ron. On a second thought, I am exploring if I can buy medium voltage breaker 4160V to feed this 500kW load. When I do the math, I see the amps required at 4160V for 500kW 3 phase load would be 70Amps. Should I put standard 600 Amp breaker or I should put 600Amp fuse. My load is 500kW, resistive load, continuous operation.

You don't have much to choose on MV switchgears/ breakers, The smallest I know is 630A. However, there are a lot of MV fuses in the market.

 

[Shahzad]

You don't have much to choose on MV switchgears/ breakers, The smallest I know is 630A. However, there are a lot of MV fuses in the market.

thanks. I am little bit unclean between 3C, 350 MCM cable ampacity vs 1C, 350MCM cable ampacity. As my load is 500kW, 3 phase purely resistive continuous load, located 110 meters away. The ampacity I am choosing is 796FLA*1.25=950Amps. So, my question is.

1) If I understand correctly, the voltage drop needs to be below 3%, correct? If so, then I am using few only software for voltage drop calculator.
2) should I run single conductor 350 MCM (3x1C, 350MCM per phase) or (3x3C, 350 MCM per phase). Any thoughts or recommendation?

 

[petersonra]

thanks. I am little bit unclean between 3C, 350 MCM cable ampacity vs 1C, 350MCM cable ampacity. As my load is 500kW, 3 phase purely resistive continuous load, located 110 meters away. The ampacity I am choosing is 796FLA*1.25=950Amps. So, my question is.

1) If I understand correctly, the voltage drop needs to be below 3%, correct? If so, then I am using few only software for voltage drop calculator.
2) should I run single conductor 350 MCM (3x1C, 350MCM per phase) or (3x3C, 350 MCM per phase). Any thoughts or recommendation?

I don't know where you came up with the 796 Amp number. 500,000/380/sqrt(3) is about 760 Amps, which would mean you need about 950 Amps of ampacity. 350 MCM copper is only rated at 310 Amps so 3 conductors per phase in 3 separate conduits would only give you 930 Amps of ampacity. You would be just a hair shy.

Voltage drop numbers are design considerations and not NEC requirements. However, several VD calculators I tried on various web sites suggested your VD will be under 3% with the conductors as described.

 

[Shahzad]

I don't know where you came up with the 796 Amp number. 500,000/380/sqrt(3) is about 760 Amps, which would mean you need about 950 Amps of ampacity. 350 MCM copper is only rated at 310 Amps so 3 conductors per phase in 3 separate conduits would only give you 930 Amps of ampacity. You would be just a hair shy.

Voltage drop numbers are design considerations and not NEC requirements. However, several VD calculators I tried on various web sites suggested your VD will be under 3% with the conductors as described.

Thanks, sorry my bad, I did typo. It was 760 amps. So, with 125% it is 950Amp.

when you say 3 conductors per phase in 3 separate conduits, are you referring to single core 350MCM cable or 3 core 350 MCM cable. I am assuming you mean three single conductor 350 MCM per phase not multiconductor cable?

I am inclining for 400 MCM as 350 MCM would not meet 960Amps requirement.

 

[petersonra]

We rarely use three core cable here of that size. Normally it would be three single conductors one per phase in each of three conduits.

 

[Shahzad]

We rarely use three core cable here of that size. Normally it would be three single conductors one per phase in each of three conduits.

Thanks. Need two clarification

(1) not sure why you mention 310Amps for 350MCM, where as As per Table 310.15 (B)(17), the ampacity of single insulated copper conductor rated upto and including 2000V in free air, based on ambient temperature of 30 deg C.... the ampacity of 350MCM THHW cable is 505 Amps. right? or should I use 310.15(B)(16) table. I am bit confused which table I should always refer to?

(2) Since all 9 conductors will be touching in cable tray, and from cable tray they will go to the load in their respective conduits. The ambient is 42 deg C. So I guess, I need to apply the below derating factor

Table 310.15 (B)(2)(a): 75 deg C. I have chosen the factor of 0.82 for ambient 41-45degC.
Table 310.15 (B)(3)(a): Adjustment factors for more than 3 current carrying conductors: I have chosen 0.7, as number of conductors falls in 7-9 range
So my total derating factor is :0.82x0.7=0.574

From Table 310.15(B)(16), column 75, if I choose 500 MCM, current ampacity comes: 380x0.574= 218.12 Ampsx4=872.48 Amps which is higher than 760A. OR, I have to choose 750 MCM cable, that has ampacity of 475 Amps. So after derating, it comes out 475x0.574=272.65 x3 per phase, so in total I get 817Amps which is higher than the 760 Amps. Any idea if it is better to run 4 conductor per phase of 350 MCM or 3 conductor per phase of 750MCM?

Can you or anyone check my calculation let me know as I don't want to oversize the cable and want to understand the cable sizing concept once and for all. I have seen several videos' and books, sometimes it seems easier but some times tricky. Would appreciate your inputs. Thank you