What are the requirements to add new breaker at LV if existing LV board/ LV switchgear is full

[Shahzad]

Hello

I have a main incoming breaker 5000Amps which feeds a low voltage 400V, 3 phase panel board. This bus is pretty much full. I need to put a breaker to feed 500kW (800Amp) resistive load. I have following question.

(1) Do I need to go with 1.25x800= 1000 Amps Breaker (Insulated Case Circuit Breaker) or a normal 1000 amp molded case thermal mag breaker?
(2) Or I should with exact load 800 Amps thermal mag Molded Case Circuit Breaker?
(3) Do I need a softstarer for 800Amp resistive load or not needed? Any recommendation how can I reduce the inrush? It is on 400V, 3 phase system.
(4) Do I need to buy a new panel and bolt it with existing on the same incoming? or do I need to have separete incomer?

 

[Jraef]

You have 2 issues:
A. Your 5000A switchboard is "full", you need to determine if you can even add any load to that first, then how would you do it if the board is full.
B. Sizing the breaker. First you have to size the CONDUCTORS based on the nature of the load; i.e. continuous or not, then you size the breaker to protect the conductors.

Inrush for resistive loads is not the same as motor starting. There might be some inrush current if the type of resistive load has a Positive Temperature Coefficient (PTC) of resistance, meaning the resistance goes up as the temperature goes up, so when cold, it has low resistance. Generally though, that is so brief that it does not affect breaker or conductor sizing. So "soft starting" it would not be something that you would do (although there may be other reasons to use Solid State control).

Not sure what you mean by #4. "Incommer" can mean several things; utility feed? Feeder from primary switchgear? Different transformer? We don't know, we can't see what you have there.

 

[Shahzad]

You have 2 issues:
A. Your 5000A switchboard is "full", you need to determine if you can even add any load to that first, then how would you do it if the board is full.
B. Sizing the breaker. First you have to size the CONDUCTORS based on the nature of the load; i.e. continuous or not, then you size the breaker to protect the conductors.

Inrush for resistive loads is not the same as motor starting. There might be some inrush current if the type of resistive load has a Positive Temperature Coefficient (PTC) of resistance, meaning the resistance goes up as the temperature goes up, so when cold, it has low resistance. Generally though, that is so brief that it does not affect breaker or conductor sizing. So "soft starting" it would not be something that you would do (although there may be other reasons to use Solid State control).

Not sure what you mean by #4. "Incommer" can mean several things; utility feed? Feeder from primary switchgear? Different transformer? We don't know, we can't see what you have there.

Thanks, very good point. I have discussed with site and they said the running load on 5000A (3.5MVA/380V, 3Phase, TX) is roughly 2500Amps so we do have room available. I got below questions, would appreciate if you can help clarify.

1) We do not have any empty cell or cubicle or breaker. So, I guess I will have to buy a section and bolt it on the side? It is a Square D LV panel board now Schneider, so I am assuming I can buy a set of feeder cabinet and bolt it on the site? correct?

2) Regarding loading, you are right, it is 500kW electric air heater, required for dryer, so it will have electrical heating elements installed in the duct. From conductor size point, since it is a resistive load of 500kW, at 380V, that gives me 760Amp, do I need to use 125% multiplication factor or should I just consider it like 800Amp load. the cable length is roughly, 400 ft. As per voltage drop calculator for 400 ft, I am getting 350kcmil (3 conductors per phase). As 3C, 350kcmil at 75 degC, is rated for 310Amps, so I have to use 3 cables, but it will be very bulky to handle. So, should I consider using single conductor 350kcmil per phase instead of combined 3 set of conductors? Any recommendation.

3) Regarding breaker, it will be as per cable ampacity, so based on above cable size, is it suitable to choose 1000Amp thermal mag breaker? any recommendation on breaker size?

 

[zbang]

Extending the switchboard-
Your best answer is to call Schneider with the model & serial# and talk with them; it may be physically possible to extend but whether you can get the section is a whole different question.
Do all the clearances even allow for adding a section?
Might have to shuffle existing loads to make space, maybe even offload some to a new separate switchboard .

The load-
It probably counts as continuous, does the manufacturer specify the overcurrent size?
Is this really a branch circuit or a feeder?

Cables-
That's a little confusing what you wrote; 350kmil THHN = 310amps @ 75 deg without any derating, but are the terminations rated for 75 or 60 deg? Might look at using XHHW aluminum instead; price of cable could be a significant driver here.

 

[paulengr]

Thanks, very good point. I have discussed with site and they said the running load on 5000A (3.5MVA/380V, 3Phase, TX) is roughly 2500Amps so we do have room available. I got below questions, would appreciate if you can help clarify.

1) We do not have any empty cell or cubicle or breaker. So, I guess I will have to buy a section and bolt it on the side? It is a Square D LV panel board now Schneider, so I am assuming I can buy a set of feeder cabinet and bolt it on the site? correct?

2) Regarding loading, you are right, it is 500kW electric air heater, required for dryer, so it will have electrical heating elements installed in the duct. From conductor size point, since it is a resistive load of 500kW, at 380V, that gives me 760Amp, do I need to use 125% multiplication factor or should I just consider it like 800Amp load. the cable length is roughly, 400 ft. As per voltage drop calculator for 400 ft, I am getting 350kcmil (3 conductors per phase). As 3C, 350kcmil at 75 degC, is rated for 310Amps, so I have to use 3 cables, but it will be very bulky to handle. So, should I consider using single conductor 350kcmil per phase instead of combined 3 set of conductors? Any recommendation.

3) Regarding breaker, it will be as per cable ampacity, so based on above cable size, is it suitable to choose 1000Amp thermal mag breaker? any recommendation on breaker size?

A panelboard like an ILine panel is not expandable and is limited to 1200 A bus capacity. A switchboard can be larger but again not expandable. Switchgear can be added on. In switchboards typically you’d replace one existing breaker with a bigger one and mount a subpanel (another switchboard/panelboard) next to it and move the original load to the new panel plus add your new loads if it makes sense. Otherwise you could add another switchboard with a new main and separate grounds and neutrals so that the old switchboard becomes a subpanel to the new main. It all depends on what is easiest to do. Siemens and Schneider generally have the cheapest panelboards and switchboards.

For resistance heating again you need to decide if it is continuous load (runs continuously for more than 3 hours) or not. If it runs 3+ hours it’s a continuous duty load and gets the 125% factor for sizing conductors. There are several other factors for conductors. Since you are well over about 300 A you will have multiple parallel conductors such as 3 350 MCM but you need to consider the raceway, derating, etc.

As far as insulated case vs molded case you can get up to around 1200 A or even larger molded case breakers. The springs get very large and tough to physically handle which is why insulated case breakers become very desirable but the price goes up significant. You can buy thermal magnetic but at these sizes the trip element gets pretty large. It is generally cheaper in some breakers to go to electronic trip units. You can get these in either type of breaker depending on the manufacturer. First size your conductors. Then pick out where you want to put things and pick out your switchboard. That will determine your breaker design. At each stage you have options. Unless you’ve done this a lot you will have to run down each option.

 

[Shahzad]

A panelboard like an ILine panel is not expandable and is limited to 1200 A bus capacity. A switchboard can be larger but again not expandable. Switchgear can be added on. In switchboards typically you’d replace one existing breaker with a bigger one and mount a subpanel (another switchboard/panelboard) next to it and move the original load to the new panel plus add your new loads if it makes sense. Otherwise you could add another switchboard with a new main and separate grounds and neutrals so that the old switchboard becomes a subpanel to the new main. It all depends on what is easiest to do. Siemens and Schneider generally have the cheapest panelboards and switchboards.

For resistance heating again you need to decide if it is continuous load (runs continuously for more than 3 hours) or not. If it runs 3+ hours it’s a continuous duty load and gets the 125% factor for sizing conductors. There are several other factors for conductors. Since you are well over about 300 A you will have multiple parallel conductors such as 3 350 MCM but you need to consider the raceway, derating, etc.

As far as insulated case vs molded case you can get up to around 1200 A or even larger molded case breakers. The springs get very large and tough to physically handle which is why insulated case breakers become very desirable but the price goes up significant. You can buy thermal magnetic but at these sizes the trip element gets pretty large. It is generally cheaper in some breakers to go to electronic trip units. You can get these in either type of breaker depending on the manufacturer. First size your conductors. Then pick out where you want to put things and pick out your switchboard. That will determine your breaker design. At each stage you have options. Unless you’ve done this a lot you will have to run down each option.

Thanks Paul. I have mostly dealt with LV switchgears, very little on panel board so I will take the serial number and model of LV panel board and share it with Schneider to ask my option.

I am evaluating if it would be cheaper to go with LV option or I should explore MV option. I am at similar situation at MV, there is no spare feeder, so will have to buy 4.16kV breaker, cable to feed transformer, stepdown transformer from MV/LV then LV breaker panel then cables to feed the electrical air heater so this option is not viable per say.

Second option I am evaluating to buy a stand alone swich board (5000A bus like existing is also 5000A bus) and feed the new switchboard through cable from existing switch board main bus. I have seen similar set up in several place where transformers secondary are connected at one switchgear, and a set of cables are tapped off the that switchgear main bus and feed other stand other switchgear 20 ft away. Not sure if that might be done for switchboard too so I am wondering do I need to have main incomer or I can connect LV cables directly on the bus of new switch board?

On cables, actually after applying derating factors, ensuring 3% voltage drop, I have come up 3C, 350MCM (3 runs, means 1 conductor per phase) to feed the heater. As it is continuous load.

On breaker, I might get 1000amp breaker with LSIG electronic trip to feed the load for electric air heater to ensure cables are protected as well.