Sizing a Fuse or Installing a Circuit Breaker and Fuse

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eeee

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Consider that I am installing an external switch between a pad mounted transformer and a switchboard so that I can split power to the switchboard and MDP I plan to install.

I now have a new circuit that is feeding the external transformer with additional ampacity; that being the newly installed MDP and its circuit which now feed the external transformer.

I would much rather protect the building connected to the switchboad than the external transformer. Should I increase the size of the fuse on the primary side of the transformer or install a new circuit breaker and fuse on the switchboard in order to better protect the building connected to the switchboard?

The newly installed MDP is not connected to the switchboard but is located inside the building.
 

charlie b

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Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

I am sorry, but I do not follow your description. What is installed now? What are you adding?

If you can't draw, scan, and post a picture, try describing it in a way that lets us draw it. I might suggest using language something like this: From utility, "Line 1" goes to XXX. From XXX, you have two lines, with "Line 2" going to YYY and "Line 3" going to ZZZ. Now I will install a new component "WWW" to be located in the middle of "Line 2." Thus, we will now call "Line 2A" between XXX and WWW, and "Line 2B" between WWW and YYY. I need to size "Line 2B." etc.
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

My description is hard to understand.

My external pad mounted Transformer is connected to the utility line. An external switch is connected to this transformer. A switchboard is connected to the external switch and a new MDP is connected to the external switch. The switchboard runs loads in some of the building. The new MDP runs loads in the rest of the building.

I can upsize the primary side fuse of the external pad mounted transformer so that the part of the building connected to the switboard has more power in case of faults. The side of the building connected to the MDP would also have more power, but that is not the concern.

I could add a circuit breaker and fuse to the switchboard to protect the loads emanating from the switchboard in case of a fault emanating from the MDP side of the circuit. Or maybe resize the circuit breaker and fuses on the switchboard knowing that the MDP loads change the combined ampacity draw seen by the external transformer.

The problem with adding a circuit breaker and fuse to the switchboad or resizing them I see is that no additonal power is obtained from the external transformer should it be required to sustain loads the the switchboard requires. Only additional protection is added. This option costs much more also.
 

charlie b

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Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

Originally posted by eeee: My description is hard to understand.
Yes it is. It still is.

Here is what I am reading:
</font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Power comes from utility, to primary protection, to primary of transformer.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Power goes from transformer secondary to "external switch."</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">Power goes from "external switch" to existing switchboard in the building.</font>
<font size="2" face="Verdana, Helvetica, sans-serif"></font>
  • <font size="2" face="Verdana, Helvetica, sans-serif">You wish to add a MPD. It will get power from "external switch." It will supply additional loads in the same building.</font>
<font size="2" face="Verdana, Helvetica, sans-serif">Do I have the situation right?
I can upsize the primary side fuse of the external pad mounted transformer so that the part of the building connected to the switchboard has more power in case of faults.
Why do you want it to have more power during a fault? You should want it to trip, and to pass as little energy as possible during the fault (i.e., up to the time of the trip).
I could add a circuit breaker and fuse to the switchboard to protect the loads emanating from the switchboard in case of a fault emanating from the MDP side of the circuit.
Every load "emanating" from the switchboard should already have a fuse or circuit breaker. What is it that you are suggesting might be added?

Are you concerned that adding the MDP will overload the transformer? Have you done (or has anyone else done) a load calculation, to prove that the transformer would not be overloaded? Other than adding the MDP, are you adding new load to the building? Or are you keeping the existing loads within the building, and adding the MDP to redistribute some of the loads off of the switchboard?
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

Thanks for answering and you have the layout of the design correct.

We are adding new loads to the building.

I would imagine that the primary side fuse for the transformer is already sized for it's maximum capacity as standard designs go. I am going to get the fuse size off the drawings and see that it is the the maximum rated fuse for a 300KVA external pad mounted transformer.

The issue is that by increasing the size of the primary side fuse to the transformer, more operating capacity can be realized from the transformer before tripping if it is indeed not sized for maximum operating capacity now.

The designer indicated that adding a circuit breaker to the existing switchboard would be the best solution in that adding a primary fuse (or increasing the size of the primary fuse on the transformer would only be a temporary fix??). His comment and another EE on base supporting this comment makes no sense to me. Like you say, the switchboard already has fuses.

The issue is that by adding the MDP, the overcurrent protection of the MDP and switchboard combined now exceeds the overcurrent protection of the primary side fuse on the transformer. Maybe the designer wants to reduce the overcurrent protection on the switchboard to better protect the transformer. The other engineer on base wants to upsize the primary fuse on the transformer to protect the building-the more valuable asset. I would think the primary size fuse of the transformer is already maxed out based on standard desing procedure as I indicated above. I am going to verify this now.
 

charlie b

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Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

This seems to boil down to a simple question of fact: Does the existing transformer have the capacity to add the new loads (i.e., on the new MDP)? You can go nowhere, until you answer that question.

I see three options. They all involve some work. They all wind up giving you a measure of the existing load. That in turn will tell you if you can add load to your transformer.

Option 1 is make a list of everything that is connected, and finding out how much power each item uses. The most common way to assemble such a "list" is by creating a "Panel Schedule" for every power panel in the building.

Option 2 is to perform a "Service Load Calculation," in accordance with NEC Article 220.

Option 3 is to measure the existing load. You install a recording device at the main service point. You take measurements every 15 minutes for a 30 day period. You look for the maximum reading during that period. Multiply that reading by 125%, and call that your "existing load."

But I have one more question; I would like you to clarify one more thing:
Originally posted by eeee:The designer indicated that adding a circuit breaker to the existing switchboard would be the best solution. . . .
What I am guessing here is that they are talking about providing power to the new MDP from the existing switchboard, instead of from the "external switch" (i.e., upstream of the switchboard). Is that what they mean by "adding a circuit breaker"?

If so, that will not change the amount of load, as seen by the transformer. All it will do is to add load to the switchboard, instead of having the new load bypass the switchboard.

If that is not what they mean, then what do they mean?
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

We have taken amp reading measurement for the building. This is our best solution for determining loads. I realized all the immense amount of unkown com equipment runs off the UPS, so I only need to determine the draw from the UPS. There are still a lot of unkown existing loads the designer could not identify. The amp measurement we took in June is the best bet since some or all of the chillers may have been on for a 84 degree day to include a computer room air conditioner.

I found it interesting that some (or all) of our transformers used for all of our buildings on the grounds do not have fuses sized for 125% of primary current as calculated from the maximum operating capacity of the transformer selected (that should have been based off of a load caclulation in the beginning), or at 300% of primary current as caclulated from the maximum operating capacity of the transformer-This is how I have seen primary size fuses calculated before.

I need to retrieve the information on the fuse for the building I am working with.

Most of our transformers must have been put in oversized for the load, so the primary side fuses for the transformers are sized to protect the building loads and not the maximum loads that could be alloted for the transformer.

This is why we may want to upsize the primary side fuse on the transformer to get more operating capacity from the transformer. Otherwise the tranformer will trip before it reaches operating capacity.
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

I should have mentioned to answer your question it is my understanding that the designer wants to add a breaker to the switchboard. I think you understand that.

He is not permitted to have the MDP be dependent on the switchboard. That is part of our scope. The MDP circuit must be completely independent from the circuit resident on the switchboard.

If he adds a breaker to the switchboard,the MDP must be connected to the external switch in such a way as to not possibly intefere with the operation of the switchboard.
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

I am sorry. I just determined that the load study I was looking at showed labeled the primary amps that feed the transformer, not the primary amps calculated from the operating capacity of the transformer, which in this case is 300KVA.

I found out that a 45 amp fuse is installed on the primary side of the 300KVA external pad mounted transformer. That is over 300% of the calculated 13.97 amps capacity for the primary side as would be expected.

This brings me back to the point, why upsize the transformer fuse on the primary side here? That would be exceeding the operating parameters of the transformer it seems? It is already more than 300% greater than the maximum available primary operating current. I don't see how any additional capacity will be realized?
 

charlie b

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Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

Although you have not said this before, I infer that the primary voltage is 12,470 volts. Therefore, NEC Table 450.3(A) applies. You are allowed to have a primary fuse rated at 300% of the primary's rated current. I calculate that as 41.7 amps. Since that is not a standard size fuse rating, you are also allowed to use the next higher standard size fuse. That would be 45 amps. So you are already at the maximum allowed size for your primary fuse.

HOWEVER. You need to keep in mind that we are allowed to fuse the primary so much higher than the standard rating in order to permit large loads to start, without tripping the primary fuse. That does not give us permission to load the transformer beyond its rating. If you have more than 300 KVA of load on this 300 KVA transformer, then you are already in trouble. If this is the case, you are not permitted to add any more load to the transformer, so you can forget the MDP (for now).

I will say it one more time, and then I will leave this thread alone. You must know the load, or you can do nothing else. Nothing!

If the transformer is presently overloaded, then you may have to buy a bigger transformer.

Two more points:
Originally posted by eeee: We have taken amp reading measurement for the building. This is our best solution for determining loads.
That does not count, unless you take readings for a continuous period of 30 days. If you have one year's worth of utility bills available, then that would also be an acceptable basis for determining the existing load. Refer to NEC article 220.87.
. . . the designer wants to add a breaker to the switchboard. I think you understand that.
I do not understand that. If breaker that the designer wants to add is not a breaker to the MDP, then what will that new breaker do?
 

bob

Senior Member
Location
Alabama
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

I found it interesting that some (or all) of our transformers used for all of our buildings on the grounds do not have fuses sized for 125% of primary current as calculated from the maximum operating capacity of the transformer selected (that should have been based off of a load caclulation in the beginning), or at 300% of primary current as caclulated from the maximum operating capacity of the transformer-This is how I have seen primary size fuses calculated before.
eeee
You seem fixed on that 125% fuse size for the transformer. Using table 450.3A you can install a fuse sized at 300% of the full load primary amps. If 300% does not correspond to a standard fuse you may goto the next size fuse.
A fuse may need to be oversized to prevent blowing the fuse due to large motor starts or other large load being temporarily energized.
Just because yuo have the fuse set a 300% does not allow you to use the transformer beyond it capacity.
This brings me back to the point, why upsize the transformer fuse on the primary side here? That would be exceeding the operating parameters of the transformer it seems? It is already more than 300% greater than the maximum available primary operating current. I don't see how any additional capacity will be realized?
You are NOT upsizing the transformer capacity.
In your situation the primary fuse, 45 amps, is not protecting the transformer from overload. Thats why the code insists that only trained personnel operate and maintain the system. Thats why you do the load studies to determine when increases in transformer capacity is required.

[ September 15, 2005, 02:15 PM: Message edited by: bob ]
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

Thanks,

You are completely right on every point.

In fact the transformer primary size fuse protects against faults for both intermediate trip and long trip, not overloading. Having a properly sized transformer matched against calculated or properly measured loads protects against overloading.

I think I can now respond to my colleagues.

This primary side fuse is already correctly sized. There is no reason to upsize it, unless to protect against some faults for large starting loads.


Many thanks. I am surprised to see that a website like this exists.
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

eeee: In reading all the posts, I'm concerned that there has been no mention of conductor sizes.
The ratibgs you keep quoting seem to realte to Art 450, Transformer protection. I hope you are also keeping Art 240 in mind so the conductors are
properly protected. The 125%-300% rules in 450 do not address conductor protection.
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

Thanks,

The conductors have been considered by the designer when the MDP and its circuit were partitioned in the design per the panel schedule.


The switchboard is not picking up any additional operating load so it's conductors should not require resizing.


Making sure the transformer is sized correctly is the only remaining issue since additional load is being added to it. I have suggested we do this next July with an ampmeter when the chiller, computer room air conditioner and condensor fans are full on. Unfortunately, we do not have the power measuring equipment to measure this per the NEC as discussed above. Lack of funds and manpower is the issue.
 

charlie b

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Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

Check around. There are a number of companies who perform this type of work. They will connect their logging meters, pick them up a month later, give you a printout, and send you a bill. I don't know how much it would cost.
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

I appreciate your comment.

Unfortunately, I could not get operations to have funding for this since operations has no budget for maintenace testing equipment I am continually told. They have been trying for years to fund for testing.

I am in design, so I have no authority to get funding for this.

A separate project would have to be approved and funded for this. This is a remote possibility should this project be funded. It is more likely that an emperical analysis would be requested from us engineers. This is an analysis I would not want considering the difficulty of identifying all the loads, much less getting spec sheets on them for a proper analysis. There are many routers?, E-thernet siwtches, Gateways, Servers unidentified; but those reside on an UPS-May only require identifying the power requirements of the UPS. Then I need to identify items such as a data storage module (unkdown manufacturer and model number), a computer room air conditioner (probably have no model number) and another large device I forget what it is.
 

charlie b

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Location
Lockport, IL
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Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

Can you obtain 12 month's worth of utility bills? If so, and if they show the peak demand each month, then you can use that as a basis for assigning a value to the "existing load."
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

We don't pay utilites here.

We do have a meter on this building, although it is rare that we have a meter on a building. Our metering shop does keep meter readings on our meters I am told. They only know what the meter readings are between meter readings. Don't know what the span is between readings. I will find out.
 

eeee

Senior Member
Re: Sizing a Fuse or Installing a Circuit Breaker and Fuse

I was just remembering that the meter is for a different building. This building for this discussion has no meter.
 
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