Transformer primary overcurrent protection.

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pete25

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I have a 30kva 480V/277transformer with a 200 amp disconnect switch fused at 100 amp on the primary side and the transformer feeding a panel with 100 Amp M.C.B.. I know the primary fuses should probably be set at 50 amp however, is the current configuration an issue. Can the we have 100 amp fuses on the primary? Please advise.

Thanks
 
I think it meets code by less then a few decimal places.

It would not be hard to get fuse reducers for that 200 amp switch that would allow the use of 65 through 100 amp fuses.
 
The 100 amp fuse is more than 250% of the rated primary current and I don't see a provison that permits rounding up to the next standard OCPD size.
 
don_resqcapt19 said:
The 100 amp fuse is more than 250% of the rated primary current and I don't see a provison that permits rounding up to the next standard OCPD size.
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Did you check with the calculator on the math. 35000/830=42.16x250%=105 amps. 100 amp fuses would be less than 250% by 5 amps.
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RUWired said:
Did you check with the calculator on the math. 35000/830=42.16x250%=105 amps. 100 amp fuses would be less than 250% by 5 amps.
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I am not sure that 'cheat' is how you would really want to make a calculation.:smile:

30*1000/480/1.73=36.1271 amps

36.1271*2.5=90.3177 amps.
 
iwire said:
I am not sure that 'cheat' is how you would really want to make a calculation.:smile:

30*1000/480/1.73=36.1271 amps

36.1271*2.5=90.3177 amps.
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Why are you using 250%? The OP was asking for the primary OCPD. Why not use 125% from table 450.3 (B)? I thought I had transformers down, maybe not. :smile:
 
The Iceman said:
Why are you using 250%? The OP was asking for the primary OCPD. Why not use 125% from table 450.3 (B)? I thought I had transformers down, maybe not. :smile:
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If the transformer has secondary overcurrent protection then Table 450.3(B) allows us to go as high as 250% with the primary protection. We don't have to go that high but we can. :smile:
 
iwire said:
I am not sure that 'cheat' is how you would really want to make a calculation.:smile:

30*1000/480/1.73=36.1271 amps

36.1271*2.5=90.3177 amps.
Click to expand...

To be more accurate and use SQRT(3) @ 480V

36.08439182 and
90.2109796

if the utilization voltage is 460V, then

30000/(460*SQRT(3))=
37.65327843
and
37.65327843*3=
94.1331961


and for 440V these would be:
39.36479108
98.4119777

So one coud argue that the 100A fuse was put in a long time ago where the utilization voltage was 440V.
 
weressl said:
To be more accurate and use SQRT(3) @ 480V

36.08439182 and
90.2109796

if the utilization voltage is 460V, then

30000/(460*SQRT(3))=
37.65327843
and
37.65327843*3=
94.1331961


and for 440V these would be:
39.36479108
98.4119777

So one coud argue that the 100A fuse was put in a long time ago where the utilization voltage was 440V.
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Anything is possible in the past but today we follow 220.5.
 
iwire said:
Anything is possible in the past but today we follow 220.5.
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He did not say that this is a new installation or even new equipment.Nor did he list what is the nameplate current data in the OP. Is this a special K-rated transformer? Dunno.... I don't think 220.5 is applicable to the transformer. First you size the fuse to the transformer primary protection Table 450.3(B) THEN you select your feeder size.

220.5 Calculations.
(A) Voltages. ​
​
Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.

I think the only dispute is; if you calculated over 90A, can you use the next higher size OCPD. Based on previous arguments, since it is not prohibited it is premitted.​

 
iwire said:
The voltage specified was 480 but figure it however you want to I really don't care. :smile:
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He said the transformer is, not what the operating voltage is. There are areas in this country that still has 440V as the operating voltage.

The point was that 220.5 - which you cited and where the voltages listed - does not belong to the issue.

The OP's question was if a 100A fuse in place could be a right one? I am pointing out that there could eb a combination of factors that would make that to be compliant with the Code. (Even though OSHA does not require that area of the NEC to be retroactive.)
 
weressl said:
I think the only dispute is; if you calculated over 90A, can you use the next higher size OCPD. Based on previous arguments, since it is not prohibited it is premitted.
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It has to be specifically permitted. The code section says that the maximum permitted OCPD is 250%. If there is not a code section that says you are permitted to round up, then you have to round down or find a OCPD that exactly matches the maximum permitted OCPD.
 
weressl said:
He said the transformer is, not what the operating voltage is. There are areas in this country that still has 440V as the operating voltage.
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As I said, I don't care how you want to figure it, I choose to figure it using the voltage provided by the poster.

Heck he might be connecting it to DC for all we really know. :wink:


The point was that 220.5 - which you cited and where the voltages listed - does not belong to the issue.
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You may be right, I may have jumped the gun on that.

The OP's question was if a 100A fuse in place could be a right one? I am pointing out that there could eb a combination of factors that would make that to be compliant with the Code.
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Of course that is what your doing.
 
292

292

i think that you can take your primary times 125% when protecting the primary only then you are allowed to use 240.4 which states you can increase to the next standard sixe if under 800 amp
 
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