What Size Conductor

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joebell

Senior Member
Location
New Hampshire
I was having a discussion with my boss on Friday about an installation of a 112.5 KVA delta/wye transformer in a computer server room. I guess the root question is about the sizing of the secondarys. My opinion is that the neutral/grounding conductor of this installation should be considered current carrying, due to the the loads being served are considered nonlinear, there for derating of the secondarys is required. I believe 600 KCMIL conductors should be installed.

He says the 3 phase 4 wire system neutral is not to be counted as current carrying and he thinks 500 Kcmil should be installed.

I would normally agree with his assement on the 3 phase 4 wire system but the loads are all computer room server equipment


Do we need to apply derating factors to these secondarys?
 

chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I would count it.

310.15(B)(4)(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.
 

infinity

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Location
New Jersey
Occupation
Journeyman Electrician
The non-linear loads would have to exceed 50% of all of the load on the transformer. If you can establish that then I agree that the neutral would be a CCC. But depending on the size of the OCPD protecting the secondary conductors 500's or 600's may work.
 

joebell

Senior Member
Location
New Hampshire
The non-linear loads would have to exceed 50% of all of the load on the transformer. If you can establish that then I agree that the neutral would be a CCC. But depending on the size of the OCPD protecting the secondary conductors 500's or 600's may work.


All the circuits out of the panels are feeding receptacles that feed server racks in the room.
 

infinity

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Location
New Jersey
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Journeyman Electrician
500's (380 amps THHN) can't be used for a 400 amp OCPD so that point is moot. Looks like 600's will be needed.
 

joebell

Senior Member
Location
New Hampshire
Actually the adjusted ampacity is lower than that if we agree the neutral is current carrying


If you were to use the 380 amps listed in Table 310.16 wouldn't 240.4 (B) "next higher standard size" come into play?
 
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Cow

Senior Member
Location
Eastern Oregon
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Electrician
I really need to learn how to do that multi quote thing :)

If you'd like to respond to several of the posters in your thread, click the quotation marks in each of their posts. Except for the last post you'd like to quote, instead click the "quote" button, now you'll have all their replies in one post that you can respond too.:wink:

................
 

augie47

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Location
Tennessee
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State Electrical Inspector (Retired)
Actually the adjusted ampacity is lower than that if we agree the neutral is current carrying


If you were to use the 380 amps listed in Table 310.16 wouldn't 240.4 (B) "next higher standard size" come into play?

Back in the good old days possibly, but now:
240.21 Location in Circuit.
(C) Transformer Secondary Conductors. A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

600 kcmil required.
 

ohmhead

Senior Member
Location
ORLANDO FLA
Well i dont agree with this if your transformer is a 112.5 kva its secondary is only good for 312.5 amps i agree with your boss 380 amps is 500 mcm .

your only going to load that transformer to what it can put out kva wise .

Your not going to get 400 amps out of it yes i see the breaker size can increase do to its not the correct next size but thats only if its a 800 amp or larger or if its 1000 amps or larger now?
 
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joebell

Senior Member
Location
New Hampshire
Well i dont agree with this if your transformer is a 112.5 kva its secondary is only good for 312.5 amps i agree with your boss 380 amps is 500 mcm .

your only going to load that transformer to what it can put out kva wise .

Your not going to get 400 amps out of it yes i see the breaker size can increase do to its not the correct next size but thats only if its a 800 amp or larger or if its 1000 amps or larger now?

I agree with the 312 amp figure but 450.3(B)allows 125% for the secondary OCPD whichnow brings the total to around 391 amps


I don't understand your last sentence you are allowed to increase the breaker size according to 240.4(B) if you can meet the 3 conditions. If we are going to say tat 312 amps is the value 240.6 lists 350 amps as the next standard size OCPD
 

ohmhead

Senior Member
Location
ORLANDO FLA
I agree with the 312 amp figure but 450.3(B)allows 125% for the secondary OCPD whichnow brings the total to around 391 amps


I don't understand your last sentence you are allowed to increase the breaker size according to 240.4(B) if you can meet the 3 conditions. If we are going to say tat 312 amps is the value 240.6 lists 350 amps as the next standard size OCPD

Well 800 amps or less sorry my mistake .

I see 500 mcm and the 400 amp breaker is ther primary protection and secondary protect both ?
 

radiopet

Senior Member
Location
Spotsylvania, VA
1100880172_2.jpg
 

infinity

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Location
New Jersey
Occupation
Journeyman Electrician
So the 500's to the 400 amp CB are too small since you cannot go up to the next size. If you consider that the neutral secondary conductor is a CCC due to harmonic loading then you would need conductors that, after derating, would be equal too or greater than 400 amps. 600 kcmil are no good either (475 amps * 80% = 380 amps) So now it looks like your secondary conductors would need to be larger than 600 kcmil. For us around here that means that your secondary conductors would be 750 kcmil THHN copper conductors (535 amps * 80% = 428 amps) since 700 kcmil conductors are not readily available. Before I would use 750 kcmil conductors I would opt for two sets of smaller conductors.
 

radiopet

Senior Member
Location
Spotsylvania, VA
Ohmhead,

Well 3/0 CU is good for 200A and (2) sets would give you 400A capacity so I don't see him as wrong. Just decided to parallel them versus running a 600 KCMIL.

Unless I am missing something on that statement....it has been a rather long day for me...lol
 

radiopet

Senior Member
Location
Spotsylvania, VA
So the 500's to the 400 amp CB are too small since you cannot go up to the next size. If you consider that the neutral secondary conductor is a CCC due to harmonic loading then you would need conductors that, after derating, would be equal too or greater than 400 amps. 600 kcmil are no good either (475 amps * 80% = 380 amps) So now it looks like your secondary conductors would need to be larger than 600 kcmil. For us around here that means that your secondary conductors would be 750 kcmil THHN copper conductors (535 amps * 80% = 428 amps) since 700 kcmil conductors are not readily available. Before I would use 750 kcmil conductors I would opt for two sets of smaller conductors.

This is something I have to say is very hard for me to determine as a plans examiner. We do not get to see the end result and 100% of the engineers we deal with will not break this factor down and since I know very clearly what MY paycheck covers versus what an engineers paycheck covers....I will always ask them the question and 100% of the time their response back to me is.....No Mr. Abernathy, the nonlinear loads are less than 50%. So I have to take them on face value since they put their licensed professional engineering seal on it...and I am no engineer.
 
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