HUGE conductor size increase based on OCPD rating

[hill900505]

Hey all,
I am sizing conductors for an OCPD and it turned out that by making sure the OCPD could protect the conductor terminated, the wire size increased from 700 kcmil (Based on 125% of continous load) to 1500kcmil (checking with OCPD after coming up with conductor size). Here are my calculations, if anyone could point out if this is really true, I would be really appreciated.
Here are my calculations.
1. 1000kW three phase load, 400VAC, which gives load current of 1443A. 125% of continuous load=1804A, the OCPD size is 2000A.
2. 4 current carrying conductors, 36 degreeC. Adjustment factor is 0.8, correction factor 0.91.
3. Using 4 parallel copper conductors rated for 90C, 75 degree C terminal temp rating gives us the wire size of 700kcmil copper
4. Here is the problem, in order for 2000A breaker to protect the conductors, I would need at least 4 sets of 1500kcmil copper (Ampacity 705A*0.8*0.91*4=2052A>2000A.
To me, this is a HUGE wire size increase, could anybody explain to me if this is real or I am just wrong in the calculation?

 

[electrofelon]

First, do you really have 4 CCC? Second, try more sets of smaller conductors. You get very little increase in ampacity above 500.

 

[hill900505]

First, do you really have 4 CCC? Second, try more sets of smaller conductors. You get very little increase in ampacity above 500.

Thanks for the help!! What surprised me is that 700 kcmil conductor will just do the job to carry the load, but we have to double the wire size to 1500kcmil just to make sure the OCPD can protect the conductor after coming up with the minimum conductor size for this application.

 

[tkb]

I also question 4 ccc. Also if you can get to 35c it would help.

750cu @ 90°c = 535a * 0.96 = 513a
750cu @ 75°c = 475a * 4 = 1900a

The calculated load = 1804a

I would use 4 sets of 750cu.

edit: I forgot about the over 800 rule. 240.4(C)
Can you adjust the trip setting to 1900a? 240.6(C)

 

[topgone]

Hey all,
I am sizing conductors for an OCPD and it turned out that by making sure the OCPD could protect the conductor terminated, the wire size increased from 700 kcmil (Based on 125% of continous load) to 1500kcmil (checking with OCPD after coming up with conductor size). Here are my calculations, if anyone could point out if this is really true, I would be really appreciated.
Here are my calculations.
1. 1000kW three phase load, 400VAC, which gives load current of 1443A. 125% of continuous load=1804A, the OCPD size is 2000A.
2. 4 current carrying conductors, 36 degreeC. Adjustment factor is 0.8, correction factor 0.91.
3. Using 4 parallel copper conductors rated for 90C, 75 degree C terminal temp rating gives us the wire size of 700kcmil copper
4. Here is the problem, in order for 2000A breaker to protect the conductors, I would need at least 4 sets of 1500kcmil copper (Ampacity 705A*0.8*0.91*4=2052A>2000A.
To me, this is a HUGE wire size increase, could anybody explain to me if this is real or I am just wrong in the calculation?

You don't use the 90 degrees conductor rating because your terminal temp rating is 75 degrees! Use the 75 degree rating instead. Though your figure came out as 1500MCM, the computation is as follows:
size = 625A (75 deg rating of 1500MCM) X 4 X 0.81 X 0.9 = 1,823A! Which is about 127% of your load amps

 

[hill900505]

First, do you really have 4 CCC? Second, try more sets of smaller conductors. You get very little increase in ampacity above 500.

Sorry for the confusion, I meant 4CCC per phase, 4 sets of ABC phases and I used three conduits.

 

[tkb]

Sorry for the confusion, I meant 4CCC per phase, 4 sets of ABC phases and I used three conduits.

In that case, 4ccc at 36°c, it calculates to 1500cu.

 

[topgone]

Sorry for the confusion, I meant 4CCC per phase, 4 sets of ABC phases and I used three conduits.

IDK, but I think you needed to use 4 conduits! Each conduit carries A, B, C, and the N conductor! Did I miss a lot here?

 

[GoldDigger]

IDK, but I think you needed to use 4 conduits! Each conduit carries A, B, C, and the N conductor! Did I miss a lot here?

You are right.
The parallel rules do not allow you to have one set in each of two conduits but two sets in the third. The paths would not be physically identical.

Sent from my XT1585 using Tapatalk

 

[JFletcher]

I dont see where the OP said the eqpt needs a neutral, and isnt it permissible to run AAAA, BBBB, CCCC in 3 conduits?

You cannot use the 90* column, so at 75*C, 1500kcmil is 625A x 4 x .8 (4 conductors/conduit) x .88 (36*C @ 75*) = 1760A, amiright?

6 sets of 750MCM either run AAAAAA, BBBBBB, CCCCCC or ABCABC (x3) or with 2 non CCC neutrals each conduit, would give 475 x 6 = 2850A, x .8 (4-6 CCC/conduit) = 2280 x .88 (temp derate_ = 2006.4A and be fine on a 2000A breaker. if my 4am math is correct.

Are these 6" conduits? and what load is 1MW? :blink:

 

[kwired]

I dont see where the OP said the eqpt needs a neutral, and isnt it permissible to run AAAA, BBBB, CCCC in 3 conduits?

You cannot use the 90* column, so at 75*C, 1500kcmil is 625A x 4 x .8 (4 conductors/conduit) x .88 (36*C @ 75*) = 1760A, amiright?

6 sets of 750MCM either run AAAAAA, BBBBBB, CCCCCC or ABCABC (x3) or with 2 non CCC neutrals each conduit, would give 475 x 6 = 2850A, x .8 (4-6 CCC/conduit) = 2280 x .88 (temp derate_ = 2006.4A and be fine on a 2000A breaker. if my 4am math is correct.

Are these 6" conduits? and what load is 1MW? :blink:

If you ran AAAAG, BBBBG, CCCCG (where allowed and must be non ferrous raceway) you may have easier time getting each phase conductor same length, but you do introduce the 80% ampacity adjustment for number of current carrying conductors in the raceway as well. If you can make it so you have ABCG, ABCG, ABCG, ABCG, you don't have an adjustment for more then three current carrying conductors in a raceway - you still will have ambient temp adjustment though, which would get you to only needing four sets of 900 kcmil conductors.

 

[petersonra]

just to be ornery, is there some reason you cannot put (2) each of 400 KCM per phase in each conduit?

that would give you 6 400s in parallel

 

[hill900505]

If you ran AAAAG, BBBBG, CCCCG (where allowed and must be non ferrous raceway) you may have easier time getting each phase conductor same length, but you do introduce the 80% ampacity adjustment for number of current carrying conductors in the raceway as well. If you can make it so you have ABCG, ABCG, ABCG, ABCG, you don't have an adjustment for more then three current carrying conductors in a raceway - you still will have ambient temp adjustment though, which would get you to only needing four sets of 900 kcmil conductors.

I agree. By using 4 conduits, the cable size can be reduced to 900 kcmil from 1500 kcmil. One one note, I kinda disagree “You cannot use the 90* column, so at 75*C, 1500kcmil is 625A x 4 x .8 (4 conductors/conduit) x .88 (36*C @ 75*) = 1760A, amiright? “
Instead I need to use 90 degree C column for derate, is that right?

 

[JFletcher]

If you ran AAAAG, BBBBG, CCCCG (where allowed and must be non ferrous raceway) you may have easier time getting each phase conductor same length, but you do introduce the 80% ampacity adjustment for number of current carrying conductors in the raceway as well. If you can make it so you have ABCG, ABCG, ABCG, ABCG, you don't have an adjustment for more then three current carrying conductors in a raceway - you still will have ambient temp adjustment though, which would get you to only needing four sets of 900 kcmil conductors.

He only has 3 conduits at present.

and yes, on further perusal, the temperature derate is from the 90*C column, not the 75*

 

[electrofelon]

Why is no one derating from the 90 deg, column? Wouldn't 6 isophase sets of 600 do it? 90 degree for derating: 475*6*.8*.91=2075=good, 75 degree for terminations: 420*6=2520=good. 9 sets of 300 might do it too.

Who planned (or didn't plan) this pipe work and conductor size out??

 

[Smart $]

...
1. 1000kW three phase load, 400VAC, which gives load current of 1443A. 125% of continuous load=1804A, the OCPD size is 2000A.
...

Why 400V?

Why are you saying 1MW is a continuous load? I've never seen a 1MW continuous load*.


*At least not on the distribution end of the electric industry. Have on the generation end.

 

[hill900505]

Why 400V?

Why are you saying 1MW is a continuous load? I've never seen a 1MW continuous load*.


*At least not on the distribution end of the electric industry. Have on the generation end.

That is right, it is on the generation side. The equipment is actually a 1MW solar inverter with 2000A AC breaker. I do not know why the AC output set by this inverter is 400V, I found it on the inverter’s nameplate.

 

[ggunn]

That is right, it is on the generation side. The equipment is actually a 1MW solar inverter with 2000A AC breaker. I do not know why the AC output set by this inverter is 400V...

I'm pretty sure it's because large inverters like this one are designed to be paired with medium voltage transformers instead of interconnection at low (e.g., 480V) voltage.

 

[kwired]

I agree. By using 4 conduits, the cable size can be reduced to 900 kcmil from 1500 kcmil. One one note, I kinda disagree “You cannot use the 90* column, so at 75*C, 1500kcmil is 625A x 4 x .8 (4 conductors/conduit) x .88 (36*C @ 75*) = 1760A, amiright? “
Instead I need to use 90 degree C column for derate, is that right?

Absolute minimum conductor size can never be below what is required for the termination which will usually be 75C these days, old equipment may be 60C but is usually still 75C if over 100 amps. You then need to apply 90 deg C ampacity tables to temperature corrections (which includes number of current carrying conductors in a raceway) - whichever conductor is larger (termination temp sized or the temperature corrected size) is the minimum required size conductor for the application. If you have no temp related adjustments to make the conductor sized for termination temp should normally be the larger conductor, if you have temp related adjustments you need to check to see which requires a larger conductor.

 

[nvpowerdoc]

Article 215.2(A)(1) Execption 1

Article 215.2(A)(1) Execption 1

You might want to consider using the Exception no. 1 to Article 215.2(A)(1) and utilize 100% rated breakers. The exception states:

If the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the feeder conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.

In this case, you are not required to apply the 125% multiplier to the calculated circuit ampacity (1443A) when selecting your conductor size. Bear in mind, the ENTIRE assembly containing the breaker must be rated for 100% operation as stated by the equipment manufacturer.

Second consideration - you may use the 90DegC column when selecting conductor sizing when applying derating factors, such as for temperature [Table 310.15(B)(2)(a)] or multi-conductor systems [Table 310.15(B)(3)(a)]. The caveat is that the final calculated ampacity for the conductor selection cannot exceed the 75degC column ampacity value for the same size cable if you are installing the cables on equipment (breakers) rated 75degC. Some breakers and equipment are rated for 90degC operation (it just costs more money), just as 100% rated breakers (and equipment enclosures) cost more money.

We just went through this with a Data Center where continuous loads often do exceed 1MW, and most of the equipment is rated for 100% operation (breakers and assemblies).

Take a look at this paper explaining this in further detail: http://www.alliedelec.com/images/Products/mkt/pb/eaton/Pdfs/ullistedmccbs.pdf