Need help quick correct wire size for a 1200kw 440 volt generator

[ryandumas]

I am working on a crane that is run by a 440 volt 60\hz 1200 kw generator. The company wants a backup generator on the deck ready to wire in. I did the calculations and I belive the amps is 1670 or about 556 amps a leg (by the way this is 3 phase) The electrician above me says we only need 350 mcm the length of the leads will be 120' in thru a barge and up inside the crane in free air with an ambient tempature of 100 degrees F. With the ambient tempature I think I would need at least 500mcm that does not include a voltage drop calculation am I way off and if I'm right should I paralell some wire or just run 500mcm

 

[bob]

Is your load calculations for the actual connected? The generator will supply about 1900 amps.

 

[ryandumas]

Thanks for the reply how did you get 1900 amps

Thanks for the reply how did you get 1900 amps

What were your calculations. I figured 1670 but maybe I'm wrong again this is a three phase 1200 kw 440volt 60hz generator what size wire do I need for each leg 120'

 

[bob]

What were your calculations. I figured 1670 but maybe I'm wrong again this is a three phase 1200 kw 440volt 60hz generator what size wire do I need for each leg 120'

1200 kw/(.44 x 1.73 x 0.8 pf) = 1970 amps on each phase conductor. Don't divide by 3.
You may not need the full capacity of the generator. What is the total load?

 

[ryandumas]

The only information they are giving me is the kw

The only information they are giving me is the kw

I don't have the load amps. Only the kw they want me to wire it for whatever the generator can handle

 

[bob]

I don't have the load amps. Only the kw they want me to wire it for whatever the generator can handle

600 kcm is good for 420 amps x 5 ckts = 2100 amps. Does the generator have a breaker mounted ?

 

[nakulak]

I was thinking 5 sets of 500 (1900), but as others said - what's the load ? are you sure you have the right number of zeroes ?

 

[bob]

ryandumas

The minimum conductors ampacity must have a rating equal to that of the generator breaker(if it has one). Voltage drop for 120 ft of 600 kcm or 500 kcm is ok.

 

[augie47]

Is the run "free air" all the way ? Depending on conditions, as others have stated, if you want full generator output you are looking at more like a minimum of paralleling (3) 500s and thats giving every consideration (free air, etc)

 

[bob]

Is the run "free air" all the way ? Depending on conditions, as others have stated, if you want full generator output you are looking at more like a minimum of paralleling (3) 500s and thats giving every consideration (free air, etc)

Augie
Are you referencing table 310.20?

 

[augie47]

Augie
Are you referencing table 310.20?

No. 310.17 which may not be applicable in his case. I was trying to give him every benefit of the doubt and yet show a single 500 was nowhere near adequate.

 

[bob]

No. 310.17 which may not be applicable in his case. I was trying to give him every benefit of the doubt and yet show a single 500 was nowhere near adequate.

If you use that table I think the 100F temp he mentioned would force a temperature adjustment of 0.82 x 620 amps = 508 amps x 3 = 1524 amps.
If you use the 1200 kw w/o the Pf that would be close to FLA. Install a 1500 amp breaker.

 

[augie47]

Bob, You pointed out an error in my post (which I appreciate). Amazingly, in my 2002 Code which happened to be handy, the temperature adjustments for 310.17 were on the next page. I seldom do free air calculations and I foolishly assumed there were none (since it was free air). That leads me to another question, however, if he used THHW would the correction factor for 100?F not be .91 (.88 for THWN) ?
If it were THHW (700 x .91 = 640..110.14 limiting us to 620 ).

All is pure conjecture I realize since we don't have the details. From the original post I only saw (1) 500 mentioned and my primary point was to show that one wouid be very inadequate.

I am glad you caught my error, but question your .82. Where does that derive ?

 

[nakulak]

I wonder what happened to OP

 

[e57]

The electrician above me says we only need 350 mcm the length of the leads will be 120' in thru a barge and up inside the crane in free air with an ambient tempature of 100 degrees F.

I don't have the load amps. Only the kw they want me to wire it for whatever the generator can handle

Sound like the guy above you is sizing for the load, and after a disconnecting means with OCP. That said - sizing conductors for full capacity of the generator - if the load is only a percentage of that is a waste IMO Especially at this size and length. And I question the 'Free Air' on that type of path.

 

[bob]

Bob, You pointed out an error in my post (which I appreciate). Amazingly, in my 2002 Code which happened to be handy, the temperature adjustments for 310.17 were on the next page. I seldom do free air calculations and I foolishly assumed there were none (since it was free air). That leads me to another question, however, if he used THHW would the correction factor for 100?F not be .91 (.88 for THWN) ?
If it were THHW (700 x .91 = 640..110.14 limiting us to 620 ).

All is pure conjecture I realize since we don't have the details. From the original post I only saw (1) 500 mentioned and my primary point was to show that one wouid be very inadequate.

I am glad you caught my error, but question your .82. Where does that derive ?

0.91 is correct. 0.82 came from the 60C Row and Col for AL wire. Came down in the wrong location.

 

[ryandumas]

I was doing as asked

I was doing as asked

I was told to get the size wire for a 1200kw generator. I now have found out that it has a 2000amp breaker. Sorry I have been very busy and have not been able to reply. any help would be appreciated thanks Ryan

 

[chaterpilar]

i would suggest 4 runs of 240 sq mm swa/xlpe/cu for each phase.

 

[topgone]

More data needed!

More data needed!

What's your ambient temperature range? You need to know the ambient temperature range to know the correct temperature correction factor which will be multiplied to your cable ampacity to match your load requirement(use Table 310.17 (conductors in free air).
The number of conductors per phase will also have to be decided by you to adjust the ampacities properly(use Table 310.15(B)(2)(a) Adjustment Factors for More than 3 conductors in a raceway).
Your case:
If your temperature range is 87 to 95 deg. F, from Table 310.17, the temp correction factor is 0.94 (if you use THHW or RHW).
If you decide to have 4 conductors/phase (to make it easier when conductors pass bends), you will have 12 current-carrying conductors so you will refer to Table 310.15(B)(2)(a) and find the percent adjustment to be 50%(10-20 cond.).
So, your choice will be:
Try 800MCM; A = 815: Wire size amps = 815 X 4 X 0.94 X 0.50 = 1532A - bad; lower than the 1970A.
We need a bigger conductor; try 1250MCM:A = 1065
Wire size amps = 4 X 1065 X 0.94 X 0.50 = 2002 A - hit!
You could play around with the number of runs per phase( try 5 per phase if you like) but remember when you have more conductors in a small place, chances are you will have to derate some more to compensate for proximity effects.
If you think you could arrange the conductors such that the spacing is maintained and there is no temperature problem, you could drop the adjustment factor and use smaller conductors; in our case you could use:
400MCM: ampacity = 545 A (free-air);
Wire size amps = 4 X 545 X 0.94 = 2049 amperes.
Hope this helps.

 

[bob]

I was told to get the size wire for a 1200kw generator. I now have found out that it has a 2000amp breaker. Sorry I have been very busy and have not been able to reply. any help would be appreciated thanks Ryan

Ryandumas
You have been given enough information to design this job. The conductor must have at least the ampacity of the 2000 amp breaker. I believe
1. You must get the local inspector involved
2. Will he allow the use of table 310.17 for this installation
3. If any of the conductors are in conduit the information given will not be valid.
4. How are you going to manage all of these conductors w/o conduit
5. If not in conduit does close grouping of the conductors require further adjustments