Utility Feeder Calculations

[baumancl]

Does anyone know what procedures Utilities follow to size their secondary conductors to a service? Do they follow 125% * continuous load + non-continuouse load like the NEC. I have heard that they size conductors based on demand factors as well. This is in the case where the utility installs and owns the secondary otherwise I the electrical contractor would obviously adhear to NEC becuase I know that some utilities require the electrical contracotr to supply the secondary conductors.

I have a example that I recently ran into. The connected load on a pump station with a proposed 120/240, 1-ph, 3W service was as follows:

15 H.P. on 25 H.P. VFD (converting 240V 1-ph input to 3-ph, 240 output) = 140 A

5kW Electric Unit Heater @ 240 = 21 A
Lighting = 2.5 A
Dehumidifier = 3.0 A
Receptacels = 2.0 A

Total Connected Load = 168.5 A.

Since it has a motor load I would assume to size the conductors for motor load plus all other loads per 430.62 correct?

Since actually the VFD draws 140 FLA then
125% * 140A = 175A
+21A+2.5A+3A+2A = 203.5 A which means that #4/0 min 70 deg C conductors must be used (ampacity = 230 amps)

Can I still size the conductors like and look at the VFD as the true motor load? What article would address this if not?

For overcurrent protection I would assume to adhear to 430.62 and size the OCPD as follows:

The max. recommended overcurrent protection for the VFD per the manufacturer is 175A so I would add this to the FLA of all the other loads?

I would use 175A+21A+2.5A+3A+2A = 203.5 A

And I would be permitted to go up to a 225A Breaker or fuse for the main disconnect ?

I am unsure I am totally correct in how I arrived at the conductor and overcurrent protection sizing but I guess the question comes back to how does the Utility look at this and size their conductors. I was told that the Utility is providing a 200 amp service with a #3/0 triplex secondary. Does this mean that typical Utilties insure their service upto their rating so in this case they assume that the transformer (unknown size) and the #3/0 secondary can carry a continuous load of 200 amps or do they apply a multiplying factor to the connected load that differs from NEC?

 

[jim dungar]

Utility service, and conductor, sizing has nothing to do with the values the NEC requires us to use. Do not base your service entrance equipment on the POCO, except for the requirements they outline in their service manual.

I believe you are mixing 240V and 120V loads in your formulas totals.

 

[iwire]

As Jim said ...

Utility service, and conductor, sizing has nothing to do with the values the NEC requires us to use.

The power companies base the service on years and years of real life usage. They have no interest in what the NEC predicts a building to require, the power company already knows very closely what the real load will be.

 

[bob]

Have you checked to see if 3 phase service is available from the utility?

 

[Smart $]

As Jim said, you appear to be mixing 120V and 240V load amperages. You must calculate loads in VA then sum, then divide by 240V for the purpose of conductor sizing. Of course, doing it this way assumes you are balancing your 120V loads. If the loads are not balanced within a reasonable amount for the conductor size chosen, you will have to add them like you were doing per each line, and size the conductors for the line having the larger current draw.

 

[baumancl]

It was assuming a general worse case scenario i guess. Assuming the single phase 120 volt loads were balanced on the two phases or the best that you could balance them would leave a worse case of 5A of the 120 volt loads on either phase which would amount to 166 amps connected load. It is also assumed for worse case scenario that all the connected loads would run continuous at some point. I knew that the Utilities didnt follow anything the NEC requires I guess I was just wondering how they size thier secondaries and at what point would they require the size of the service to be upgraded from 200A to the next typical size of 400A.

Single phase power to my knowledge was avaible near the site but the cost for installation was extremly high due to a need to extend overhead primary and drop a couple new power poles.

Does anyone have any additional comments on my reasoning for the calculations assuming that the new connected load would be 166A (not much difference)? In particular the application of using the FLA of the input on the VFD in respects to sizing the service entrance conductors after the meter pedestal and the main OCPD per article 430.24 I believe and 430.62?