This total power to site does not add up.......

[dionysius]

I am looking at a new job to bid. Went there to scope it out.

There are 2 bldgs on the site: A and B each separately metered.

Bldg A has a 400A Disconnect 480Y/277V Service. So it is 400x277x3/1000 = 332.4 kW

Bldg B has a 100A Disconnect 480Y/277 Service. So it is 100x277x3/1000 = 83.1 kW

The total for Bldg A and B add to: 332.4kW + 83.1 kW = 415.5 kW

Now the utility pole supplying the entire 2 buildings has 3x75 kW pots (clearly marked 75) up on the pole which is only 225 kW.

Why is the supplied total so much less??????

 

[GoldDigger]

I am looking at a new job to bid. Went there to scope it out.

There are 2 bldgs on the site: A and B each separately metered.

Bldg A has a 400A Disconnect 480Y/277V Service. So it is 400x277x3/1000 = 332.4 kW

Bldg B has a 100A Disconnect 480Y/277 Service. So it is 100x277x3/1000 = 83.1 kW

The total for Bldg A and B add to: 332.4kW + 83.1 kW = 415.5 kW

Now the utility pole supplying the entire 2 buildings has 3x75 kW pots (clearly marked 75) up on the pole which is only 225 kW.

Why is the supplied total so much less??????

The nominal capacity of the POCO pots is less than the sum of the service sizes for two reasons:

1. POCO sizes to the calculated or measured load, not the service size. It is doubtful that either building uses anything near the nominal service size.
2. Even the calculated load of the buildings and the actual load are likely to be very much time dependent with short peak loads. POCO's oil filled transformers have a high heat capacity and can deliver more than 100% over their rating for short periods of time.
If a transformer fails from overload, POCO will replace it with a larger one.

 

[dionysius]

Thank you GoldD for this. What you state in essence is that if any site is examined one is likely to find that the delivered power as read from the pots or XFMR kVA stamps is significantly less than the service discos added up??? There are so many margins built in (e.g. 80% max of brkr rating etc.) that it is surprising to me at least to see margins built on top of more margins. I hope that makes sense.:?

 

[don_resqcapt19]

Thank you GoldD for this. What you state in essence is that if any site is examined one is likely to find that the delivered power as read from the pots or XFMR kVA stamps is significantly less than the service discos added up??? There are so many margins built in (e.g. 80% max of brkr rating etc.) that it is surprising to me at least to see margins built on top of more margins. I hope that makes sense.:?

The service size is based on the very conservative (inflated) loads that result from the application of the rules in Article 220. The utility transformer sizing is based on decades of real world information.

 

[dionysius]

While that is possibly true, please consider this.

The customer at the receiving end has paid for an expected capability. Will the amount on the pole be honored if push comes to shove???? In this case I am trying to see if I can accommodate the customer with a significant load increase that would not yet stress the POCO but will or may take the POCO to where the installed amounts are at which in this case are 500A on each and every one of the three phases?????

Another way of asking the same question is if this customer, who has limited resources, "stresses" the system by simply using at or close to 500 Amps per phase will he be beaten up and told he has to up his service by installing a pad trafo which will cost him $50,000 or more dollars??

This customer possibly may be able to afford an increase to his existing usage but it will place him right on the margin if he is forced to expend $50k plus to upgrade to a new trafo. What do you all think????

 

[GoldDigger]

In general as long as the customer is not required by NEC to upgrade the service and is not upping it voluntarily POCO is likely to fund any upgrade required in their equipment.
If the customer originally gave POCO an estimated load and they are going to go significantly above that then POCO may ask for improvement money.

 

[topgone]

I am looking at a new job to bid. Went there to scope it out.

There are 2 bldgs on the site: A and B each separately metered.

Bldg A has a 400A Disconnect 480Y/277V Service. So it is 400x277x3/1000 = 332.4 kW

Bldg B has a 100A Disconnect 480Y/277 Service. So it is 100x277x3/1000 = 83.1 kW

The total for Bldg A and B add to: 332.4kW + 83.1 kW = 415.5 kW

Now the utility pole supplying the entire 2 buildings has 3x75 kW pots (clearly marked 75) up on the pole which is only 225 kW.

Why is the supplied total so much less??????

@dionysius,
The total loads of the feeder will be analyzed and the economic size of the distribution transformer will be known. You need to know the operating costs relative to using a certain size of transformer, the usefull life when loaded as such, the cost of replacement and compare which size is more economical in the long run.

IIRC, a rule of thumb exists, that sizing of distribution transformer supplying a number of loads also depend on the number of downstream users . For a load group consisting of not more than 20 users, the usual diversity factor used is 53%. Does your building occupancy fit the number of users?

 

[GoldDigger]

If the customer owns a transformer with primary voltage service from POCO, its their pocket to buy a larger transformer.

 

[dionysius]

The total for Bldg A and B add to: 332.4kW + 83.1 kW = 415.5 kW

Now the utility pole supplying the entire 2 buildings has 3x75 kW pots (clearly marked 75) up on the pole which is only 225 kW.

Why is the supplied total so much less??????

When I told the customer that the POCO has essentially limited him at 225 kVA he felt bad since his plan can be accommodated within his installed 415.5 kW. In this case should I advise him to speak to POCO and they will change out the pots to higher values for him since that was their original intent??? It seems unfair that he should have to pony up money to them that should have given him what was contracted to begin with.

I hope each person is being impartial on this question. I feel it is a fair question.

 

[mgookin]

Have you done an existing and a proposed load calculation? That's where you need to start.

 

[dionysius]

Yes, I have which is precisely why I raise this question. You might also ask if the POCO has done theirs when the install was done some years ago.

 

[topgone]

Yes, I have which is precisely why I raise this question. You might also ask if the POCO has done theirs when the install was done some years ago.

The only card the HO has is to prove that the existing transformer fails to meet the power quality requirements! If the supply voltage is lower than what's stipulated in the supply contract or is very erratic, he can cite the PoCo for that, IMHO. Other than that, IDK if what you considered "fair" flies.

 

[dionysius]

The only card the HO has is to prove that the existing transformer fails to meet the power quality requirements! If the supply voltage is lower than what's stipulated in the supply contract or is very erratic, he can cite the PoCo for that, IMHO. Other than that, IDK if what you considered "fair" flies.

This issue is about power quantity only (kVA). It is NOT about power quality or voltage level which are both fine at the level he is currently using. It is an industrial user not a home owner.

 

[topgone]

This issue is about power quantity only (kVA). It is NOT about power quality or voltage level which are both fine at the level he is currently using. It is an industrial user not a home owner.

Please be reminded that problems with insufficiency of power manifests in power quality issues also. That said, ask for a voltage recording that he will use to convince PoCo that there's a need to upgrade the transformer capacity. Been there, done that, got the t-shirt thing!

 

[mgookin]

Yes, I have which is precisely why I raise this question. You might also ask if the POCO has done theirs when the install was done some years ago.

Years ago it would have been up to the customer to tell POCO what load they are going to have; same as today. You tell POCO what you need and they tell you how much it's going to cost.

So what load do you need then? You came on here telling us what the main OCPD's are rated and said nothing about what load is existing or what you need.

Nobody on here is trying to give you a hard time. We're trying to get to the bottom of it to help you with your project.

 

[Smart $]

IMO quit dancing around the issue. Call up the POCO. They will work with you and let you know exactly where the customer stands.

 

[dionysius]

The total for Bldg A and B add to: 332.4kW + 83.1 kW = 415.5 kW

Now the utility pole supplying the entire 2 buildings has 3x75 kW pots (clearly marked 75) up on the pole which is only 225 kW.

Why is the supplied total so much less??????

Sorry. I should have stated up front that the customer's expected average load will be, by my calculations, around 450 kVA. He is desiring to expand his operation from where he is at currently which is measured at circa 250 kVA. His fear is that the entire service will fail once he brings the extra 200kVA on line.

 

[JoeStillman]

All you have to do is notify the utility that the load will increase soon and how much. They'll take it from there. The customer doesn't get to/need to tell the utility what size transformer to use. The utility will actually allow some overload of their transformers on purpose. The customer isn't limited by the existing transformer size - maybe by the primary (but rarely).

 

[mgookin]

Sorry. I should have stated up front that the customer's expected average load will be, by my calculations, around 450 kVA. He is desiring to expand his operation from where he is at currently which is measured at circa 250 kVA. His fear is that the entire service will fail once he brings the extra 200kVA on line.

No problem. It's the nature of forums where it's tough to understand at one end what's being conveyed from the other end.

It's logical that if you nearly double the load, it very well may compel upgrading farther up the line. But has has been stated by others, loads don't run at peak nameplate rating continuously and some are not even considered concurrent (heat & a/c for example). There are some really good resources (smart people) on here who will likely help with your load calcs to see if there's any kVA savings anywhere if you want to post your spreadsheet. Dialing in those numbers may end up saving your client considerable cost.

 

[junkhound]

utility will actually allow some overload of their transformers on purpose

Poco here subcontracts out their xfmr installs to Potelco, they will nearly always use 150% xfmr rating, often 200%. Change out if any undervoltage problems later.

Local Habitat 5 unit (gas furnace and WH but 100% electrical other) the poco put in a 25 kVA xfmr that served ALL FIVE units. Just the kitchens if all on together would be close to 50kW. 5ea 200A service panels.

Habitat engineer complained to poco, they said the 25 kVA would be sufficient, and has been for the last 4 years.