Sizing KVA of Transformer

[augie47]

I agree the terms "utility and service conductors" should not be used in this context.
transformers have to be sized per a article 220 load calc.
the motors fall under 220.50

educational moment for me.. Code reference ?
I know it's just common sense but I get hung up on "have to" statements.
As long as the transformer is protected (Art 450), what Code reference actally requires you to size it to the calculated load ?

 

[tortuga]

Augie I would be so happy to be wrong about this!
450.3 requires overcurrent protection, and 215.2(A)1(b) says the feeders have to be sized to the calculated load.
Article 100 def of feeder.

 

[jim dungar]

Augie I would be so happy to be wrong about this!
450.3 requires overcurrent protection, and 215.2(A)1(b) says the feeders have to be sized to the calculated load.
Article 100 def of feeder.

You are correct, but neither of those actually say how to size the transformer itself.
There is nothing in 450 that says you need to be able to supply the load calculated per 215. Similarly there is nothing in the NEC about sizing motor loads.

NEC does not result in a optimum, or even adequate, system, it just cares that it is safe.

 

[petersonra]

transformers are relatively cheap to upsize upfront. if it is a close call better to make it beigger.

 

[tortuga]

You are correct, but neither of those actually say how to size the transformer itself.
There is nothing in 450 that says you need to be able to supply the load calculated per 215. Similarly there is nothing in the NEC about sizing motor loads.

NEC does not result in a optimum, or even adequate, system, it just cares that it is safe.

The transformer is part of the feeder no?

Feeder. All circuit conductors between the service equipment,
the source of a separately derived system, or other power supply
source and the final branch-circuit overcurrent device.

 

[Sparky2791]

If there is no power company (utility) there is no true service entrance which would follow NEC article 230.

Well technically there is a power company delivering service at 12470V to a primary switchgear. We take 'what would than be considered a feeder ( I get it now) from the primary switch to the pad mount and to the building. Some rules would still apply as if it were a service such as main disconnect at entrance of ungrounded conductors. Also GES similar to the way you would ground a service

 

[electrofelon]

The NEC has rules for sizing service entrance conductors, feeder conductors, branch circuit conductors and their protective devices. The NEC does not provide guidance on sizing transformers themselves. Given proper terminations it is allowable to have a properly primary side protected 10kVA transformer feed a secondary 1000A switchboard.

You are correct the OP is dealing with feeder and branch circuits not service entrance circuits.

I agree the NEC doesn't address transformer sizing but considering 450.3 I don't see how you can oversize a service by more than 125%. Sure you can feed any size panelboard or switchboard with a small transformer, but somewhere (either on the primary or secondary depending on details) there is going to be a device rated no larger (plus next size up) than 125% of the transformer.

 

[tortuga]

OK educational moment for me too..
So OP says he has 100 5HP motors @480V.
215.2(A)(1) says we have to do a art 220 load calc for the secondary feeder supplying those motors but not the transformer.
that is now understood.
So to keep it simple we use the art 430 table 430.250 for the motors rating and, our art 220 load calc is add up the motors and 25% of the largest ( one 5HP):

​	​	Amps​	Qty​	Extended​	VA​	Amps​	kVA​
​	5hp Motor 208V 3PH​	7.6​	100​	760​	631834​	760​	632​
​	25% largest motor​	1.9​	1​	1.9​	1580​	1.9​	2​
​	​	​	​	​	633413​	761.9​	633​

Then say OP's boss gets a deal on a 167 kVA padmount. There is no NEC on sizing this so boss gets to use it.

So we have a 600% OCPD on primary side and 300% on secondary right?

Primary side can be sized 6X nameplate of 167 so 7.73 Amps X 6 ~47 Amp min OCPD.

Now the secondary side is a 480V feeder OCPD and the 300% OCPD for the transformer.

What size OCPD do you all come up with for the secondary side?

I made a little graphic, pardon any end of the day mistakes.

Cheers and all happy friday.

 

[tortuga]

Correction 'Supervised location' secondary less than 1000V = 250% not 300% sorry

 

[jim dungar]

The transformer is part of the feeder no?

How many feeders is a transformer allowed to supply?
Where is the method for determining how to sum these multiple feeders? Where are the rules that address load diversity or non-coincident loads.

 

[tortuga]

@Jim dungar what size secondary OCPD do you come up with in my sumup of OP's scenario in post #28

 

[jim dungar]

@Jim dungar what size secondary OCPD do you come up with in my sumup of OP's scenario in post #28

I am not in position to confirm any of your calculations or the NEC text.

In general, I would size the transformer at 1kVA per anticipated online HP. Based on your supplied data I would start with a 500kVA unit. If I was told theses motors would be run at 50% load, I might look at a 300kVA unit with an option for fan cooling.

 

[electrofelon]

Correction 'Supervised location' secondary less than 1000V = 250% not 300% sorry

Right, you can get up to 250% for supervised locations, for a MV to LV transformer. If one also used note 5 you could skip the secondary protection and just have the primary at up to 600% I guess that would allow a transformer to be sized "utility style"

 

[tortuga]

Note 5 is interesting, yeah I was trying to point out there is some calculus between 215.3 and T450.3(A)
In my example 450.3(A) would require the OCPD to be 600A.
However 215.3 would kick in and require the OCPD have a minimum rating of 762.

 

[tortuga]

 

[electrofelon]

Note 5 is interesting, yeah I was trying to point out there is some calculus between 215.3 and T450.3(A)
In my example 450.3(A) would require the OCPD to be 600A.
However 215.3 would kick in and require the OCPD have a minimum rating of 762.

IMO 215 does not apply to the transformer due to 310.1

 

[tortuga]

IMO 215 does not apply to the transformer due to 310.1

Agreed
However both 215.3 and T450.3(A) apply to the secondary overcurrent protective device depicted in post 35 no?

 

[electrofelon]

Agreed
However both 215.3 and T450.3(A) apply to the secondary overcurrent protective device depicted in post 35 no?

I would say yes. But We can make the conductors as large as we want, so the limiting factor to undersizing the transformer would be 450.3

 

[tortuga]

I would say yes. But We can make the conductors as large as we want, so the limiting factor to undersizing the transformer would be 450.3

In the example based on the OP, a 450.3 would only require a 600A breaker,
Since the secondary is also a feeder overcurrent device

215.3 Overcurrent Protection:
Where a feeder supplies continuous loads
or anycombination of continuous and
noncontinuous loads, the rating of the
overcurrent device shall not be less than
the noncontinuous load plus 125 percent
of the continuous load.

Would require a 800A breaker.
That would in effect re-size the transformer.

 

[electrofelon]

In the example based on the OP, a 450.3 would only require a 600A breaker,
Since the secondary is also a feeder overcurrent device

Would require a 800A breaker.
That would in effect re-size the transformer.

Ok hold on I am lost and confused now. Need a sign on me that says "if found please return to 184 palmer road". It seems the OP was talking about a 12.47KV to 120/208 500kva transformer. That would be a FLC of 1390. IS that what we are talking about?