Sizing Conductors

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es1451

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I'm sizing conductors for a 75kva 480/208/120v 3phase 4wire Delta-Wye transformer.
Is the neutral a current carrying conductor ?
4 wire secondary
voltage = 120v
amps = 361A
wire size = 500kcmil THNN 90c
disconnect or ocpd = 500A

I have attempted about three different ways at arriving the correct conductor size taking in
consideration the ampacity, the ocpd, adjustment for current carrying conductors, 75c on
terminations. When do you divide the ampacity by adjustment factor, when do you multiply the ampacity by the adjustment factor.... you can be going along pretty good when "BAM!!
you are lost.... I really need a step by step process to sizing conductors so I can find my way back home... s.o.s s.o.s !!!! es1451
 
Good Question!!

Good Question!!

This is what I remember for quick reference....and it is my short answer ......

114.14 especially 110.14(C) pay close attention to the last sentence... "....Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both. but continue to read the rest of 110.14(C)(1)(a) and (b)
To properly size conductors you must own 110.14, it can be confusing enough sometimes even when you mastered this section.To paraphrase 110.14(C), this is how you know which column to use in 310.16

The table for conductor ampacities that is generally referred to is 310.16
Where is the ampacity adjustment table....you ask? 310.15(b)(2)(A)

(Almost forgot......You never divide the adjustment factor...it is a multiplier)

I hope this helps............again just my disclaimer......this is a very short way to keep your bearing.:smile:
 
I'm alittle confused a 75kVA 480 step down to 120/208 should have a 90A Pri and a 208A secondary

maybe this needs clarification before proceeding
Are you sure its a delta secondary usually these are Y secondarys. It makes a differece on neutral current???
 
cpal said:
I'm alittle confused a 75kVA 480 step down to 120/208 should have a 90A Pri and a 208A secondary

maybe this needs clarification before proceeding
Are you sure its a delta secondary usually these are Y secondarys. It makes a differece on neutral current???
Click to expand...


????es1451 makes no mention of a Delta secondary.....just looks like some of the numbers are mixed up:)
 
I must clarify this a little more. This is for the wiring for a 75 kva 480v / 208v /120v 3 phase transformer. It has a 480v Delta primary (3wire) and a 208v / 120v secondary with a neutral (4wire)
 
Take a look at Charlie's math:

75000/(480*1.73)= 90.3 amps primary current

75000/(208*1.73)= 208.4 amps secondary current
 
This changes the 4 to 1 ratio or does it ??? (480v primary) to (208/120 secondary)
primary phase current = 90.3A
secondary phase current = 361A
 
The original post was a problem out of Tom Henry's Transformer Calculations Book, I do not have a copy of this book just the example es1451
 
maybe this will help...

maybe this will help...

es1451 said:
This changes the 4 to 1 ratio or does it ??? (480v primary) to (208/120 secondary)
primary phase current = 90.3A
secondary phase current = 361A
Click to expand...

No. :smile:

480 primary to 208 secondary = 480/208= 2.3 to 1 ratio( primary line to secondary line)



As Infinity already stated...
75000/(480*1.73)= 90.3 amps primary current

75000/(208*1.73)= 208.4 amps secondary current
Click to expand...


Power stays the same( not considering impedance)
Since the voltage on the secondary goes down the current goes up
The voltage on the primary is up so the current is down.

They are inversely proportional. Since the voltage from primary line to secondary line is 2.3 to 1 voltage ratio(and power is constant) The current is inversely proportional, thus you will have 2.3 to one amp ratio from secondary to primary.

208.4/90.3=2.3 to 1
 
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Pierre C Belarge said:
Question for the learned out there.

What is the maximum size overcurrent device is the conductor on the secondary side permitted to be?
What section number?

Hint:
The info is located in Article 240. :grin:
Click to expand...

Table 450.3(B) 125% of 208.5 = 260.5

Secondary OCPD could be as high as 250 amps due to the restriction by 240.21(c)
 
For the primary conductor size you need to take 125% of the 90A primary and get 112.5A. I would therefore use a #2 AWG as a feeder for this transformer.
 
philly said:
For the primary conductor size you need to take 125% of the 90A primary and get 112.5A. I would therefore use a #2 AWG as a feeder for this transformer.
Click to expand...

You are not required to supply the transformer at 125% you are allowed to.

The size of the conductor supplying the transformer will not be based on the transformer it will be based on the OCPD chosen to supply this transformer.
 
es1451 said:
es1451 what would be the secondary conductor size and overcurrent device
Click to expand...

Are you asking the maximum allowed?

Because without knowing what the load is it is the only thing we can do.

IMO the maximum secondary over current device for this transformer is 300 amps per note 1 of table 450.3(B)

In order to use that size OCPD the conductors must be fully rated per 240.21(c) So that would be 350 kcmil copper or 500 kcmil AL
 
iwire said:
You are not required to supply the transformer at 125% you are allowed to.

The size of the conductor supplying the transformer will not be based on the transformer it will be based on the OCPD chosen to supply this transformer.
Click to expand...

O.k. I was thinking of a motor where you need to supply 125% of motor rating. I guess this in not the case for a transformer.

So what you are saying is that you first select the OCPD to protect the transformer primary based on maximums from 450.3(B) the after you have selected the primary breaker you select the feeder conductor size. This is somewhat the opposite of a motor where you first select the cable size and then select the OCPD size?

In my plant there are many transformers fed from an MCC which has a breaker and then another primary breaker at the primary of the transformer located in the field. The breaker at the primary of the transformer is sized to protect the transformer obviously but what about the breaker back at the MCC? Is this simply to protect the feeder out to the primary breaker of the transformer? Is this OCPD usually sized the same as the primary protection located at the transformer?
 
iwire said:
Are you asking the maximum allowed?

Because without knowing what the load is it is the only thing we can do.

IMO the maximum secondary over current device for this transformer is 300 amps per note 1 of table 450.3(B)

In order to use that size OCPD the conductors must be fully rated per 240.21(c) So that would be 350 kcmil copper or 500 kcmil AL
Click to expand...

I agree.

Secondary current of 208A protected at 125% gives 260A. Next largest size breaker per note 1 is 300A breaker. Would need 350 kcmil as stated above for this breaker.
 
iwire said:
IMO you can roll that up to a 300 amp Secondary OCPD as long as the secondary conductors are sized for that.
Click to expand...

I disagree based upon 450.3(B) and 240.21(C).

paraphrasing- 450.3(B)..........max OCPD 125% IMO - note 1 still has to stop at the 125% cap

240.21(C) ......The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
 
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