Large Transformer Secondary Breaker NEC Requirement

[dickricker]

Hello,
New to this forum. New to specifying large transformers.

Starting with the load and working backwards.

Two 600 HP, 480 VAC, 60 Hz, three phase motors. NEC FLC = 720 A. Nameplate FLA = 656 A.
Using NEC FLC for branch (single motor) conductor and breaker sizing
720 A x 1.25 = 900 A
350 kcmil at 75 C : 310 A. Three runs (separate conduits) of 350 kcmil = 930 A. OK!
838 A soft starter, good for 2933 A for 30 sec starting, set for 3.5 x 656 A = 2296 A current limited starting.
720 A x 1.75 = 1260 A. Using motor duty 1200 A three pole breaker (Siemens ND63B12).

One 7.5 KVA lighting transformer, 480 VAC, 60 Hz, single phase, 16 A full load current, 30 A two pole breaker

Feeder (combined load) conductor
900 A + 720 A + 16 A = 1636 A
300 kcmil at 75 C : 285 A. Six runs (separate conduits) of 300 kcmil = 1710 A. OK!

Feeder (combined load) breaker
1200 A + 720 A + 16 A = 1936 A
Next standard size lower = 1600 A
Since this is so far below, better use 100% motor duty breaker.

I think that I am good to here. Any comments?

For transformer sizing, I am using nameplate FLA (656 A) and estimated 8 hour average load (80%)
656 A + 656 A + 16 A = 1328 A
1328 A x 0.80 = 1062 A
1062 A x 480 V x 1.732 / 1000 = 883 KVA 8 hour average transformer load
So, 1000 KVA transformer. Do you agree?

1000 KVA Delta to Delta transformer. Secondary: 480 VAC, 60 Hz, three phase, 1204 A
Following NEC Table 450.3(A) or 450.3(B), secondary breaker 125% full load current, next size higher
1204 A x 1.25 = 1505 A : next standard size higher = 1600 A

This matches the 1600 A breaker from the feeder (combined load) determination;
and the conductors (1710 A) are good too.

Am I good to go with this set up?

Now, Question 2:
240.21(C)(3)
Less than 600 V primary with 125% fusing or breaker.
(This would be for a South Korean installation, 380 VAC, 60 Hz stepped-up to 480 VAC, 60 Hz)
Industrial installation, secondary conductors not over 25 feet, well protected.
Can I remove the 1600 A breaker and run the feeder (combined load) conductors directly to the transformer?

Now, Question 3:
Does 240.21(C)(3) apply to greater than 600 V primary with 125% fusing or breaker?
(This would be for a USA installation, 2400 VAC, 60 Hz stepped-down to 480 VAC, 60 Hz)
Industrial installation, secondary conductors not over 25 feet, well protected.
Can I remove the 1600 A breaker and run the feeder (combined load) conductors directly to the transformer?

Thank You! I greatly appreciate your input.

 

[iceworm]

... Using motor duty 1200 A three pole breaker (Siemens ND63B12).

... better use 100% motor duty breaker. ...

Never heard of a "motor duty breaker". I have no idea what the trip curve looks like. Unless you know the motor starting curve is inside of the trip curve - definitely perform a coordinatin analysis

... For transformer sizing, I am using nameplate FLA (656 A) and estimated 8 hour average load (80%)
656 A + 656 A + 16 A = 1328 A
1328 A x 0.80 = 1062 A
1062 A x 480 V x 1.732 / 1000 = 883 KVA 8 hour average transformer load
So, 1000 KVA transformer. Do you agree? ....

No. I've never heard of using an "estimated 8 hour average load (80%)" to down size an xfm 80% of the connected load. You are purposely overloading an industrial grade system by 10% right out of the box. Nothing I would consider reasonable design for an industrial system.

... Question 2:
240.21(C)(3)
Less than 600 V primary with 125% fusing or breaker.
(This would be for a South Korean installation, 380 VAC, 60 Hz stepped-up to 480 VAC, 60 Hz)
Industrial installation, secondary conductors not over 25 feet, well protected.
Can I remove the 1600 A breaker and run the feeder (combined load) conductors directly to the transformer? ....

I don't know anything about South Korean codes. This section applies in the US as long as you meet the entire section.

... Now, Question 3:
Does 240.21(C)(3) apply to greater than 600 V primary with 125% fusing or breaker?
(This would be for a USA installation, 2400 VAC, 60 Hz stepped-down to 480 VAC, 60 Hz)
Industrial installation, secondary conductors not over 25 feet, well protected.
Can I remove the 1600 A breaker and run the feeder (combined load) conductors directly to the transformer? ....

Yes, as long as you meet the entire section.

The entire design is incredibly thin. I can't tell that you won't get nuisance trips. Definitely you want to do a good coordination study. I don't do design work - but it is nothing I would ever recommend.

ice

 

[augie47]

I may be mis-reading, but it appears your calculations for the two 600 HP comes from 430.24. The fact that a drive is involved would lead me to think 430.120 should be used in that the drive nameplate as opposed to motor FLA would be the figure for motors incorporating drives

 

[dickricker]

Thank You, iceworm!

Never heard of a "motor duty breaker". I have no idea what the trip curve looks like. Unless you know the motor starting curve is inside of the trip curve - definitely perform a coordinatin analysis

What I mean is a breaker which is designed for motor service and has adjustable instantaneous trip which also adjusts the trip curve (like the Siemens ND63B12). We know from experience that the motor starting curve is inside the breaker trip curve.

No. I've never heard of using an "estimated 8 hour average load (80%)" to down size an xfm 80% of the connected load. You are purposely overloading an industrial grade system by 10% right out of the box. Nothing I would consider reasonable design for an industrial system.

The transformer manufacturer engineer has stated that the transformer can stand significant overloads for short periods of time without reducing its life. Motor starting current is not a problem. Heat is the potential problem; but if we keep our average current below the transformers rated load current, there will be no degradation of the transformer. The "8 hour average" is used to give some time frame to the average current. The motors may stop and start several times during an 8 hour shift; and the transformer will heat up during starting. But running at an average power of 883 KVA, the transformer will have time to cool down between starts.

Sorry, I forgot to say that this is an electric chipper; and it is not continuous 100% duty. The motors idle between logs. I used the continuous duty calculations for conductor and breaker sizing to be sure to have enough for starting.

I followed NEC guidelines in sizing conductors, soft starters, and breakers; so, I am definitely not overloading that part of the system. And my calculations show that I am running the transformer at 88%.

I don't know anything about South Korean codes. This section applies in the US as long as you meet the entire section.

Good Point! I better check South Korean codes.
If it was in the USA, following NEC, could I remove the 1600 A breaker and run the feeder (combined load) conductors directly to the transformer?
That is really my main question. I can't find an NEC section where powering motors from transformers is covered.

Thanks Again!

 

[dickricker]

Good Question.

I may be mis-reading, but it appears your calculations for the two 600 HP comes from 430.24. The fact that a drive is involved would lead me to think 430.120 should be used in that the drive nameplate as opposed to motor FLA would be the figure for motors incorporating drives

My understanding is that with a VFD, we use VFD nameplate current; but with a soft starter, since it connects across-the-line after starting, we use the motor HP and associated NEC table FLC. The soft starter is not really a motor controller. It limits current during starting; and then it functions as the overload device. The soft starter has a maximum current rating, just like a maunal starter. But that current rating does not substitute for the FLC of the motor.