480 VAC Transformer Question

[stripedbass]

I'm installing controls for a treatment facility. The power company has a bank of transformers that they said they are going to connect in a 480 VAC 3-wire delta to me.

I have a 45KVA 480 by 120/208Y transformer that I wish to connect to their supply. How would I create the neutral if the power company is feeding 3-wire? Would it meet code if I were to drive some ground rods and use them as VAC ground reference? I've grounded buildings before but without a transformer.

Thanks everyone.

striped.bass@erols.com

 

[DrivesGuy]

Typically, you can ground the 208Y120 side just as you stated. The Wye point can be directly connected to ground. Do you have any transformer protection devices installed? If not, you should consult a local engineering firm to provide you with an appropriate protection scheme.

 

[jbartos]

Suggestions:
1. The neutral of 208V/120V 3-phase 4-wire system shall have a solidly grounded neutral to some suitable grounding rods, water pipes in front of water meter (or provide jumper across the watermeter) according to NFPA 70-1999 National Electric Code.
2. Consult the transformer manufacturer for suitable protective devices. Else, stay on a very conservative side with your own selected protective devices, e.g.
Iprimaryfuse=45000VA/(3**0.5 x 480V)=54.1Amps ~ 50A fuses (or 3phase 600VAC circuit breaker) on the primary and
Isecondaryfuse=45000VA/(3**0.5 x 208V)=124.9Amps ~ 125A fuses (or 3phase ~250VAC main circuit breaker) on the secondary side. A 125A 208V/120V 3phase 4wire distribution panel with 30 to 42 circuit breaker spaces could be used, e.g. by Square D, General Electric, Cutler-Hammer, Siemens, etc.

 

[rhatcher]

The system neutral will connect to the transformer wye and should be grounded. I am not an NEC expert. However, I feel sure that you should give careful consideration before simply grounding the neutral of your distribution transformer with a ground rod and then connecting the four wires to a distribution panel. For example, if this transformer is connected to a distribution panel and utilization equipment which are separately grounded by different ground rods you may have an inadequate ground return path based on the type of soil, the distance between the rods, and the method of installation of the rods. Obviously, if the panel and equipment have no ground then there would be no ground return path.

I would think that good practice would be to use ground conductors connected between distribution transformers, distribution panels, and utilization equipment. This system of ground conductors may be connected to one grounding point (at the transformer wye) or to many depending on the size of the system.

For overcurrent protection you should use fuses or circuit breakers on the primary and secondary of the transformer and you should also consider ground fault protection on the secondary side. Finally, be sure that the primary overcurrent protective device is rated to interrupt the short circuit capacity of the utilities transformer bank.

 

[jbartos]

Comment to the previous posting: The primary side of the transformer, 480V delta, should not be grounded without consulting of the Utility. The Utility may have a source with grounded star winding. Then the 480V delta winding on the transformer primary is not supposed to be grounded. Definitely, the secondary transformer winding needs some system grounding scheme since the ungrounded systems are considered dangerous to the equipment due to overvoltages.

 

[rhatcher]

jbartos, my previous post was specific to the the secondary side of the transformer.
"The system neutral will connect to the transformer wye and should be grounded."
I apologize for any confusion caused by not specifically pointing out that the above mentioned "system neutral" would have to be on the secondary side of the referenced delta-wye stepdown transformer. Thank you for the opportunity to clarify since anyone acting on my post based on your interpretation of what was presented would be in for a nasty surprise.

 

[jbartos]

Suggestion/Comment: It is a good practice and industry standard to make a distinction between the system ground/grounding and equipment ground/grounding (not considering other types of grounding, e.g. lighting grounding, computer/instrument (quiet) grounding, etc. The system grounding is sufficient and normally done at one point close to the system grounded neutral. The equipment grounding is different. It is tying conductive surfaces and bringing them to the same potential level. It may use as many ground points (ground electrode locations) as necessary to make sure that the conductive surface are at the same potential not only among themselves but also against earth ground. This aspect seems to be missing in the above postings.

 

[CSCEE]

The delta primary of the 45 KVA transfomer will connect directly to the 480V supply. The secondary 120/208V wye will require grounding to a ground rod, structural steel, and/or metal water pipe per the NEC, and be bonded to the utility grounding conductor. The 480V supply must be a 4-wire supply (3 phases plus a grounded conductor).

 

[redtrumpet]

jbartos - re: april 25 posting - with transformer primary current of 54.1 A, are you recommending fusing the primary for less than the primary rated amperes (50 A fuse)? In your experience, does this provide the necessary margin to allow for transformer inrush current without nuisance blowing of the primary fuse? What type of fuse do you suggest using to obtain the necessary time-current characteristic?

 

[jbartos]

Suggestion: Had it been my transformer, which I had paid for (in many instances I had), I would started with a lower fuse (~50A), since the transformer is usually less than 100% loaded, mixed load may have 0.9 PF or so, cost of the blown fuse more than handsomely compensates for the tight or sensitive protection. If the fuse is time delay (dual element) then the above statement and approach is especially quickly leading to my goal.

 

[peterb]

Re the transformer fusing -
NEC Art. 450-2(b)(2) allows the following for a transformer which has primary & secondary protective devices -

- Secondary - Fuse or CB rated at not more than 125% (or nearest higher standard rating) of transformer full load current
- Primary - with secondary protection as above, primary overcurrent protection may be rated up to 250% of the transformer full load current; individual circuit protection is not required in this case

Using the above criteria -
- Secondary protection rating not greater than 124.9*1.25 = 156A; Select 150A Fuse or CB
- Primary protection rating not greater than 54.1*2.5 = 135A; Select 90A fuse or CB

I checked the coordination of the above combination (using Buss FRS-R Fusetron fuse curves) and the primary & secondary fuses will operate selectively. A 50A primary/125A secondary combination definitely doesn't coordinate - the primary fuse will blow ahead of the secondary fuse for secondary faults. Depending on the fuse type selected, there may also be issues with transformer inrush current for the 50A fuse.

The bottom line of the above is to remember that the overcurrent protection is fault protection for the transformer circuit, NOT overload protection. If you want close overload protection, you probably need to look at winding temeperature monitoring for alarm and trip.

 

[jacka]

To peterp: You chose 90A fuse or C.B. for secondary, according to NEC table 450-3(b) the secondary protection should be 250% of 54.13A = 135.3A, why not use 125, 110, 100 or 90A fuse or C.B.?

 

[peterb]

jacka -
The code allows primary (not secondary) protection sized up to 150A (135.3 A, or next larger standard fuse or breaker), where the secondary is protected with a device rated at 125% or less of rated current.
I chose 90A in order to get closer protection, although any of the sizes that you mention, plus 150A, could also be selected.

 

[jbartos]

Suggestion: The NFPA 70 makes a distinction between supervised installations and unsupervised installations. The latter have lower maximum settings on 600V and below secondaries. Also, the transformers may have impedances greater than 10%, which would lead to smaller protective device ratings.