What is wrong with this?

[twinman]

Had a discussion with a co-worker regarding the one-line and he/she still has a difference of opinion.

Let's see what others see wrong with this one-line...neither the motor load nor the control cabinet at the bottom of the one-line have overcurrent protection within the enclosure.

Fire away!

 

[augie47]

Looks like we are lacking proper protection on the primary & secondary of our 7.5 kva transformer.

 

[Smart $]

Agree with Gus for one.

Another may be step-up xfmr's secondary tap conductor protection or disconnect for gate operator. Can't tell because parameters—distances, line of sight—are not provided on one-lines.

 

[twinman]

I see...

I see...

What I see:

a) Inadequate protection of the step up transformer and its secondary feeders.
b) Tapping of single phase transformer not only does it not comply with the tap rule but also is to take place within the disconnect. Even it it were allowed, depending of where the tap is connected it could be considered a feeder or a branch circuit tap.
c) Single phase transformer lacks proper protection. Since the control enclosure does not have overcurrent protection within it, the primary side protection cannot exceed 125% of FLA.


I suggested relocating the disconnect switch to the secondary side of the step-up transformer and revising the fuse size to 45A (standard size) as per table 450.3(B). I also recommended providing a junction/splice box for tapping of secondary feeder and and adjacent fused disconnect 2P, 30AS, 20AF to protect the step-down transformer.

General comment was that the overcurrent protection sizes should be compared against table 450.3(B). The primary protection of the step-up transformer is 120% of the full primary current, thus the transformer cannot be utilized to full capacity.

 

[augie47]

that, plus since you are utilizing the 3 wire (240 + 240/120) on your 7.5 kva the secondary will need protection also (the primary protection can not serve as secondarey protection)

 

[twinman]

120/240 protection

120/240 protection

Gus,

I was wondering that myself...I looked at the square D website for transformer connections. It appears that terminals X2 and X3 are being connected (as it center tap) which would require bonding it to ground to comply with NEC.

Instead X1 and X4 are utilized to derive 240V and either X1-X2 or X3-X4 for the 120V.

Could you explain what you mena by having both voltages, the primary protection not protecting the secondary?

Thank you.

 

[augie47]

Gus,

I was wondering that myself...I looked at the square D website for transformer connections. It appears that terminals X2 and X3 are being connected (as it center tap) which would require bonding it to ground to comply with NEC.

Instead X1 and X4 are utilized to derive 240V and either X1-X2 or X3-X4 for the 120V.

Could you explain what you mena by having both voltages, the primary protection not protecting the secondary?

Thank you.

If you are deriving both 120v and 240v from the secondary it has to be a three wire secondary. 240.4(F) explains primary overcurrentr protection is not sufficient when you have this arrangement (since this is single phase..a two wire primary with two wire secondary could work with properly sized wire & primarey OCP--but not a 2 wire primary and 3 wire secondary). I beleive 240.21(C) (1) refers to that also.

 

[Smart $]

What I see:

a) Inadequate protection of the step up transformer and its secondary feeders.

After checking the numbers, the xfmr primary, secondary, and secondary tap conductors are all protected by the primary's 100A CB. [450.3(B), 240.30(C)(1)]

One question though, the one-line does not show any grounding of the step-up transformer... is the secondary grounded at all? If not, where are the ground detectors?

b) Tapping of single phase transformer not only does it not comply with the tap rule but also is to take place within the disconnect. Even it it were allowed, depending of where the tap is connected it could be considered a feeder or a branch circuit tap.

I would assume the tap is to occur on the load side of disconnect. I don't see any problem as long as the means is an approved method.

c) Single phase transformer lacks proper protection. Since the control enclosure does not have overcurrent protection within it, the primary side protection cannot exceed 125% of FLA.

Well it certainly needs protection... but the percentage is not limited to 125% since secondary protection is required as Gus has brought up.

I suggested relocating the disconnect switch to the secondary side of the step-up transformer and revising the fuse size to 45A (standard size) as per table 450.3(B).

As mentioned in my response to "a)" above, the xfmr and tap conductors are protect by the primary ocpd. The disconnect is for at the operator. The fusing is unnecessary.

I also recommended providing a junction/splice box for tapping of secondary feeder and and adjacent fused disconnect 2P, 30AS, 20AF to protect the step-down transformer.

That is a viable alternative.

General comment was that the overcurrent protection sizes should be compared against table 450.3(B). The primary protection of the step-up transformer is 120% of the full primary current, thus the transformer cannot be utilized to full capacity.

Perhaps... but is it necessary? 125% is only 4 more primary amps... so what two more amps at best on the secondary. You might be able to get more inrush capability, but then you'd have to add secondary ocpd and upsize primary ocp. Without knowing the loads, it's kind of hard to do anything but speculate

 

[Smart $]

The fusing is unnecessary.

Scratch that comment.

However the specified fuse size is too high to really protect anything that the initial ocp can't handle. So I condone splitting the feeder to supply two fused disconnects, one for the operator and one for the step-back-down xfmr.

 

[twinman]

Response

Response

Smart,

I cannot find article 240.30(C)(1) on the 2005 NEC.

I concur that the transformer should be a 208V delta to 480V wye grounded step-up transformer in lieu of what is indicated. It appears that a standard 480V delta to 120/208 wye was selected and reversed, doesn't it? Are you refering to the ground loops?

If the disconnect remains as indicated, the load side tap becomes a branch circuit tap, 240.21 and 210.19. Although I suspect that there is an OCPD within the gate controller enclosure, the fact ot the matter is that I don't have details on this "misterious" box and have based review on the diagram.

Agreed, with secondary protection, primary protection can go up to a max of 250%.

I always prefer to provide primary and secondary protection so that I can size the primary OCPD larger than 125% to account for the magnetizing current.

 

[Smart $]

Smart,

I cannot find article 240.30(C)(1) on the 2005 NEC.

Ooops! ...would you believe I made a typo?

Should have been 240.21(C)(1)

It's the same one that requires secondary tap conductor protection for the multi-voltage step down transformer... but in the case of the step up transformer, it appears to be a 3-wire delta, single-voltage secondary. Note part of the criteria is the xfmr be delta-delta connected. Even though its primary windings are wye configured, the connections will be delta-delta.

I concur that the transformer should be a 208V delta to 480V wye grounded step-up transformer in lieu of what is indicated. It appears that a standard 480V delta to 120/208 wye was selected and reversed, doesn't it? Are you refering to the ground loops?

Yes, it appears to be a delta-wye step-down wired in reverse. No, I'm not referring to ground loops. There is no indication any of the three line conductors will be a grounded conductor. The 3P disconnect suggests none are grounded as you would only need a 2P disconnect if one leg were grounded. If none are grounded, 250.21 would require ground detectors for the secondary.

 

[twinman]

Ooops! ...would you believe I made a typo?

Should have been 240.21(C)(1)

It's the same one that requires secondary tap conductor protection for the multi-voltage step down transformer... but in the case of the step up transformer, it appears to be a 3-wire delta, single-voltage secondary. Note part of the criteria is the xfmr be delta-delta connected. Even though its primary windings are wye configured, the connections will be delta-delta.



Yes, it appears to be a delta-wye step-down wired in reverse. No, I'm not referring to ground loops. There is no indication any of the three line conductors will be a grounded conductor. The 3P disconnect suggests none are grounded as you would only need a 2P disconnect if one leg were grounded. If none are grounded, 250.21 would require ground detectors for the secondary.

The lenght of the secondary feeders between the set-up transformer and the gate controller and access control is approx. 1600' and being in a duct bank (new, resolution to previous post), 240.21(C)(4) applies. Per 240.21(C)(4)(2) allows tapping of the load side of the disconnect to energize any number of additional overcurrent devices. As such, proper primary and secondary protection can be provided for the single phase transformer.

It still does not correct the lack of grounding on the secondary side of the step-up transformer.

 

[Smart $]

The lenght of the secondary feeders between the set-up transformer and the gate controller and access control is approx. 1600' and being in a duct bank (new, resolution to previous post), 240.21(C)(4) applies. Per 240.21(C)(4)(2) allows tapping of the load side of the disconnect to energize any number of additional overcurrent devices. As such, proper primary and secondary protection can be provided for the single phase transformer.

Distance and raceway type have little bearing on the transformer's secondary and tap conductor protection. If protection of such meets the specifications of 240.21(C)(1), there is no need to complicate by qualifying the tap conductors under another subparagraph, such as 240.21(C)(4). The main paragraph of 240.21(C) is not very explicit in this regard... but if you think about it, there would be no way for every installation to qualify under all six subparagraph. Logic precludes only one needs to be satisfied.

It still does not correct the lack of grounding on the secondary side of the step-up transformer.

Definitely a concern that needs resolved!!!

 

[Smart $]

...Logic precludes only one needs to be satisfied.

PS: Qualifying under 240.21(C)(1) permits you to tap the secondary conductors because there is no provision limiting its termination to a single overcurrent device.