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#190427 11/17/09 02:24 AM
Joined: Nov 2009
Posts: 2
matt_b Offline OP
New Member
So I have been lurking the site for a while and I now have an issue at work I could use some input on.

The gear we submited was 3 sections: pull, meter, main and distribution.

Instead we got gear with an extra section for the main.

soooo... we know have to move a 300kva xfmr which is fed from 500A breaker 480 delta, to 1000A 280/120Y distribution panel. we have to move it to a utility room about 50ft away.

it had to get "re-enginered" but the engineer drew up a 1200AS/1000AF for the primary, and no OCPD on the secondary?

NEC 240.21 - 10ft tap rule

all seem to tell me that if there is a 1000AF on the primary, i need a disco on the secondary within 10ft?



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Joined: Nov 2007
Posts: 763
Well... I think you should probably be using Table 450.3[B], since the voltage is 480 and not over 600V.
Also, if the transformer primary conductors are fed directly from OCP bolted to the main bus of the switchgear, I don’t believe that would be considered a tap, so 240.21 most likely wouldn’t apply here.

The engineer probably had other manufacturers data available when laying out the design, but I would still keep a change-order handy, just in case the inspector has an issue with the design. wink

KJay #190465 11/19/09 05:38 AM
Joined: Oct 2000
Posts: 2,722
Broom Pusher and

Welcome to ECN!

First off, I would like to clarify a few things, per the EE's Design.

Does the Single Line show the 300 KVA Transformer having Over Current Protection for the Feeders only on the Primary Side Feeders, or Both Primary and Secondary Side Feeders?

Since the Separately Derived System ("SDS") is "Dual Voltage", the Primary Over Current Protection cannot be used to protect the Secondary side feeder Conductors.

Since the Secondary rating is 208Y/120V, there will need to be Over Current Protection for the Feeder Conductors on the Secondary side - as well as the Primary side.

Example setup + values for OCPD (Over Current Protection Device):

300KVA Transformer;
Primary rating = 360 Amps @ 480V 3 Phase 3 Wire.
Secondary rating: 830 Amps @ 208V (208Y/120V 3 Phase 4 Wire).

Maximum Primary side OCPD: 900 Amps (250% of 360 Amps) - refer to Table 450.3(B).
Primary Feeder Size: 500 MCM THHN Cu. or equivalent parallel conductors.

Maximum Secondary side OCPD: 1200 Amps (125% of 830 Amps, brought up to the next highest common OCPD rating).
Secondary Feeder Size: (3) sets of 500 MCM THHN Cu, or equal.

Basically, if the Single Line shows OCPD on the Primary Side only, the Design is not compliant, since the Secondary is Dual Voltage (208/120V).

If the Secondary was a Single Voltage - such as 240V 3 Phase 3 Wire, then the Primary OCPD could protect the Secondary Feeder Conductors, and the design would be compliant.

Since it appears that the Secondary is Dual Voltage, the Secondary Feeder Conductors require OCPD.

This may be done in Two different ways:
  1. 1200 Amp Feeders brought directly from Transformer, to a 1200 Amp OCPD (Circuit Breaker or Fusible Safety Switch).
    This option would relate to feeding a gear section from the Transformer, and the gear contains a 1200 Amp "Main Breaker" or "Main Fusible Switch".
    *** FYI, The OCPD does not have to be an intregral part of the gear - it may be separate equipment. The OCPD must be "In Front Of" (on Line Side of) the gear section.
  2. Separate feeders - say of 400 Amp capacity, brought out of Transformer, and run to individual Panelboards - each Panelboard contains a 400 Amp Main Breaker (or Fusible Safety Switch with 400 Amp Fuses).

Review the Single Line for this information, and let us know what's indicated.

As it stands, this issue requires an RFI submitted to the EE, requesting clarification for Primary & Secondary Conductors' OCPD.
Reference NEC Article 450 & Table 450.3(B) in the RFI document.

Good luck.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Nov 2007
Posts: 763
I was under the impression that this was an industrial high bay type installation, but that was an assumption on my part so, maybe not.

KJay #190495 11/20/09 11:48 PM
Joined: Apr 2002
Posts: 7,333
Likes: 7
Could you please explain the methodology for a MOCP of 1200 amp, on three (3) sets of 500KCmil, with 75 degree terminations. 500 KCmil Cu is 380 amps, which is compliant for 400 amp OCP. 2 sets would be compliant for 800 amp OCP, but 3 sets only total 1140 amps, and being >800 amps, a lower OCP is required, right?

Joined: Feb 2003
Posts: 939
Originally Posted by HotLine1
Could you please explain the methodology for a MOCP of 1200 amp, on three (3) sets of 500KCmil, with 75 degree terminations. 500 KCmil Cu is 380 amps, which is compliant for 400 amp OCP. 2 sets would be compliant for 800 amp OCP, but 3 sets only total 1140 amps, and being >800 amps, a lower OCP is required, right?

If I did recall the NEC code reguarding of any conductors using over 800 amp the conductor have to be sized to match or excessed of the OCPD and with 3 X 500KCM ( 240mm˛) I don't think it will meet the code with 75°C rating and the only way it can meet the code you have to upsize the conductors like 3 X 600KCM ( 300mm˛) then it will meet the code.


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Nov 2009
Posts: 2
matt_b Offline OP
New Member
thanx for the input, we got the response on the RFI, now, they are goint to consider the primary protected by the breaker feeding it because it is within 50 feet, and visible, if two sets of doors remain open.

the secondary will be protected by a 1200A fusible disconnect that is 69"H, 18"D, and 36"W. this will be big enough for our 3- 3" Conduits with 4-500KCmil and 2/0 ground.

thanx again for the input.


Joined: May 2005
Posts: 984
Likes: 1
The RFI response sounds like the designers investigated your question and found that they fouled up the design...and they're grasping at straws to avoid a big change order. Counting on 2 doors to be open so that you can consider something "in sight" as a real stretch that I wouldn't allow.

But; that being said, the RFI response has given you your marching orders and if it's later found to be wrong, it's THEIR liability.

Joined: Apr 2002
Posts: 7,333
Likes: 7
Yes, what you say is what I say. I just want to see if Scott has a methodology.

Joined: Jan 2005
Posts: 5,369
Likes: 1
Cat Servant
You know, one thing missing from the NEC is any consideration of AIC and the influence the impedance of the transformer has on it.

The first point is that the transformer will limit the maximum current the secondaries will ever see. This can have a secondary effect of slowing the response of an OCPD on the primary side.

The second is that the downstream side may be eligible for less expensive, lower AIC-rated equipment, and lessened risk from arc-flash events.

I am becoming persuaded that there should always, as a matter of design, be some manner of OCPD on the secondaries for these reasons. Maybe not a 'main;' but at lugged panel with branch circuit OCPD would be fine.

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