Transformer GEC size

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hhsting

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I have 75 kva transformer 480/277V to 208/120V three phase. Secondary side has two panelboards. One set of secondary conductor #3/0 AWG goes to 200A MCB panelboard from the transformer, second set of secondary conductor #2 AWG goes to 100A MCB from the transformer. Both panels within 10 feet of each other.

Question:

1. Transformer grounding electrode conductor size should be based on #3/0 or #2 AWG or combination sum of #3/0 and #2 AWG?

2. Secondary breaker size how does code go about checking this?

3. Supply side bonding jumper #3/0 AWG would be based on #3/0 AWG or sum of 3/0 and #2 AWG?
 
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Take a look at 250.66 that table says "Size of Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors (AWG/kcmil)"
 
hhsting said:
1. Transformer grounding electrode conductor size should be based on #3/0 or #2 AWG or combination sum of #3/0 and #2 AWG?
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The sum of both.

hhsting said:
2. Secondary breaker size how does code go about checking this?
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See 450.3(B)

hhsting said:
3. Supply side bonding jumper #3/0 AWG would be based on #3/0 AWG or sum of 3/0 and #2 AWG?
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Supply side bonding jumper would be based on the #3/0 for the 200A feeder and the #2 for the 100A feeder.
 
david luchini said:
The sum of both.


See 450.3(B)


Supply side bonding jumper would be based on the #3/0 for the 200A feeder and the #2 for the 100A feeder.
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1. For the GEC size where does it say should be sum of both for transformer NEC 2014? Table 250.66 note #1 is about service conductors not transformer GEC?

2. For the breaker sum of the two panelboard breakers should not be less than stated in 450? If not then how does normally one check?
 
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T250.66 says "Size of Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors (AWG/kcmil)", so why would you add them?
Sorry I had the incorrect code reference in an earlier post that I will change.
 
infinity said:
T250.66 says "Size of Largest Ungrounded Conductor or Equivalent Area for Parallel Conductors (AWG/kcmil)", so why would you add them?
Sorry I had the incorrect code reference in an earlier post that I will change.
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But panelboards do not have parallel secondary conductors.

Secondary Panelboard A for instance has secondary conductor size A from transformer X1

Secondary Panelboard B separate location has secondary conductor size B from the same transformer X1

Conductors A and B are not parallel.
 
hhsting said:
But panelboards do not have parallel secondary conductors.

Secondary Panelboard A for instance has secondary conductor size A from transformer X1

Secondary Panelboard B separate location has secondary conductor size B from the same transformer X1

Conductors A and B are not parallel.
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Yes that was my point. I was asking David why he felt that the GEC was based on the sum of the two when they're not in parallel.
 
Attached is sketch showing configuration
regarding sizing of transformer GEC question


a2a598a10c784aca0e0c3a480cee3671.jpg
 
If the GEC were sized according to the largest ungrounded conductor, in this case #3/0 as it says at the top of T250.66, what benefit would there be if added the size of the other conductors (#2) to it? It's already large enough for the #3/0 so it would certainly be large enough for the #2's.
 
infinity said:
If the GEC were sized according to the largest ungrounded conductor, in this case #3/0 as it says at the top of T250.66, what benefit would there be if added the size of the other conductors (#2) to it? It's already large enough for the #3/0 so it would certainly be large enough for the #2's.
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Top of NEC 2014 Table 250.66 says:

“Size of largest ungrounded SERVICE entrance conductor or Equivalent area for parallel conductors”

I don’t think transformer secondary conductors are service entrance conductors. The conductors are not in parallel.

How did you figure we can use top of Table 250.66?
 
hhsting said:
Top of NEC 2014 Table 250.66 says:

“Size of largest ungrounded SERVICE entrance conductor or Equivalent area for parallel conductors”

I don’t think transformer secondary conductors are service entrance conductors. The conductors are not in parallel.

How did you figure we can use top of Table 250.66?
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Because 250.30(A)(5) tells you to use 250.66 to size the grounding electrode conductor for a separately derived system.

(5) Grounding Electrode Conductor, Single Separately Derived System. A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived ungrounded conductors. It shall be used to connect the grounded conductor of the derived system to the grounding electrode in accordance with 250.30(A)(4) , or as permitted in 250.68(C)(1) and (2). This connection shall be made at the same point on the separately derived system where the system bonding jumper is connected.
 
packersparky said:
Because 250.30(A)(5) tells you to use 250.66 to size the grounding electrode conductor for a separately derived system.

(5) Grounding Electrode Conductor, Single Separately Derived System. A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived ungrounded conductors. It shall be used to connect the grounded conductor of the derived system to the grounding electrode in accordance with 250.30(A)(4) , or as permitted in 250.68(C)(1) and (2). This connection shall be made at the same point on the separately derived system where the system bonding jumper is connected.
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So the question was how size GEC in case of post #9 attachment based on #3/0, #2?

Seems to be two different approaches but not sure what code intent: first largest ungrounded of two which is #3/0 or second Sum combination of area #3/0 plus #2 then use table 250.66.
 
hhsting said:
So the question was how size GEC in case of post #9 attachment based on #3/0, #2?

Seems to be two different approaches but not sure what code intent: first largest ungrounded of two which is #3/0 or second Sum combination of area #3/0 plus #2 then use table 250.66.
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What code language tells you to use both conductor sizes added together? You're told to use T250.66 for the GEC and it clearly states to use the largest conductor in Note #2.
 
infinity said:
What code language tells you to use both conductor sizes added together? You're told to use T250.66 for the GEC and it clearly states to use the largest conductor in Note #2.
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Well code section David quoted post #5 NEC 2014 Section 250.30(A)(5) says:

“A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived ungrounded CONDUCTORS...”

Note says derived ungrounded conductors (plural) does NOT say largest derived ungrounded conductor.
 
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hhsting said:
Well code section David quoted post #5 NEC 2014 Section 250.30(A)(5) says:

“A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived ungrounded CONDUCTORS...”

Note says derived ungrounded conductors (plural) does NOT say largest derived ungrounded conductor.
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Correct. It does say (emphasis yours) “A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived ungrounded CONDUCTORS...” 250.66 gives further instructions. The instructions say to size the GEC in accordance with the table. Also see 250.66

250.66 Size of Alternating-Current Grounding Electrode Conductor. The size of the grounding electrode conductor at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at a separately derived system of a grounded or ungrounded ac system shall not be less than given in Table 250.66,
 
packersparky said:
Correct. It does say (emphasis yours) “A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived ungrounded CONDUCTORS...” 250.66 gives further instructions. The instructions say to size the GEC in accordance with the table. Also see 250.66

250.66 Size of Alternating-Current Grounding Electrode Conductor. The size of the grounding electrode conductor at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at a separately derived system of a grounded or ungrounded ac system shall not be less than given in Table 250.66,
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In that event why wouldn’t Table 250.66 note #1 apply to transformer grounding electrode conductor sizing as sum?
 
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