12 sets of 400MCM

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Dennis Alwon

Dennis Alwon

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Retired Electrical Contractor
I just got a call from a friend and past employee of mine who is working on a very large installation. He is running 12 sets of 400MCM. The job is engineered and the specs called for a 3/0 GEC to the building steel and water pipe.

The inspectors are interpreting 250.66 to mean that 3/0 is the largest GEC if you are using one conductors. They are saying you must use the notes for multiple sets. :confused: This is true but what happens when you get beyond the Tables--

I don't think this is correct for one because I can't figure out how you can calculate it.-- 12 x 400000= 4,800,000cm or 4,800MCM. Then what? :D

Is 3/0 the largest GEC you will ever need?
 
Dennis Alwon said:
I don't think this is correct for one because I can't figure out how you can calculate it.-- 12 x 400000= 4,800,000cm or 4,800MCM. Then what? :D
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Then your 1100 kmil so use a 3/0.
 
chris kennedy said:
Then your 1100 kmil so use a 3/0.
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That's what I said but the inspectors thinks not but he did not say what to use.

Is your take that 3/0 is the largest necessary in any situation.
 
Dennis Alwon said:
That's what I said but the inspectors thinks not but he did not say what to use.

Is your take that 3/0 is the largest necessary in any situation.
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for a grounding electrode connductor, yes
for a bonding jumper on the line side of service, no
 
Sure would like to see the equipment you could land 12 sets of 400 easier then 6 or 8 sets of larger conductors.

BTW the inspectors are wrong.

Finding a way to prove that is not as easy but I would start with the heading of Table 250.66

'Size of Largest Ungrounded Service Entrance conductor or equivalent area of parallel conductors'
 
The max sixe on the gec is 3/0 as previously stated.

The main bonding jumper is based on the total area of all the ungrounded phase conductors multiplied by 12.5 percent or 600 kcmil. I am sure the gear has a busbar type bonding jumper anyhow. Remind him to "meg" out his wires before he terminates.

I am curious about one thing, can the poco's transformer handle 12 sets? I thought the limit was 10, but I have known to be wrong.


Joe Villani
 
Joe Villani said:
I am curious about one thing, can the poco's transformer handle 12 sets? I thought the limit was 10, but I have known to be wrong.


Joe Villani
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They probably just don't connect two of them :D
 
augie47 said:
They probably just don't connect two of them :D
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Actually you double them up with offset compression lugs and use both sides of the buss stab. 12 sets of conductors terminated with two-hole compression lugs use 6 holes. The same can be done at the switchgear.
 
Smart $ said:
Actually you double them up with offset compression lugs and use both sides of the buss stab. 12 sets of conductors terminated with two-hole compression lugs use 6 holes. The same can be done at the switchgear.
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Boy glad you explained that. :grin:

Now explain it to the power companies that limit us to 10 sets. :D
 
iwire said:
...Now explain it to the power companies that limit us to 10 sets. :D
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I've not run into that limitation. Besides, I'm sure you're completely capable of passing along the info...:grin:
 
Okay-- I talked with my friend and he says that the PODco has okayed the install of the 12 conductors because of the unusual circumstances. Apparently the pulls are very long and larger conductors would be difficult to pull-- Just relaying what was told to me..

Now I need help on this. I see no problem with the 3/0 to the water pipes but what about the building steel that is just bolted to the concrete foundation. The steel needs to be bonded so would that have to be sized at 600MCM??

BTW, Joe Villani I believe the fellow that is doing this work knows you. His name is Jacques Menache-- says he knows you thru the music world. Is that right?
 
Dennis,

Sorry, not me:smile:

The 600 kcmil is for the main bonding jumper, used to connect the grounding bar to the neutral bar at the service disconnect. I just finished a 2500 amp service and the main bonding jumper was a bus bar that was bolted between the neutrals and grounds, it had a big sticker on it visible for inspection. As for the grounding electrode system, bond from the gear to the steel with the 3/0, then in the area where the water enters the building bond from the steel to the water pipe(s). In these building there are usually multiple water pipes that qualify as a Grounding Electrode, so I will put a bus bar somewhere in the room where the water comes in and go from the building steel to the bus bar, then to the water pipes.

Joe Villani
 
Dennis Alwon said:
Now I need help on this. I see no problem with the 3/0 to the water pipes but what about the building steel that is just bolted to the concrete foundation. The steel needs to be bonded so would that have to be sized at 600MCM??
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Dennis .,

If my memory serve me right on this one you have to get all the phase conductors total sum then multi by 12.5% which the result you got the 600KCM size which that is the bonding conductor size and i did recall it was stated in the NEC code [ not sure if in Art. 250 area may ring the bell in that spot ]

Merci,Marc
 
frenchelectrican said:
Dennis .,

If my memory serve me right on this one you have to get all the phase conductors total sum then multi by 12.5% which the result you got the 600KCM size which that is the bonding conductor size and i did recall it was stated in the NEC code [ not sure if in Art. 250 area may ring the bell in that spot ]

Merci,Marc
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Thanks Marc --That is section 250.28- Main Bonding Jumper & System Bonding Jumper
 
Jacques sends his deepest thanks. He said he helped him sleep better thru the weekend.

Inspector called first thing this morning and apologized for being wrong. Thanks again.
 
NEC 250.28 does not apply to bonding the building steel. It applies to MBJs and SBJs.

NEC 250.104(C) is going to cover the bonding connection to the building steel and it points directly to 250.66 which says nothing about 12.5%.
 
crossman said:
NEC 250.28 does not apply to bonding the building steel. It applies to MBJs and SBJs.

NEC 250.104(C) is going to cover the bonding connection to the building steel and it points directly to 250.66 which says nothing about 12.5%.
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I agree the wire to the building steel is 3/0 max but if you needed a bonding jumper it would need to be 600KCM. In this particular case there was no bonding jumper needed.
 
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