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#49547 - 03/09/05 06:24 AM Main Power Block sizing
rad74ss Offline
Member
Registered: 04/19/04
Posts: 156
Loc: Pryor, OK USA
I regularly work on HVAC systems that end up having large amp loads. I have a question on what you would expect if you were a customer.

Example:
Disconnecting means by customer.
MCA = 550 Amps
Should I ASSume the customer will run one three wire set of 1250KcMil from the 75 degree column (75 degree column required and stated on print), or should I ASSume that the customer will run two three wires sets of 300MCM?

Our largest stock power block can handle two 500MCM's per pole, and is a three pole block rated for 780 amps at 600V.

The big factor here is that if I figure on the customer using one run it will take a special order block and a huge bend radius that takes up a good portion of the panel.

If you were the customer and I did not specify what size of conductors the power block would accept what would you have run to this unit?

Should I always specify the maximum number and size of conductors the power block is capable of handling? Would you be okay with being restricted on what wire size you could run to the unit?

Am I going to irritate my boss by bringing this issue up at our next production meeting?

Actually I know the answer to the last one, but they can't afford to fire me. I just don't like leaving dangling issues on units that are this large and expensive. Plus a happy customer is usually a repeat customer.

Thanks in advance for your insights into this problem.
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#49548 - 03/09/05 08:35 AM Re: Main Power Block sizing
winnie Offline
Member
Registered: 09/15/03
Posts: 649
Loc: boston, ma
Why bother either assuming or bringing this up at a production meeting? The specification for what sort of conductors can be fitted to your standard equipment should be on the prints supplied with the equipment, and there should be no reason to discuss putting this specification in.

Besides, with your standard block, the customer could bring in two sets of 3 conductor 500MCM _in the same pipe_ and meet your ampacity requirements. (2 * 380A * 0.8 = 608A), so even if the customer prefers to use a single conduit run, they should probably be encouraged not to use 1250MCM.

-Jon
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#49549 - 03/09/05 09:03 AM Re: Main Power Block sizing
rad74ss Offline
Member
Registered: 04/19/04
Posts: 156
Loc: Pryor, OK USA
At this time we do not show any information on our power blocks on the prints. If this information is not asked for the customer will not know the maximum size available until they get the unit on site.

That is fine most of the time, but on other occasions the customer runs their power wiring before the equipment gets on site. If they have a run that exceeds the size of our power block it turns into who blames who and why wasn't that information conveyed to the proper people.

It also plays an important role in laying out the panels i.e. bend radius.

I am all for showing the maximums for the power block on the print, however I usually get a lot of flak whenever I suggest putting more information than standard on the prints.
My view is that if we specify the block information then the customer will not get a nasty surprise.
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