Feeders and Voltage Drop

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DMG_1

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Boston, MA, USA
Hi All.

I have a question regarding voltage drop. A 120/208V, 400A, 3-Phase/4-Wire panel is installed 600' from the source. The feeder to the panel is sized at 400 Amps. If the panel is loaded to 80% capacity I would expect that the voltage drop on the feeder would be <= 3%. If the voltage drop on the feeder was greater than 3%, could it be said that this is a design flaw?


Thanks.
 
By my calculations you would have 8% voltage drop with #500 MCM copper and 7.5% voltage drop with #600 MCM copper, with a load of 320 amps. Would I call that a "design flaw"? Yes. Does it violate the Code? No. Well, not unless your AHJ is like the City of Los Angeles and interprets the fine print notes as being part of the language of the Code (which they're not). It always makes me angry when I come across a setup like this, though -- here's a perfectly good 400 amp panel with only 125 amps connected to it, and I can't add any more load to it because of voltage drop. :rant:

The only caveat I would add is that you didn't mention the size of the overcurrent protection. If it's a 400 amp panel with a 150 amp MCB (or fed from a 150 amp MCB), then I have no problem with it. I've seen this before, when long distances force you to use VERY oversized conductors to avoid excessive voltage drop. If nobody makes a 225 amp panel that will accept #600 MCMs, then you don't have much choice but to use a 400 amp panel.
 
fyi...

fyi...

If the enforcing authority or later on a failure study confirms that you designed a system without
accounting for voltage drop the Energy Code, ASRAE or ICC can be cited as an enforceable requirement
for voltage drop...(or so I have been led to believe)
 
JDBrown said:
.... If nobody makes a 225 amp panel that will accept #600 MCMs, then you don't have much choice but to use a 400 amp panel.
Click to expand...

Actually, you DO have a choice: One could install a j-box or wireway ahead of the panel, and splice the 600mcm to a 250mcm, and that would fit nicely into a 225 or 200 Amp main ...
 
kbsparky said:
Actually, you DO have a choice: One could install a j-box or wireway ahead of the panel, and splice the 600mcm to a 250mcm, and that would fit nicely into a 225 or 200 Amp main ...
Click to expand...

True. Sometimes i forget not everyone "gets" to deal with the same government agencies I usually work with. They tend to prohibit a lot of little things like this that are allowed by Code and would save the customer some $$. They also do nice things like requiring ALL panelboards with 120V to ground to have a 200% rated neutral bus. But I digress...
 
2x12x600x320/208x3%= 4608000/7= 658,286 Cm Energy code requires max 2% for feeders = 1,107,692 Cm. No matter which school of thought you use doing it properly will lead you to parallel runs. 320 Amps @ 3 phase leads me to think this is a fairly large structure. Is this a what if question?
 
IMHO, even 3% drop in a feeder may be too much to be a good design, though not specificly prohibited.
I would consider that in most cases there will be voltage drop between the service and the start of the feeder, and also that branch circuits supplied from the feeder will also have voltage drop.
The combination of these voltage drops could result in undesirably low voltage at the load.

Of course if the feeder was supplied directly from the service equipment, and the branch circuits were short it would probably be fine.
 
Design flaw depends on what design specs were.

If spec was 3% and you have more than 3% then there is a design flaw.

3% in the informational note in NEC is just that - an informational note or suggestion, not a design requirement.

If the supplied equipment can withstand 10% voltage drop with no significant performance issues, then 10% could very well be considered acceptable.
 
Wouldn't you need to protect the conductors per 240.6 -- 350 Amp overcurrent device for the design of 320 amp rating of the conductors.
 
JDBrown said:
By my calculations you would have 8% voltage drop with #500 MCM copper and 7.5% voltage drop with #600 MCM copper, with a load of 320 amps. Would I call that a "design flaw"? Yes. Does it violate the Code? No. Well, not unless your AHJ is like the City of Los Angeles and interprets the fine print notes as being part of the language of the Code (which they're not). It always makes me angry when I come across a setup like this, though -- here's a perfectly good 400 amp panel with only 125 amps connected to it, and I can't add any more load to it because of voltage drop. :rant:

The only caveat I would add is that you didn't mention the size of the overcurrent protection. If it's a 400 amp panel with a 150 amp MCB (or fed from a 150 amp MCB), then I have no problem with it. I've seen this before, when long distances force you to use VERY oversized conductors to avoid excessive voltage drop. If nobody makes a 225 amp panel that will accept #600 MCMs, then you don't have much choice but to use a 400 amp panel.
Click to expand...

Or you live in Florida where it is written in to the Florida Building Code that feeder drop si 2% max and branch drop is 3% max.
 
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