voltage drop question........

[Fulthrotl]

i gotta voltage drop question....

two 3/4" runs, x10's in them, two full houses and a ground
two conduits, for a total of 12 circuits....

now, connected load for calc purposes would be 80% of 20 amps, or sixteen amps....
so, my little voltage drop calc app tells me that a 208 volt 3ph 4w 75 degree application
will give me 2.98% voltage drop at 180 feet.

now, the problem is the run is 625' long
my little app tells me that to make the 3% code requirement for branch circuits, of the 5% total,
i'd need to pull #4's......

they are plugging in a PC at each location. that's it. but they want a quad outlet, and a dedicated 20 amp circuit for each
anyone got any insight on this... ? am i just screwed on this?

i'm looking at a subpanel as the only viable option at this point.... is there any magic derating fine print i'm missing?

thanks for any help....

 

[iwire]

You don't know the load, it sounds like the load will be in the neighborhood of 2 to 5 amps so why would you do the calc using 16 amps?

BTW, as far as the NEC there is no voltage drop requirement for the circuit you describe. I realize some areas do have VD requirements.

 

[Strife]

The voltage drop gets calculated on load, not the circuit ampacity.
By PC I assume you mean a personal computer, it doesn't make much sense why they want 2 dedicated circuits for each one, so you might want to go with the subpanel.

i gotta voltage drop question....

two 3/4" runs, x10's in them, two full houses and a ground
two conduits, for a total of 12 circuits....

now, connected load for calc purposes would be 80% of 20 amps, or sixteen amps....
so, my little voltage drop calc app tells me that a 208 volt 3ph 4w 75 degree application
will give me 2.98% voltage drop at 180 feet.

now, the problem is the run is 625' long
my little app tells me that to make the 3% code requirement for branch circuits, of the 5% total,
i'd need to pull #4's......

they are plugging in a PC at each location. that's it. but they want a quad outlet, and a dedicated 20 amp circuit for each
anyone got any insight on this... ? am i just screwed on this?

i'm looking at a subpanel as the only viable option at this point.... is there any magic derating fine print i'm missing?

thanks for any help....

 

[Fulthrotl]

You don't know the load, it sounds like the load will be in the neighborhood of 2 to 5 amps so why would you do the calc using 16 amps?

BTW, as far as the NEC there is no voltage drop requirement for the circuit you describe. I realize some areas do have VD requirements.

just got off the phone with my EE, and the short version is, it's a 20 amp circuit, so 80% of that is the maximum
connected load per NEC, which comes to 16 amps. and with this being a warehouse, safe to assume people will
be plugging in bun warmers to keep their feet warm....

so the voltage drop allowed in this part of the country is 5% total, that is feeder and branch circuits, and you are permitted
to use 2% for feeders and 3% for branch circuits is customary, but you can do it 3% feeders and 2% branch circuits
as long as you don't go over 5% total voltage drop.

time to contact customer and look for value engineering solution... they need twelve dedicated 20 amp outlets? not when they
find out what it's gonna cost, i bet.

thank god that the wifi repeater plugs are 20' up in the air, so they can be calculated at actual connected load of the device.
the run for those is 1,100 feet.... :-/

a 60 amp panel to feed those 12 outlets will have to be four #2/0 copper.... it's not my fault they have a big warehouse.
800k sq.ft.

 

[iwire]

Ah, the max load on the circuit per the NEC is 20 amps not 16 so if you insist on using the max load you are really screwed.


You could just as easily and correctly use the 180va from article 220 for this as the actual load is unknown

 

[ramsy]

I'd be more concerned with overheated equipment, according to 110.14(C).
Load side of sub panel, when voltage drop is no longer an issue, 20A branch wire exceeds 60?c at 15A, if using #12. Temperature calcs & derating tables for 8 ccc's in conduit show a high-temperature design with no room for derating.

 

[Fulthrotl]

Ah, the max load on the circuit per the NEC is 20 amps not 16 so if you insist on using the max load you are really screwed.


You could just as easily and correctly use the 180va from article 220 for this as the actual load is unknown

that's what i asked my electrical engineer, and he said 80% which is the 16 amps.....
now, the solution that seems to look best, is to take an existing panel fed by a 25
kva xfmr, feed that panel as a subpanel off the 225 amp panel i'm putting in that
area, take the 25 kva, move it to the distant area, and set a 60 amp subpanel.
that'll take three #4's to feed it the 700' it has to go, providing 30 amps primary with
under 3% voltage drop.

that way, in the winter when there are 7 bun warmers plugged into those outlets
in cubicles, cause the women doing data entry are cold, we shall have more than
70 volts available to run the computers. talked to the customer, explained the
issue and the reasoning, and he agreed.

 

[ramsy]

take the 25 kva, move it to the distant area, and set a 60 amp subpanel. that'll take three #4's to feed it the 700' it has to go, providing 30 amps primary with under 3% voltage drop.

Ask your engineer if 40A works for those feeders. If that 25-kva xfmr is 480/240 1? #4cu seems good for 80% of xfmr nameplate / 80A sub panel.

If that 25-kva xfmr has taps to set +2.5% then 40A on #6, sharing a conduit w/6 ccc's, still delivers < 3% VD < 60?c to 80A subpanel.

Engineers may not design to max thermal limits, but setting OCP's to trip there should be fine. Especially if it works with the Knucklehead-Electrician Canon (NEC) 110.14(C)(1)