# Calculate KVA Transformer



## C_951 (Nov 4, 2015)

Hello, i'm a little confused about how to calculate the amount of KVA i need for 2 20 amp 120 circuits, here's the scenario. 

Location A has 208v single Phase on 30 Amp Breaker, i need 2 20 amp 120v circuits approximately 600 feet away at Location B. The idea is the transform the 208 to 480 and run the 600 foot length then transform back down from 480 to 240/120, the problem is do i have to use the SQRT 3 in my calculation since this is deriving from a 208 circuit? I used the single phase formula and came up with 5 KVA but using the SQRT 3 would give me roughly 8kVA. I deal with mostly instrumentation and its been a long time since i had to calculate a load .


----------



## electricalwiz (Mar 12, 2011)

what is the actual load of the those two 120 volt circuits
Does that 208 volt load have a neutral?


----------



## telsa (May 22, 2015)

Why perform any calculations at all ?

Open up Ugly's to page 64 where all of the full load currents for NEMA transformers are laid bare in a table.

Right there it spells out that a 5kVA single-phase NEMA transformer is designed to pass 41.7 Amps at 120 VAC, more than enough for you, as you will never load this puppy all the way up. 

A more studied effort might prove that even 3 kVA is plenty if your loads are trivial. 

A 480/120 single phase transformer is common stuff, a perfect match for you. 

All NEMA designs can go into (mild) overload for three-hours. Just how robust a unit is will be spelled out in its cut sheet.

So these are your step-up & step-down transformers, the 'twins.'

Replace the existing breaker with a single-pole 40 A C/B.

This 480 V run needs 1 Hot 1 Return 1 Grounding conductor. 

( Brown, Grey, Green: customary -- but Black, White, Green are Code acceptable)

You must absolutely label the voltage (480 VAC ) where present so that those coming after realize just how high the voltage is, and that it's not 277 V over ground potential.

Pipe and wire, please. 

At the step-down end you will have to set a Ufer// grounding rod and have your dinky panel close by the transformer -- as OCPD. The two C/Bs will be as adequate as a Main breaker and need not even be inter-tied.

Other schemes could work -- they'd just get even more complicated.

I leave the wire sizing up to you.


----------



## RePhase277 (Feb 5, 2008)

No, since it is a single phase 208 volt circuit, you don't need the square root of 3. 5 kVA is good for the load. Don't forget a fuse in the 480 volt line as well.


----------



## hardworkingstiff (Jan 22, 2007)

Depending on the loads, it might be more cost effective to install some larger conductors (#4?) instead of a couple of transformers and the additional grounding.


----------



## oliquir (Jan 13, 2011)

i would check cost of big al conductors compared to the xformer setup


----------



## 37523 (Dec 30, 2012)

You also have to ask yerself: is 2* 20A 120V really what he wants? If "B" is a 6'x8' chicken shack, or a plug for a portable saw, maybe. If a 5HP compressor or big buzz-welder would look good at "B", I'd be thinking 10KVA from the start (or a pledge that 4.8KVA is really all he needs for a long time). 

> _transform the 208 to 480 and run the 600 foot length then transform back down from 480 to 240/120_

The supply is single-phase. The load is 4.8KVA. 

Even though the 208V is probably derived from a 3-phase service, the root-3 calculation does not apply in your part of the job. Since the 208 is surely from a much bigger service, a 5KVA load is not enough 3-phase unbalance to worry about. 

> _transform ... and ... transform_

I too question the double transformer against the cost of fat conductors. 

I have 240 come in on 500 feet of #2 Al wire. Voltage drop happens, but I am running a whole house! A 35A well-pump peak dims the lights, but the PC and VCR never reboot. We hardly notice the sag from a 20A dryer. 

There's thousands of dollars here any way you do it. You maybe should find a nearby expert and pay him to define the problem and compare alternate solutions against your catalogs and labor costs. 

However my rough-guess below shows no great difference: what you save on the wire you spend on the transformers, and vice-versa. 

This long line WILL sag. 

Adding transformers reduces line sag but each 5KVA lump adds 1%-2% sag. 

If you must work to under 2% total sag, two transformers blows the deal right there. 

10KVA iron working at 5KVA will sag more like 1% each (2% for 2), but almost double the cost. 

OTOH if you can tolerate 10% sag (fine for a chicken-coop with heat and plucker), you can skimp a lot of dough. 

Double the sag means half the copper, and nearly half the cost! 

First figure your *real* allowable voltage drop. The "2% rule" is for "no" perceptible lamp-dim when the vacuum-cleaner (or well-pump) starts-up. I tell you that 15% dips are very livable when you are far from the source and have to pay for the wire. That's also 15% added to the electric bill, which is significant for large always-on loads, negligible for most small or short-time loads. (Anyway you won't will see the electric bill, you just broker the cost of the wiring.) 

If the customer will complain about lamp-dim, or runs several KW day and night, work to 5% and tell them that a no-dim low-loss job will cost twice as much. If they won't complain, aim toward 10% drop.

You do need a transformer somewhere, to turn 208 to 120V. (Maybe...) 

Except! I assume this 208V has 120V-to-N legs. If you run the line at 120V, no transformers needed! And no transformer sag. But the wire is fat. Running 208/120V 3-wire cuts costs some more. 

If you run the 208 the 600 feet, line current is 23 Amps and any mechanically sound wire will carry that. 

If you step-up to 480V, line is only 10A and #16 will carry that, but for mechanical reasons we never go below #14 and better #12, so you "must over-buy". 

Obvious alternates: 
208V/480V --- 10A 2-wire(?) --- 480V/[email protected]
208V ---- 23A 2-wire(??) ---- 208V/[email protected]
208V/240V --- 20A 3-wire ---- [email protected]
208V/120V --- 40A 2-wire ---- [email protected]

Wire resistance and gauges for 4.8KVA with X% sag over 600 feet (1200 both ways) 
("240V sag" computed on 120V side) 
480V line -- 5% = 2.5r = #13 copper -- 10% = 5.3r = #16 copper
208V line -- 5% = 0.48r = #5Cu/#2Al -- 10% = 1.0r = #9Cu/#6Al
240V line -- 5% = 0.32r = #4Cu/#2Al -- 10% = 0.7r = #7Cu/#4Al
120V line -- 5% = 0.16r = #1Cu/00Al -- 10% = 0.33r= #4Cu/#2Al
(Typos ARE probable!)
(You can't buy #13 etc, so next-up gauge will be a little less sag.) 

Very-very-VERY rough costs for 10% sag: 
480V line: trans=$2000 wire=$300 ====$2300
208V line: trans=$1000 wire=$1000 === $2000
208/120V line: trans=$0 wire=$1000 === $1000
240V/120 line: trans=$1000 wire=$1000 === $2000
120V line: trans=$0 wire=$2000 === $2000

I did not really sharp-pencil these: number of conductors including ground. I've never run line-line 208 and am not sure how many conductors makes it safe/legal. I assume trenching is the same, though conduit will vary from 1/2" to 2" which adds much cost to the low-volt plans. The several options need different fusings and I did not figure that. Invoice prices will be higher. 

But you can only buy transformers "each", while wire comes in small-step gauges, so avoiding even one transformer can justify burying somewhat fatter wire. 

The 480V line "could" be run as plowed-in #12/3 UF (scares me to death).

The 120V 40A line has the advantage of simplicity, but you are burying a LOT of Aluminum. Around here, it might get stolen. 

The 240v line seems obvious but has problems. Loads are 120V and sure to be unbalanced. That means each _side_ of the 240/120V split has to not-sag at 120V. (Because of sag, the 240V line must be sized so it can run a 30+A breaker, so the 20A breakers at "B" will trip first.) 

208/120V 3-wire needs the same as 240/120 3-wire. 

I'd do a real work-up on the 208/120V 3-wire option. It needs 3/G #2Al(5%) to #4Al(10%) but meets spec and avoids transformers.


----------

