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Old 03-04-2016, 10:30 AM   #1
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Default Calcul maximum electric panel size

Hi guys,

I got a electric panel (Siemens EQ424225) 225amp 120/240vac 3ph 4wire 24/48 circuit. which is about 75% full of 15 amp breaker single pole. That panel is powered by a dry transformer 15kva 600vac to 208vac 3 ph.

I want to add a air conditioner (208v 3ph 19amp total load sayin on the nameplate) and was wondering if this panel could take it ? i got 3 slot left but i was wondering if that transformer was delivery enough power to take it ?

Other than taking a amp reading is there any way to calcul this ?


So if im right....

My transformer:
which is 15kva at the output 208vac (15000va / (208*1.732) = 41.6 amp...


Total load of my panel:
(15 circuits) times (15 amps) times (80%) times (120 volts) = 21,600 VA
Total load = 21,600 VA
((21,600VA / 208) / 1.732) = 60 amps

So even if im right o my estimation without mesurement that my panel take only 60amp on 225amp total.

The part i dont understand, how a 15kva transformer can feed this 21.6kva panel ?? there most be a part i didnt calcul because i see alot of big panel connected to a 15kva which seem to be fine.
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Old 03-04-2016, 11:53 AM   #2
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I did my calculation again and it seem like this transformer is way too small for this size of panel no? we are prety lucky that the actual load isnt really at 80% seem like i got way too much load for the size of this transformer......
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Old 03-04-2016, 11:57 AM   #3
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I would think the CEC has info on calculating loads. You don't just add up the breakers because you may have a 15 amp breaker with 3 amps max on it. IDK.... You have to calculate the exact loads

In the nec for a commercial job each receptacle is rated 180va for calc purpose. If there are specific loads then you have to add them in and then there may be a demand factor
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Old 03-04-2016, 01:28 PM   #4
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Take a look at Section 8, Circuit Loading and demand factors. It's pretty close to a design manual for this kind of thing.
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Old 03-04-2016, 02:00 PM   #5
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The otherr issuse will arise is the transformer imo it on border line especaly if you have good load on that panel and A/C start up it may cause the lights momonentary dimmed when it kick on..
If 25 KVA yuh it will be fine..

Double check the load as other mention to make sure..
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Old 03-04-2016, 03:29 PM   #6
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Quote:
Originally Posted by frenchelectrican View Post
The otherr issuse will arise is the transformer imo it on border line especaly if you have good load on that panel and A/C start up it may cause the lights momonentary dimmed when it kick on..
If 25 KVA yuh it will be fine..

Double check the load as other mention to make sure..
good observation... the server of the company are all in this panel i dont want to mess with any lack of power when my A/C start
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Old 03-04-2016, 03:52 PM   #7
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Quote:
Originally Posted by martindag View Post
good observation... the server of the company are all in this panel i dont want to mess with any lack of power when my A/C start
I think it better off is get other transformer for A/C load plus whatelse they will throw ( additional circuits )..

That way the servers do not get crashed and reboot it..
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Old 03-04-2016, 04:13 PM   #8
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As a side note: Maybe they should consider a UPS for those servers?
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Old 03-04-2016, 11:14 PM   #9
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I could almost replace it for a 30kva with very minor change to the circuit, even my wire is all fine but this panel is on the emergency circuit on our generator so i need to make sure first i got also enough power left from it to supply my new transformer.
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Old 03-04-2016, 11:15 PM   #10
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yeah already asked for a UPS ... and they would rather buy a new generator than buying a UPS for this unit.

But thanks ! i was thinking the same actualy ... why not just a UPS anyway it's only 1 unit... oh well !
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Old 03-05-2016, 11:09 PM   #11
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How are they going to power the server until the Gen set comes up? I would opt for a separate transformer for the motor loads, leave the computer stuff separate.
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Old 03-06-2016, 08:01 AM   #12
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hmm yeah good idea ! didn't think about this option, this way i could keep the small transformer and just buy a new one to supply my A/C. should be alot faster also to add on the circuit and reduce the downtime.
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Old 03-06-2016, 06:23 PM   #13
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NEMA dry-type transformers can go into 'over load' for hours on end and still come out sweet.

They are THAT conservatively rated.

Go to the manufacturer to find out how much loading the transformer can take.

Mine can go three-hours -- easy -- at 125% of nameplate.

( MGM Transformers -- the Big Boys in Los Angeles -- they dominate the California market.
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