Help in Calculation 1

[kingbambina]

I have a 400V, 10kVA UPS that will be fed from a 480V panel through 15kVA, 480/400V delta/wye transformer. I just want to confirm my calculations. I will take the power from 25A breaker with a 3C 10mm² power cable to the transformer. From the transfomer I will run 3C 10mm² + 4mm² ground to 25A disconnect switch and lastly to the UPS. Is this good?
Thanks

 

[LionelHutz]

25A is a little light for feeding a 15kVA transformer at 400V since it should only be loaded to 80%. For wire sizing follow your local electrical codes.

 

[che12345]

Dear Mr. kingbambina
1. From the voltage (400V) and the wire size (xx mm²), it seems to be in the IEC world,
2. Please tell us to complied with which standard/regulation/Code/location e.g. IEC (International), BS (Great Britten only), NEC (US), CSA (Canada).... etc. Any local regulation/national regulation/Code that is applicable at the location of the installation,
3. I raised this request because the requirements differ.
Che Kuan Yau (Singapore)

 

[kingbambina]

Dear che12345,

We're following IEC.

 

[7anoter4]

In my opinion, you are in USA and have to supply IEC standard equipment -the source it is 480 V as standard in U.S.A. and the UPS supplies 400 V. We don't know the frequency-60 Hz in USA and 50 in Europe. We don't know the length of cables. We don't know what is the actual load on the transformer 480/400- if except the UPS you need 400 for other loads and if the cable run in conduit or other.
I think the cable 10 mm^2 it is a PVC insulated [ 70 oC rated temperature] copper conductors, 4 conductors [3 lives and a neutral].For grounding you have another 10 mm^2 copper -insulated or bared.
The UPS rated current has to be 14.5 A
According to BS7671/2008 Table 4D2A for multicore 70 oC thermoplastic insulated and thermoplastic sheathed non-armoured cables (COPPER CONDUCTORS) one cable in conduit the ampacity [current carrying capacity] will be 39 A.
From table 4D2B the impedance [in mV/A/m] is 3.8 [0.0038 ohm/m] This impedance includes the factor √3 for three-phase system.
If the permissible voltage drop will be 3% [400*3%=12 V] then the maximum length will be 12/0.0038/14.5=217.7 m

 

[che12345]

Dear Mr kingbambina
@ Mr. 7anoter4
1. Thank you for your learned advice. BS 7671 complied with IEC. Therefore, it is applicable in the IEC world.
..... " .....I think the cable 10 mm^2 it is a PVC insulated [ 70 oC rated temperature] copper conductors, 4 conductors [3 lives and a neutral].For grounding you have another 10 mm^2 copper -insulated or bared....The UPS rated current has to be 145 A...". are in order.
2. Mr.kingbambina FYI: Per BS 7671/IEC [earthing conductor size S_p] = [phase conductor S =< 16mm²]. That is to say that the earthing conductor shall be 10mm². Your proposal earthing conductor of 4mm² is NOT acceptable.
Che Kuan Yau (Singapore)

 

[LionelHutz]

Oops, 25A is fine for a 15kVA transformer being fed from 480V.

 

[cherryg]

Considering the Ampacities as given in IEC 60364-5-52, and the derating factors given in this std, you are still good with a 4 sq.mm cable, based on ampacity. Method D - laid in ducts, Method D2 - laid in ground and Method E - laid in tray give ampacities 35A,39A and 42A reply. You may take 6 sq.mm to keep it safe. I do not know the specific laying conditions to eomploy the derating factors.