# Correct generator size

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• #10331
Keymaster

Supposing I have 3phase power installation, 80A/phase, with 420V/phase as well, what is the calculation for generator to be installed as standby?

#11724
erickench
Participant

Since you already know the ratings of the generator I would refer you to article 445 of the National Electrical Code. Section 445.13 contains the rules for sizing the conductors. 80A X 1.15 = 92 A. If you look at NEC Table 310.16 the conductor size would be No. 3 AWG at 75'C.

#11713
Keymaster

Puka said:

Supposing I have 3phase power installation, 80A/phase, with 420V/phase as well, what is the calculation for generator to be installed as standby?

Dear Puka :

This need a complete calculation but as a guide you must consider the followings :

1-KVA/KW:       P=√3xVxIxcosφ =√3x420x80x0.8≈46000=46 KW , S=P/cosφ=58 KVA

2- as ISO3046 for standby load amount must be about 75% then you need 58/0.75 =77 KVA

3-above is valid only when you have resistive load if you have electromotor its related to :

– biggest motor

-starting to rated current ratio of it

-power factor of it

-permissible voltage deep(ΔU)

-direct on line or star-delta connection of it

-……

as a guide for starting of 1kw electromotor you need 3 to 5 kw og gen if ΔU=20%.

e.g. if you have 2×15 kw motor and if you can start them one by one you need about 90 kw but if you want start them together you must have 150 kw gen.

with best regards

#11579
jatin333
Participant

Puka said:

Supposing I have 3phase power installation, 80A/phase, with 420V/phase as well, what is the calculation for generator to be installed as standby?