Amp Capacity of Electric Service

You do not make any adjustments. Add the load up in kw. convert the load to amps using kw/(.225 volts x 1.73)

If I remember my EE correctly, 225 Delta is 225 RMS hot to hot, 115 Hot to neutral. Each pairing of hot to hot will carry a given current (amps) set by the transformer/drop. Say it is a 300 AMP single phase rated service --then each conductor will carry 100 AMPS of 115 volt phase to neutral. If it is 300 AMP 3 phase rated, that is the sum of the currents across each phase pair. A-B, B-C, C-A

Because of the phase angle, each hot conductor will 'see' 167 amps of current from the sum of 2--'100 amp' loads it is connected to. (check the formula please). Any 115v loads are additive to the phase to which it is connected. For instance , a 30 Amp- 115 volt load is connected Phase A to neutral. That is 30 amps load on Phase A AND the neutral conductor. A 30 AMP 225 volt load, connected to Phase A and Phase B represents a 30 Amp load on BOTH of those conductors. So the electrician must calculate both the phase-to phase currents, AND the phase to neutral currents. Conductors will handle only so much CURRENT (AMPS) without over-heating.

Wiring in the building distribution must distribute the loads fairly equally to maintain power factor efficiency.

I thought it was 208 delta that was designed to give 115 from one leg to nuetral,,,, I think 220 gives you 120 from the A and C phases to ground, B phase is slightly hotter around 125-128 vac to neutral. Or am I confusing delta with wye again?

if 250 Amps is the total loads in delta,you have to divide these equally to the 3 phases to balanced the current flowing in each phase.

IΦ=250/3

IΦ=83.33Amps. since you have told that the system is connected in delta, the current line to line is = to √3 x current per pahase.

therefore to solved for the line current:

IΦ-Φ=√3 (83.333)

IΦ-Φ=144.1609 Amps, this current will be your reference to decide your cable size refering it to the NEC cable ampacity table.

this will give you an idea if the existing cable can handle the additional loads,considering the demand and service factor of the loads it serves.

hopefuly this simple idea of mine gave you some answer.

Short answer, you don't. You let an EE spech the service....

Or you could spech the service and let the EE design the structure....

Contact a licensed electrician of the proper class, and he should be able to calculate your proper load and service size.

In most states,only an EE or a licensed electrican are allowed to do such calculations.

For you to do so may be illegal, and dangerous, not to mention risky from a liablilty standpoint.

HTRN