How many amps for a 10 unit apartment building

[Jordan F.]

Dear BP,

We are in the process of researching an electrical upgrade for a 1950 apartment building with 8 one bedrooms and 2 studios, taking into account the following:

• We have no dishwashers or garbage disposals and won't install any,
• Stoves, ovens and heaters are gas,
• We are in Southern California,
• In one bedrooms: 10,000 BTU a/c for living room and 6,000 BTU for bedroom,
• Central water heater is gas,
• We'd like 2 electric vehicle chargers connected to house circuit (currently only has LED lights on it),
• We have no washers/dryers currently but might get one of each or a pair in the future,
• Our insurance company states each unit should have 150 amps.

How many amps should we put in the main breaker?  The current electrician wants to keep it at 400 amps max because if it goes above then we have to higher an engineer or something like that.  But we also want to "future-proof" the building over the next 40 years or so.  Nothing's been done to it; we still have fuses and original wiring inside metal conduit.

Thank you all for your help!