How many amps for a 10 unit apartment building

Question

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!

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Kenneth Garrett · Answer

Account Closed

The National Electrical Code has a calculation to determine the minimum amps required.  This doesn’t mean you can’t go higher for future use.  See Article 220.

Account Closed · Answer

I just spoke to a city electrical inspector.  He says 40 amps per apartment, 100 amps for the house and 400 amps for the building is more than enough.

Account Closed · Answer

Thanks for responding.  Insurance requires 150 amps/unit but I was wondering about how many amps for whole building.  We'd have to rewire because most of our wiring is original copper with cloth insulation from 1950.

Mike McCarthy · Answer

@Jordan F. The proper way (per NEC) is a demand load calculation. It’s not easy, but doable as there are some calculators online for it. It takes into account heating, square footage, and appliances and would be used if the building was being built new.

But if your electrician and building inspector are good with 400A, that sounds reasonable.

Your electrician is right, a 400A service is the highest ‘standard’ service the POCO will install. Over that you’ll be getting into more switchgear ($$$) and approvals.

Steve Morris · Answer

Check with code.  On new units in Portland, something like 120 AMP service for 1 beds is min.

Probably won't cost that much more to go higher, so I'd do more than min IF you're going to rewire.  What type of wiring do you have now?  Alum is OK IF the right gauge.