With respect to commercial properties, is the landlord or the tenant responsible for repairing/upgrading the electrical wiring of a premisis?
There's a long story behind this. No lease in place. As being a commercial property, is it generally the tenants responsibility for repairs?
As far as I know virtually all commercial Industrial leases are the rent + outgoings, mine are.
Structural repair work is the responsibility of the owner.
As said above it depends on the lease, even if there is an expired one, but certainly upgrades are paid for by the tenant in most cases.
All that have been said in today's market of empty shops and factories, many owners will bend a long way to keep a tenant, myself included!!
Usually a commercial lease is for the bare shell premises. As such it is the lessors responsibility to maintain the switchboard and make sure it is compliant with safety switches etc and that the power outlets and lights as originally fitted are up to scratch.
If a previous tenant has put in dividing walls or other modifications and left them there, it is up to the new tenant to accept them or modify them to suit his/her purposes and repair/ maintain them.
With commercial/industrial premises fairly much everything except building compliance is negotiable.
You have no lease in place. I strongly suggest you start negotiating for a lease that spells out lessor and lessee responsibilities.
If you are already in a dispute situation you need to assess how valueable this tenant is, how desireable the property is in the market place and decide how flexible you will be.
Remember, flexible people do not get bent out of shape. 
