If the overall main fuse box has two 100A cartridges in it does that mean the amp service is 100A or is it 200A and what is the 'hydro meter'?

Answer for USA, Canada and countries running a 60 Hz power supply service.

(LIFE SAFETY WARNING! [disclaimer]

Electricity is dangerous!

You can be injured or killed!

Improper installations can cause fire, injury and death!)

The fuse or over-current device in the service entrance equipment which limits current flow is the determining factor in the service's rating [assuming that the over-current protection is properly sized for the conductors used!]

Therefore, if the main fuses are 100 Amp, it is a 100 Amp service, even if it has 3 fuses. The exception to this is a service having more than one disconnect [allowed under the six-disconnect rule], wherein you might have a 100 Amp service with the largest fuse or breaker being 60 Amps! ... or two 100 Amp panels fed with a 200Amp cable - is it a 100 Amp or 200 Amp service?

Barring seeing a picture or being there physically, my guess is that you have a 100-amp service.

I can only guess what the "hydro meter" is.

Is there more than one meter?

It could be that the "hydro meter" is the off-peak service meter for your water heater power! [In the Philadelphia area, PECo has historically provided the option to have some loads powered through a second meter at a reduced rate, controlled by a clock or other means, which is shut off during peak demand hours.]

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS

WORK SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.