Is the garage considered the same structure because its attached? It would remain standing if the house was demolished.
Yes, in order for it to be considered a detached garage it must be a stand alone structure and not structurally attached to the house.
So it sounds like what you are saying is I need to run a 3 wire cable with a braided ground to a 90 amp breaker in the main panel.
Then remove the ground rod at the sub panel and ensure that the neutral bar is not bonded.
Yes, 3 conductor with ground . If you use copper wire and want 100 amps using table 310.16 you need #3 awg. The ground will be smaller probably a #8 or #6 if you use a 100 amp rated cable. If you use aluminum and want 100 amps you will need #1 awg. If by chance your juristiction allows the dwelling main feeder table 310.15(B)(6) then #4 copper or #2 aluminum will give you 100 amps. Check with your local codes department this will save you a good chunk.
As an added note you do not have to feed the panel 100 amps you could feed it 60 amps with a 60 amp breaker in the main panel. It just depends on what your load requirements will be in that garage. If you just want to run some power tools and lights, and a few outlets , maybe an air compressor then 60 amps will be plenty. That would save you a bit on the cost of the breaker. If you use the #2 al for your 3 conductors and a #8 ground wire then you could always up-size the breaker later if needed.
Btw you never said what kind of wire you have.....is it a cable or individual wire?
Shouldnt the breaker in the sub panel be a 90 amp or less then?
Nope, in fact you could have used a MLO panel (main lug only) with no main breaker. As it is your 100 amp breaker will simply serve as a convenient disconnect for the sub. The 90 amp or less that you use in the main panel will determine the allowable load that the sub-can draw and protect the feeder. So essentially any one of the two legs draws more than 90 amps long enough the 90 amp will trip out and the 100 amp will not notice. If both were 100 amps then it would just be a race as to which one would trip first. Personnally I prefer mlo panels for your situation with a stand alone 100 amp disconnect next to the panel. but what you have is fine.
I guess not being familiar with residential wiring I'm stumped as to why the ground in the sub panel needs to be ran from the service panel
.
I believe you may be confused as to the purpose of ground rods (lightning and power surges) and the equipment ground (human safety).
The main panel is the only place where you bond the neutral and ground. Remember you have equipment ground and the grounding electrode system (metal water pipe & ground rods) bonded in the main panel to the grounded conductor (service neutral). These are entirely different systems each serving a different purpose. Essentially the equipment ground is for human safety and bonds all metal parts in the electrical system that are likely to be energized in case of a fault such as a short to ground. This allows fault current to flow on the equipment ground back to the main panel where it then travels the service neutral to the transformer center tap. Remember the two are bonded in the main panel. This low impedance path allows so much current to flow that the branch circuit breaker will trip on over current. The fault current
will not flow to the ground rods or water pipes as they offer a much higher impedance. If you relied only on a ground rod for your fault current then a breaker will never trip as they arent there to serve that purpose & will not allow current to flow in sufficient amps to trip a breaker. The GES (ground rods. metal water pipes, ground rings etc .. are there primarily for huge voltage events like lightning or power surges. The metal water pipe and ground rod is where that big bare copper wire runs to from the main panel neutral bar. BTW the neutral bar and ground bar are one and the same in the main panel.
Finally if all you had going to the sub-panel was the neutral along with the 2 ungrounded conductors and no equipment ground then the neutral plays double duty both as the return current carrier and the equipment ground fault path to the grounded conductor (neutral) of the service. If you lose the feeder neutral to an open or fault then you lose both. You have no way for a breaker to trip in the sub. A ground rod at the sub is no help.
When you run a 4 wire feeder you have a seperate equipment ground and provides better safety than a 3 wire feeder. The key is to not bond the two in the sub-panel. If you did then return current on the branch circuit neutrals would travel (split evenly) on the feeder equipment ground and the feeder neutral. You do not want any current on the equipment ground unless it is fault current serving to trip a breaker.
This may help....
In this graphic from Mike Holt the metal conduit is serving as the equipment ground but just look at it as an individual wire like the top graphic. The enclosure marked service is your main service equipment whether a disconnect or panelboard with main breaker. The graphic labeled panelboard is your sub-panel. Notice the grounding symbol at the service panel... this is your grounding electrode...ie metal water pipe, ground rod/rods ....notice no current is flowing to the grounding electrode. Also note how all return currents and fault currents end up on the main service panel neutral bar and then travel on the service neutral (due to bonding) to the center tap of the transformer (not shown). Also note there is no ground rod at the sub-panel (both panels are in the same structure). The sub-panel btw is a mlo panel.
In this graphic the main panel is a disconnect
main breaker only but could be a main panel with main breaker and branch circuit breakers. The first disconnect the service entrance wires hit is the service equipment everything on the load side after that is a sub-panel.
Hope this clears things up some
Stubbie aka Roger