New Subpanel

[Al G]

I have a detached RV garage with a subpanel fed from a 100 amp breaker in the main panel on the house. We're extending the RV garage and adding on a pool house. An electrical engineering firm prepared a load calculation and riser plan as part of the permitting process. They show adding a 70 amp breaker to the subpanel on the RV garage to feed a new subpanel in the new space. All that makes sense.

Their plan also says to remove 3 #2 and 1 #8 (bond) between the 100 amp breaker and existing subpanel on the RV garage and replace with 3 #1 and 1 #6 (bond). If the conductors are already properly sized for a 100 amp breaker why would they need to be increased? The 100 amp breaker isn't changing. We're just adding to the load in that subpanel. The total conductor length from the 100 amp breaker to the subpanel it feeds is about 20 feet, all in conduit. The electrician doing the work doesn't understand either why the conductors need to be changed.

 

[Stuff]

Aluminum or copper wires? In conduit or NM cable? #2 aluminum is only good for 90 amps when used as individual wires in conduit. Common mistake as is good for 100 amp when handling the entire residential service.

 

[alfredeneuman]

Aluminum or copper wires? In conduit or NM cable? #2 aluminum is only good for 90 amps when used as individual wires in conduit. Common mistake as (it) is good for 100 amp when handling the entire residential service.

NM can't be run outside especially buried. Don't you mean UF cable?
#2 Al is rated at 90 amps whether in conduit, MHF cable, or USE cable. UF is 75amps

 

[Al G]

Aluminum in conduit. So it never should have been #2 on a 100 amp breaker? It was installed when the house was built in 2004.

 

[pattenp]

Aluminum in conduit. So it never should have been #2 on a 100 amp breaker? It was installed when the house was built in 2004.

Yep, should have been protected with a 90A breaker to the letter of the NEC.

 

[Al G]

Now I wonder if I need to ask the electrical engineer if swapping the 100 amp breaker for a 90 is an option instead of pulling new conductors. By my understanding of the load calculations 90 amps is sufficient.

 

[alfredeneuman]

The electrical engineers should have known better. They were wrong. You should have no problem convincing them to let you put in a 90.

 

[wyliesdiesels]

yeah someone screwed up. shouldve been 90a breaker from the getgo

Id ask them what their load calcs came out to. you may be able to get by with just changing the breaker

 

[mike93lx]

If 100a works, 90a will almost certainly be fine.
I would hire a new electrician. This isn't complex and if he didn't know that the 100a breaker isn't allowed, I wonder what else he will miss

 

[Stuart in MN]

Just call the engineer and ask them about it.
I was an EE for 43 years and often got calls from clients asking questions about one thing or another on a design; sometimes my explanation was all that was needed, and sometimes after talking over the issue I would agree that a change was warranted. We aren't privy to all the details, so there may be a reason he called for replacing the wire that isn't obvious to us.

 

[Terry D]

Aluminum in conduit. So it never should have been #2 on a 100 amp breaker? It was installed when the house was built in 2004.

Check with your local codes. There are some areas that override the code and allow #2 aluminum for 100 amp sub panels also. Here, we are allowed to do it.

 

[bamawildcat]

What is the length from the subpanel to the new pool subpanel? The upsize in conductors, albeit 20 feet, may be to account for voltage drop down the line.

 

[Norcal]

What is the length from the subpanel to the new pool subpanel? The upsize in conductors, albeit 20 feet, may be to account for voltage drop down the line.

It’s not a upsize when the existing overcurrent protection is 10 amperes over what the conductors are rated for.

 

[wyliesdiesels]

What is the length from the subpanel to the new pool subpanel? The upsize in conductors, albeit 20 feet, may be to account for voltage drop down the line.

upsize? how is it an upsize? the conductors are too small for the breaker rating. if they were 1/0 AL on a 100a breaker, then it would be an upsize...

and at 20' voltage drop is not even an issue.... nowhere near being an issue...

 

[dcg9381]

The electrical engineers should have known better. They were wrong. You should have no problem convincing them to let you put in a 90.

Or just put in a 90A.
EEs aren't really used for residential around here... Master electricians are.. This sort of thing, in my experience, is definitely in the wheel house of a good electrician and shouldn't be trusted to us desk-jocking engineers..

I assume you've got a 50A RV connection... Even running 2 ACs, you're pulling about 60% of that circuit.

 

[u2slow]

Its a top-down (EE) vs bottom-up (electrician) perspective. On a drawing, it's equally easy to change the wire size as the OCP size... so why reduce the ampacity?

 

[Al G]

I got some more information today. First, my earlier conversation had been with the electrician apprentice (he didn't understand the #2 vs #1 issue). Today I spoke with the actual electrician on the job. He is fully aware that the existing conductors are undersized for the 100 amp breaker. He says it's no big deal to upgrade the conductors and keep the 100 amp breaker. In his opinion the load calculations justify the 100 vs 90 amp breaker. He assumes that's why the engineer called for upgrading the conductors instead of downgrading the breaker.

There is a 30 amp RV receptacle in the RV garage. I won't be running the RV AC units in there so 30 amp is all I need for running the battery charger and the refrigerator at times. That receptacle is part of the new 70 amp subpanel.

Thanks for the responses. They gave me enough information to have an intelligent conversation with the electrician.