My house has 400amp service (not really sure why, but it does). At the main panel I have two circuits which feed our two HVAC units, one runs to our electric range, one for our dryer, and then there are two 70amp breakers for subpanels which handle everything else in the house.
What is the bus rating of the subpanels? Is it 70A or is it larger? Coz 70A busing is fairly rare. I would expect 100-125A busing.
Are the subs a scary-dangerous type like Zinsco or Federal Pacific?
An obsolete type like Pushmatic or really old GE?
If the panel takes modern breakers, and has 100A busing, and has enough spaces for you, I see no earthly reason to replace it. Replacing a panel is stupid amounts of work, and paying to do it merely because of the word “old” is like buying a MacBook pro and throwing it away. Naturally an electrician will do everything in his power to make you give your money to him instead of the Apple store. So I’m sure the electrician is singing songs about how much better it’d be to do it “right”.
The subs are old and the ground and neutrals bonded. Half of the house isn't grounded, etc. so I wanted to update everything.
As of NEC 2014, you are allowed to simply retrofit ground to anywhere that doesn’t have it. That is super easy to do in conduit, which you say you have.
If the conduit is metal and non-flexible, the metal conduit is a valid grounding path, so all you need to do is yank the N-G bond and yer done.
Contacted a local electrician and told him I planned to install two 100amp subs in place of the 70amp subs. He told me that #4 copper would be sufficient for this, but the more I'm reading I'm getting some mixed results on if that is accurate. The run from the main to the sub is just shy of 40'.
I suppose you can “nuke it from space” and start over, if cost is no object.
But the #1 mistake made with subpanels is getting too-small subs. Get one that is “just big enough” and you’ll have an expensive problem when you want to expand later. (That problem works out nice for electricians). As such, watch your electrician like a hawk: he will probably aim to leave you only 1-2 extra spaces. You want the panel to be about half full when the electrician is finished, and not depend on any “double-stuff” breakers (twin, tandem, quadplex etc.)
Also watch that the electrician doesn’t try to fill your panel with expensive AFCI/GFCI breakers you don’t need because you are grandfathered. (Although they can be nice for fault detection on old wiring).
So, to summarize, is #4 copper too small for feeding a branch from the main to a sub 40' at 100amps?
Yes, it’s too small for feeder. Feeder does not benefit from the 83% derate found in NEC 310.15(B)(7), which means 4 is allowable as service wire but not feeder at 100A. Note that copper THHN/THWN in conduit must use the 75C column of Table 310.15(B)(16). That is because the terminations at breaker and panel are 75C.
#4 copper is alowed For Services & feeders for residential
They’re allowed at 100A for services but only 90A for feeders. The difference is discussed in NEC 310.15(B)(7).
Why should it be stranded?
Because solid #6 and solid #4 do not exist
Sorry for my ignorance in terminology, but is the wire running from the main to a sub considered a feeder?
Yes. Feeder is wiring between panels.
Service wire is wiring between the meter and the first disconnect. However if the feeder acts like service wiring, in the sense that it carries 100% of the load of the service, i.e. between pole/disconnect and house... then it is allowed to use the 310.15(B)(7) derate.
There is also an exception for “feeder acting like service wire” that is tapped, e.g. if you have #4 to from your pole/disconnect to your house, then you add a 30A breaker to feed #10 wire from pole to a new shed, that doesn’t suddenly disqualify the house run for the derate. However this doesn’t help you in the slightest.