How do I calculate circuit load, when I don't own the tools yet???

[1SlowFormula]

I am getting ready to not only fully re-wire the house (built in 1915 + 3 major renovations over the years) but also install a sub panel for the detached garage built in 1980. The issues the property has electrically are crazy, it has good new run from the street to the 200a box then from there it goes downhill fast. Things like:
1 12-3 line with the red connected to a 15a breaker and the black to another 15a breaker and the neutral to the neutral bar and the ground to the ground bar (those last to as expected), this runs through an older metal conduit back outside the building and along the foundation (not buried) to the front of the house then up the wall to the original 1915 fuse box that has been gutted and turned into a junction box. From that junction box those 2 power circuits get joined to 2 separate hot knob and tube lines and the neutral knob and tube lines join to the single neutral and the box is then grounded. Those 2 knob and tube circuits run 50% of the upstairs lights and outlets, illegally spliced (someone cut the sheathing and just wrapped the romex onto it then just threw a little electrical tape around that) with romex, and random outlets and lights in 3 rooms on the first floor some with romex at the boxes so there must be more illegal splices somewhere.

I have run electric many times before but mainly as a laborer where someone else figured out the circuit layout. I am a software engineer by trade so I have the ability to figure it out, I just never had the chance before, so my questions might sound basic in the beginning, but it will be well thought out before applying it to the house.

Anyhow that was just background and not the full purpose of the thread. Here are a few of my questions:

I want to take the 1 15a circuit I currently have running out to the garage and put in a sub panel, in consulting a local electrician he said all I needed was a 100a sub panel. But the question is if I take 100a away from the 200a in the house does that only leave 100a for the house, or is it a subset that isn't really calculated from the main house panel?

When laying out the circuits in the garage, before actually buying the tools/appliances how can I accurately calculate the load? Is there a general rule of thumb that x tool/appliance needs y size circuit?

Here are the list of tools I own and a list of tools I will get in the future, how do I figure out the circuit load or do I just run the circuits for what I have then run the others once I buy the tools?
Own:
-- 120v 1.5hp 33gal air compressor (will probably have a dedicated run since it pops the breaker now if anything other than a radio is running at the same time, lol)
-- 120v 20a welder (same as the compressor, it will probably have a dedicated line, but doesn’t pop the breakers now like the compressor does)
-- 120v 6hp 16gal shop vac
-- 120v 1/2hp bench grinder
-- 3/4hp drill press that could be wired 120 or 240, currently wired 120
-- misc wood working tools (table saw, 2 routers (1 table and one hand held), small planer, compound miter saw, and a few misc sanders) more than likely only a couple would ever be running at one time and even if I did have a helper it would be one person cutting while one was sanding so the load there shouldn't be that bad.
-- Small refreshment fridge
-- 2 garage door openers

On the 'wish' list:
-- good 240v mig welder
-- good 240v tig welder
-- good plasma cutter, probably 240v
-- better air compressor, probably 240v

I figured there would only be (2) 240v circuits for the wish list stuff, one for the compressor and one for either the welders or plasma cutter to be plugged into as needed/used.

That doesn't count standard lighting and outlets (well kind of counts outlets since most need to be plugged in, lol) and the electrician said a 100a panel should handle all that. I know the 240v items need separate dedicated runs, but how should I calculate the runs for the other tools and should I assume that they would never really be running at the same time or calculate it like a house that it should be calculated as if all were running at the same time even if they are not permanently plugged in?

 

[Rick S.]

Very interested in advice to this thread. I have an almost identical situation minus the wiring problems in the house. I have a 36x36 pole barn about 100' behind the house that I would like to run a 100 amp service to off the 200 amp main panel that need updating in the attached garage. Same wish lish plus a lift someday. The only 240v tools that I could imagine running at the same time would be the plasma and compressor. I was planning to estimate the future cicuit load by researching the tools/welders that I would someday like to buy. My budget does not allow 200 amps and all the "overkill" that I used to plan for before the kids and $4 dollar gasoline came along... Thanks for letting me tag along 1SlowFormula.

 

[jvitez]

You're on the right track. Add up everything you could possibly ever want to run at the same time, in wattage not amperage, then make sure it's under 100Ax240V=24,000 VA or watts. You're 100 amp subpanel is therefore adequate.

For individual branch circuits, others familiar with the NEC should chime in. I'm in Canada. Our CEC requires one dedicated 15 amp circuit per vehicle stall, and only one 15 amp branch circuit for interior and exterior lights including one over the man-door. We don't use much 20 amp circuits here. But calculating subpanel loads is the same on both sides of the 49th parallel AFAIK.

BTW: for total load for your main service panel, do a proper NEC load calc, then add everything you could foresee running at the same time in the garage. Is this total number under 48,000 watts? Then you're OK. For a garage subpanel you use the actual anticipated max load, not a proportional load calc like for a residence.

 

[theoldwizard1]

Just a quick "rule of thumb". A 120V 15A breaker will "hold" a "hard working" 1 HP motor (like on a compressor) as long as there is nothing else on the line. A 120V 20A breaker (and 12 gauge wire) should be good for up to about 1.5 HP.

Compressors are kind of a "worst case". When running, they are working hard (drawing lots of amps) for minutes at a time. Saws and welders have lower "duty cycle". That is, they work hard for a few seconds and then are off (or not working hard) for more seconds. Based on this, you can probably run a 2-3 HP table saw on a 120V 20A circuit.

Lower HP motors obviously draw less current.

Here is some "free advice" (worth every penny). When wiring a shop, first put lights on a separate circuit (I do run my garage door opener on a light circuit). 15A/14 gauge is fine. The rest should be 20A circuits. Put in as many 20A/12gauge circuits as you can afford, one outlet per circuit is best (this is code in kitchens and bathrooms).

If you don't have any 240V tools, don't install the circuits (unless you just want to install the conduit; size for 8 -10 gauge), but make certain the breaker box has plenty of extra space. Plasma cutters draw lots of current and require constant air flow, so you probably need at least 2 - 240V/20-30A circuits. You aren't like to be running a table saw and a welder, or 2 welders at one time.

Depending on the size of the shop, you might want to install one of the 240V circuits as a drop cord from a central point in the ceiling. 30A would require 10 gauge SOOW "cord". Also you want a locking receptacle (Like NEMA L14-30R) on that cord.

 

[TheGrooveking]

Not to be a ball buster, but the 120v 6hp 16gal shop vac is not 6 hp actually, it more like 1/2 hp. 110 Volt circuit will not handle 6 hp of actual running motor, at 6 hp that vacuum would be glowing pile of red metal and burning plastic. Typical manufacture hype.

TheGrooveking

 

[ddawg16]

Not to be a ball buster, but the 120v 6hp 16gal shop vac is not 6 hp actually, it more like 1/2 hp. 110 Volt circuit will not handle 6 hp of actual running motor, at 6 hp that vacuum would be glowing pile of red metal and burning plastic. Typical manufacture hype.

TheGrooveking

Yep.....the 6hp # is a BS # created by taking the initial current inrush and calculating what the HP would be 'if' the current was that high. Read the label...it will say 6HP "Peak".

One thing to remember, your not going to be operating all those tools at the same time....so you have to look at what your typical peak loading would be.

So....assume the following would be on worst case:
1. Lights - Just add up the total wattage you want, divide by 120....that is your amps.

2. Compressor - Assume it might be on while your using one of your tools

3. Mig Welder - Besides your compressor, this will most likely be your hightest current user

4. Fridge - Gotta have a place for cold beer storage

5. Tunes - Need music

In real life, I doubt your going to be using more than 50A @ 240Vac

 

[big.jim]

in the uk we use what is called diversity, all your tools might add up to 100amp but you are only likely to have half powered up at any one time so 50 amp would be enough , i have a 45amp 240 volt supply into my garage running compressor, welders,lights and i have run the hot tub from there and still plenty spare .