Moved here a few months ago and the heat pump is junk. It has a 100 amp breaker out of the main box. Thought about putting in a sub panel off of this breaker. Looked at the wire and it is a 3 wire (hot hot ground) and not a 4 wire (hot hot neutral ground). Not a pro sorry for the weird wording. Would I just connect the 2 hots to the sub and the ground to the bar and that's it or would this be a problem?
Need to add a sub panel
subpanel
Related Solutions
According to Table 250.122 of the National Electrical Code, a 10 AWG copper conductor is fine as an equipment grounding conductor for circuits with up to 60 ampere protection.
National Electrical Code 2014
Chapter 2 Wiring and Protection
Article 250 Grounding and Bonding
250.122 Size of Equipment Grounding Conductors.
(A) General. Copper, aluminum, or copper-clad aluminum equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122, but in no case shall they be required to be larger than the circuit conductors supplying the equipment. Where a cable tray, a raceway, or a cable armor or sheath is used as the equipment grounding conductor, as provided in 250.118 and 250.134(A), it shall comply with 250.4(A)(5) or (B)(4). Equipment grounding conductors shall be permitted to be sectioned within a multiconductor cable, provided the combined circular mil area complies with Table 250.122.
I'm fine with the idea of a main panel with only 2 things in it feeding a sub-panel with everything in it.
Question #1 - I'd rethink putting a service panel outdoors at all. Weather is rough on panels, even if they claim to be outdoor rated. I'm a little nervous about a 100A breaker supplied from the normal bus bars, but if the manufacturer stands behind it, okay. The 100A wires are going to be a mother to wrestle onto that 100A breaker. Are you quite sure the power company has provisioned you 125A service? 100A is more common.
Question #1 (the second): You're gonna want more slots than 24, since this box powers pretty much your whole house. Nobody ever installed an addition and went "Gosh, that job was sure made harder and more expensive by having too many slots in the panel". It's a false economy, especially since bigger boxes are often bundled with more breakers. Your house may be ok now, but do a kitchen remodel and lookout!
Question #2 (the second): Don't bond your grounds to random plumbing that happens to be going by. It's not code, and someday you might have a plumbing problem and the plumber replaces a downstream chunk of it with PVC. Whoops. Also, they've been upgrading customers to PLASTIC water meters. Double whoops. Bond properly and to code.
Question #3 (the third): Bond ground and neutral only in the (singular) main panel. As such, you need 4 wires between main and subpanel.
Just for your edification, it's only a sub-panel if it's fed from a main panel. If it's fed directly from a transformer, it's a main panel.
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Best Answer
Is it really ground?
Look closer. Some cables, that wire can be used for either neutral or ground. If so, ask your AHJ whether they will authorize to re-classify it as neutral and retrofit an actual ground. The best plan is to ask your local AHJ if they are comfortable with that. You can't retrofit neutrals, but you can retrofit grounds.
If you don't get AHJ permission, you will get a very, very rude and expensive surprise when you sell the house.
All subpanels must be fed with entirely separate neutrals and grounds. Avoiding this is out of the question. But you don't need neutral for 240V-only loads. All your heat-pump loads are 240V, that's why neutral was never run in the first place!
Is it really 120V/240V?
If it's a subpanel for the heat pump, then it's very likely that every single load is 240V, with no 120V or 120/240V loads present.
In that case, install a 240V-only panel. Use a subpanel of your preference, get lots of spaces because every breaker will need 2 spaces, install a ground bar, remove the ground-neutral strap since it is a subpanel, and connect nothing to the neutral bar, it will simply float. Then you install your usual 240V breakers - a 20/30 for the heat pump proper, 30/40/50/70A for the emergency heat, etc.
Label the panel very clearly "240V only" so some future handyman or HVAC tech doesn't lose his mind or do something stupid.
You could make it a 120/240 subpanel later by running a new main cable.
Oh, but I need a little 120V too
First, try changing your loads to 240V. Motors can be had easily either way. It's easy to get modern lighting that happily runs on 120-277V, 240V being within that range. Old sodium/mercury/halide lighting often has transformer jumpers for that.
Second, OK, you really need 120V. How much? For loads up to 1200 watts, obtain a 1.5KVA transformer, or forget that, just bring a branch over from your main panel. For loads up to 4000 watts, obtain a 5 KVA transformer used on Craigslist, typically $100. Feed the transformer 240V primary from a 240V dedicated circuit (so you can turn it off). Feed the 120V secondary to a tiny 30-space* subpanel with both 120V rails jumpered together. Fit your 120V circuits.
If you really need more than 4000 watts of 120V at that location, knuckle down and replace that heavy cable with /3+ground cable. And get aluminum - the lugs are aluminum and this will reduce corrosion, also it's far cheaper.
* That's an inside joke. I am one of several here who recommend rather large panels, so you avoid the major hassle of running out of space. Feeding a 1.5KVA transformer a 2-space panel is plenty, from a 5KVA, a 4-space panel will suffice.