I have a 240v 30a circuit with a 4 prong 14-30 receptacle installed in my garage. I'd like to connect a 240v baseboard heater that requires L1 (hot) and a neutral and a ground. Since the circuit consists of 2 120v leads (red and black) is there a way to safely connect both to the single lead on the baseboard heater?
NEMA 14 is the universal donor
Your NEMA 14-30 connector is the type you want. It has
- neutral (white or gray, 120V to L1 or L2),
- Safety Ground (green, green/ehite or bare, near 0V to neutral, near 120V to a hot)
- both hots (any color yet unnamed, same color ok, black and red commonly, L1 and L2, 240V across them)
What voltage is this thing?
You need to look at the machine's nameplate and see what it actually is. What's more, get the manufacturer's instructions for the unit and see what it says. It is illegal to connect it contrary to the manufacturer instructions, and that makes it illegal to fail to read them.
US-market large heaters (2000W and up) are made for 240 volts, needing to connect to L1 and L2. They do not use neutral. Many smaller heaters made for permanent installation are also 240V.
US-market small heaters are often 120V. If they are 120V, they have a hot (either L1 or L2, never both) and neutral.
European and rest-of-world heaters are all 230/240V, because everything there is 230/240V. Most will tolerate 240V because they're built to be compatible with the UK and some Asian nations which are 240V rather than 230V. Unlike us, they have 240V between hot and neutral. They use brown for hot, and blue for neutral. First, you must check the instructions or with the manufacturer as to whether they allow North American style hot-hot connections. If they do, connect brown to L1 and blue to L2, or vice versa.
You also connect a ground wire, always. Doing it bare 2-wire is not allowed, unless the heater is listed as double-insulated, which it will say rather directly if it does, because they had to pay a lot of money to have it certified as such. Don't assume this.
Cord-and-plug connected, or hard-wired
You can only go cord-and-plug if it makes sense for the thing to be portable. But then, you can fab it yourself. You need to visit a real electrical supply house and obtain proper cordage, which is special electrical cable made to be a cord. The size of the required wire depends on the rating of the heater. Then you need proper strain relief on the appliance end, and a good fitting plug on the wall end. Getting right stuff that matches is the reason to go to a proper electrical supply; they can do it; big-box can't.
If it's hard-wired, you will need to go to your town and pull a permit, then have it inspected when you're done. If you're not super good at this (and you aren't yet), the second one may be the way to go, because a set of smart eyes will look it over for you. On the other hand, if you're doing something you should not, he can shut the whole thing down. Also in most jurisdictions you must hire an electrician for units you are renting out, or if you have been banned for being really bad at it.
If you hardwire it, you need to run 10 AWG wire, since your circuit breaker is 30A.
Since the circuit consists of 2 120v leads (red and black) is there a way to safely connect both to the single lead on the baseboard heater?
There's no safe way to directly connect your two US split-phase 120V leads together. That is a short circuit and your main panel's circuit breaker would try really hard to save your house from burning down.
It might be possible to connect the appliance's "neutral" to one of your 120V leads but to be sure you should check the appliance manual, contact the manufacturer or edit your question to include make and model of appliance. Your appliance might be unhappy that "neutral" is 120V from ground.
You NEED 4 wires not 3, you simply have a 110 service on that receptacle. Get an electrician to upgrade this receptacle. Check to see if your heater needs 4 wires to work or is it a 110 heater that only needs 3 wire. Make sure you've got 12 gauge wire...really get an electrician to do this...unless your regulations say 'Anyone can do electrical with inspection and approval'...I've never heard of homeowners getting away with doing their own electrical work or their friends and having their homeowners insurance good with that kind of work. What are the rules/regulations from your city's buildings and code?