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Shared HPWC Circut
- Thread starter ChicagolandMX
- Start date
The HPWC manual calls out for an unusual cable spec: 18 gauge, shielded, twisted pair.
The signal travelling over that cable is RS485, which is a balanced line communication protocol. Unless you are in an industrial environment, shielded wire is probably overkill. 18 gauge is ridiculous - its a signal wire, carries hardly any current. The most important part of the spec is twisted pair. I've seen other threads here on TMC specify cable that isn't twisted pair, and that is just inviting signal interference.
I'll bet you if you used good quality cat 6 or cat 6a cable between the HPWC for the signal cable, it would work. Something like this (use only one pair): 333ft Cat6A PVC Cable (Reel) - WHITE
If you do use shielded twisted pair (the twisted pair part is important), make sure to ground the shield. The install manual doesn't make mention of that, but an ungrounded shield is almost useless.
Depending on data rate used, the RS485 protocol can usually be used for hundreds and even thousands of feet.
The signal travelling over that cable is RS485, which is a balanced line communication protocol. Unless you are in an industrial environment, shielded wire is probably overkill. 18 gauge is ridiculous - its a signal wire, carries hardly any current. The most important part of the spec is twisted pair. I've seen other threads here on TMC specify cable that isn't twisted pair, and that is just inviting signal interference.
I'll bet you if you used good quality cat 6 or cat 6a cable between the HPWC for the signal cable, it would work. Something like this (use only one pair): 333ft Cat6A PVC Cable (Reel) - WHITE
If you do use shielded twisted pair (the twisted pair part is important), make sure to ground the shield. The install manual doesn't make mention of that, but an ungrounded shield is almost useless.
Depending on data rate used, the RS485 protocol can usually be used for hundreds and even thousands of feet.
If you want shielded cable (and this one's cheaper), here's another one:
Monoprice Cat6 Ethernet Bulk Cable - Solid, 550Mhz, STP, CM, Pure Bare Copper Wire, 23AWG, No Logo, 250ft, Blue - Monoprice.com
When grounding the shield, ground it at one end only.
Monoprice Cat6 Ethernet Bulk Cable - Solid, 550Mhz, STP, CM, Pure Bare Copper Wire, 23AWG, No Logo, 250ft, Blue - Monoprice.com
When grounding the shield, ground it at one end only.
dhrivnak
Active Member
While our shared circuit worked well we were side by side. I would give it a try with a good quality twisted cable.
gnuarm
Model X 100 with 72 amp chargers
If you don't know the details of the internal design of the devices, I would stick to the recommendations. Yes, RS-485 is robust and often will work with longer cables, but there are many other factors. The one I would be concerned about is the grounding. With AC power there can often be significant voltage on the neutral. The safety ground should not be used for any aspect of the electrical circuit. I would hope the RS-485 interface is optically isolated which means you don't need a common ground other than the neutral even with the full 240VAC on it. But who knows what's inside the box? Anyone have schematics?
What's wrong with the recommended 18 ga? It can't be all that expensive. But yeah, I can't see how 23 ga wouldn't be just as good for this application as long as there aren't safety issues.
gnuarm
Model X 100 with 72 amp chargers
Ok, I still want to see the details of the design before I tell people to not listen to Tesla's recommendations. There are significant safety issues involved. A search didn't find any info.
Why would anyone want to cheap out on an 18 ga wire pair?
Because the performance might be better. Typical twisted pair 18 gauge wire uses at most a very lazy twist which doesn’t handle common mode noise rejection very well. A cat 5e, or cat 6 cable is designed for high frequency data transmission and might enable longer than 50’ distances.
I gotta say that the 50’ distance limit and the 18 gauge spec are both bizarre. RS485 normally works over hundreds of feet, and normally runs over 24 gauge wire. Sometimes 22 gauge. But 18???
Here’s a catalog link to various Belden cables that are meant for RS485. Belden Global Catalog
gnuarm
Model X 100 with 72 amp chargers
I thought the spec was for the distance between units. What makes you think it has anything to do with the RS-485 signal? The issue might be with the power side.
gnuarm
Model X 100 with 72 amp chargers
That one I know. Because the signalling wire is in the same box as the high voltage wire, it has to be treated as high voltage as well... so conduit. That's an electrical code requirement and the reason why they don't put phone/network/cable TV wiring in the same outlet box with the AC cables.
I'm not going to second guess the reasons for the various requirements for installations. Compared to everything else the cost of the signalling cable is in the noise and I expect performance just won't be an issue. The line is balanced, low impedance terminated I expect, so noise is typically not a major issue, especially in most of the electrical environments it will be installed in.
It's going to be the best practice for sure. Exposed wiring has a greater chance of getting cut or damaged.
Don't think you want any possibility of both HPWC trying to push out 72 AMPS each on a 100 AMP circuit due to a failure to communicate.
gnuarm
Model X 100 with 72 amp chargers
The worst that should happen is both units fail to negotiate and draw full power blowing the circuit breaker. Circuit breakers don't blow right away if pushed a little over their limit, but at 50% overload they will pop quickly I believe. That said, I expect it will work just fine.
gnuarm
Model X 100 with 72 amp chargers
Let us know how your installation works out. I'm thinking of installing a pair of HPWC, one outside (which is more convenient day to day) and one inside my single car garage. So I would likely share a circuit breaker and use the comms connection. The connection may well be a bit over 50 feet even if it's not 100 feet like yours.
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