Wiring for Nest

Alan (California Radiant) Forbes

Just finished adding two Nest thermostats to an old SR506-2 pump control. Taco says that the onboard transformer can handle up to two, but more Nest stats will come online later, so I’ve wired a larger transformer in series to handle the future load. 

My question is, once I’ve charged the “R”  terminal with the additional transformer on the first thermostat, do I have to do the same on all the others or have I now increased the energy available for all of them?

hot_rod

Does the 506 have two 40va transformers? If so I think they are split 3 on one transformer 3 zones on the other.

Alan (California Radiant) Forbes

There’s just one transformer on the pump controls. 

Jamie Hall

I hope you wired them in parallel, @Alan (California Radiant) Forbes ! But if they really are in parallel they will act as a single power source.

HomerJSmith

Ganging class 2 transformers, the transformers secondaries need to be wired in polarity (phase). The easy way to determine if the transformers secondaries are correctly wired together is to take one secondary of each transformer and tie them together. Take the other secondary of each transformer and strike them together quickly and they will spark. Reverse one secondary lead and connect them together again and strike the other untied leads which will spark. The connection that produces the smallest spark is the connection that is in the correct polarity.

hot_rod

I'd use the @EdTheHeaterMan fuse trick when wiring transformers together

Alan (California Radiant) Forbes

STEVEusaPA

Here's what I was taught...
There is no hot or ground on the secondary side of a transformer. It is floating until one side is grounded.
Ground one side of the secondary, measure L1 to R, should read about 96v.
If it reads 144v, then you have the wrong side of the secondary grounded.

ratio

Phasing single phase transformers is easy. Connect the common of both transformers together & measure the voltage between the two Rs. If it's zeroish (2-4 volts is common) they're in phase, if it's 50ish volts (2x nominal 24 VAC) they're out of phase. You shouldn't need to involve the primaries unless it's three phase. (Phasing transformers on 3 phase involves getting onto the right primary too. It does make a difference.)

Alan (California Radiant) Forbes

I'd use the @EdTheHeaterMan fuse trick when wiring transformers together

Yeah, I liked ETHM’s trick as well. I went the easy route with a circuit breaker on the transformer. 

EdTheHeaterMan

hot_rod said:

I'd use the @EdTheHeaterMan fuse trick when wiring transformers together

There is a minimum service charge for that idea. $2.50 for the fuse or $4.99 for the circuit breaker.

pecmsg

EdTheHeaterMan said:

I'd use the @EdTheHeaterMan fuse trick when wiring transformers together

There is a minimum service charge for that idea. $2.50 for the fuse or $4.99 for the circuit breaker

Operators are standing by. 

place your order in the next 10 minutes and we’ll ship 2 for the price of one. Just pay the extra shipping and handling. 

hot_rod

There will come a time when the Caleffi switching relays will come with a snap in 40Va, a second 40 can be snapped in. Both circuit breaker protected. Gone will be the small 15 Va transformer populated on the board.

Plenty of power for stats and light arc welding projects

Alan (California Radiant) Forbes

So, when I buy a transformer with the primary wiring hot and neutral (black and white) and the secondary wiring marked "R" and "C", can't I assume that it's properly phased?

And when I wire it in parallel to the transformer on the SR control, can't I assume that it is properly phased as well since the terminals are marked "R" and "C"?

ratio

IF the transformers are the same mfgr, it's probably a safe assumption that they're the same polarity so that wiring the primaries & secondaries in parallel will be uneventful. It's not a given that any old transformer will be in phase with any other. Transformers are inherently uncaring about things like polarity and even power input—a 120:24 transformer will happily (mostly, there is some risk of the insulation breaking down at the higher voltage) give you 600 volts out if you feed the 24 volt side with 120 volts.

Jamie Hall

Alan (California Radiant) Forbes said:

So, when I buy a transformer with the primary wiring hot and neutral (black and white) and the secondary wiring marked "R" and "C", can't I assume that it's properly phased?

And when I wire it in parallel to the transformer on the SR control, can't I assume that it is properly phased as well since the terminals are marked "R" and "C"?

No. Not only is electricity colour blind, it's illiterate. As someone once said -- "trust, but verify".

The Steam Whisperer

Also, be aware that Honeywell has 120/24 transformers out there now where the neutral is black and the hot is white.

EdTheHeaterMan

The Steam Whisperer said:

Also, be aware that Honeywell has 120/24 transformers out there now where the neutral is black and the hot is white.

We are all trying to get along in this new politically correct world. Nice to know that Honeywell is allowing the roll reversal of the black and the white wires.