24v transformer questions


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Old 04-18-22, 08:31 AM
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24v transformer questions

I've been lurking on these forums for the great info and now have a question. I'm looking to replace my old 40VA transformer as it started buzzing with a new Honeywell AT72D-1683 (40VA). It will power 3 x Honeywell V8043E1012 zone valves (0.32a each) and up to 2 x Ecobee 3 Lite t-stats (heat-only, 3.5va each from my research)...all going to a Weil McLain gas hydronic boiler. The third t-stat will be powered by the air handler as this one has both a/c and heat.

1) I wanted to confirm the transformer is properly sized for this application? I believe I'll have a total of 30.04VA at full load with the 3 x zone valves and 2 x t-stats.

2) On the Honeywell AT72D for the primary connections, it says black is common and white is 120V...with respect to transformer windings only and not the external circuit. On the primary side, do I connect the transformer's white to black-hot (from switch/breaker) and the transformer's black to white-neutral (from switch/breaker)?

Thanks!
 
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Old 04-18-22, 09:18 AM
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Assuming the transformer has a 24 vac output, a 40va transformer maximum current draw is 40/24 or 1.6 ac amps for 24 vac loads. (3x 0.32)a + (2 x 3.5/24)a = 1.25 ac amp. The transformer can power the five loads if they are 24 vac. Primary winding of a transformer is not polarity sensitive so either connection can be hot/neutral. Secondary winding of a transformer is not polarity sensitive unless there is a need to phase output with input. It is not in your application. Either connection can be output/output return. Just make sure you connect the primary winding to 120 vac.
 
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Old 04-18-22, 10:12 AM
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Thanks for the info! Very helpful.
 
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Old 04-18-22, 08:04 PM
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Welcome to the forums.

That has been a question asked many times.
Typically they use a multivoltage diagram where the black as common makes sense.
If you only see a single voltage diagram.... black as common makes no sense.

Here in the full diagram you can see that the black wire is the common wire when referenced to the white, red or orange wires. If you read note 2 it says black is common with respect to the transformer winding but not the connections.


 
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Old 04-19-22, 05:59 AM
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AT72D-1683 (40VA) is a single voltage step down transformer 120/24 vac so a multi voltage input transformer diagram doesn't apply here. The multi voltage input transformer shown in the diagram will operate with the black and white wires connected either way to 120 vac hot/neutral. The diagram is misleading as 240 vac has no common.
 
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Old 04-19-22, 01:58 PM
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Yes.... thank you. The AT72D is a 120vAC input only transformer.
I posted that diagram to explain where the black as common fit in.
Notes 1 and 2 are also printed on the single voltage diagram but make no sense without knowing the full picture.
 
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Old 04-19-22, 04:07 PM
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PJMax, I agree with you. Marketing is a bigger cause than dandruff for people scratching their heads after reading the instructions. lol
 
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Old 04-20-22, 07:40 PM
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A 24 volt 40 VA transformer is marginal for many systems. It is good practice to use one transformer to power all 24 volt power all items on a home heating/AC system. Having just one avoids many “C” wire/connection issues.

Below is 70 VA unit similar in price to 40 VA Honeywell AT72D-1683 (40VA).

https://www.zoro.com/rib-transformer...YaAkvNEALw_wcB

Common wiring practice is black for hot and white for common/neutral. In wiring jargon there are 2 power buses.
120 volt AC black is hot and white common, 0n 240 volt single phase red is other phase.

For 24 volts thermostats use black for R and RT RC and white for C common.

On old burner controls with T T connection, it much be determined which is 24 volts for R RT. and other to activate burner with second T to W for heat. In some burner controls there can be issues and problems may arise.
 

Last edited by doughess; 04-20-22 at 09:00 PM.
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Old 04-21-22, 06:17 AM
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Either of the 2 connections on a transformer 24 vac secondary can be used as the common for that circuit. Wire colors for R, RT, RC and C connections are a HVAC convention and don't apply to power wiring complying with National Electrical Code.
 
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Old 04-24-22, 02:04 PM
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Thank you all! I replaced my thermostat successfully using the info in this thread and a diagram I found on another thread from PJMax. I used 1 conductor of a separate 18/2 thermostat wire to connect my C wires to the transformer.


 
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Old 04-24-22, 02:14 PM
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Good job ! ........................
 
 

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