×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Relay Panels3

## Relay Panels

(OP)
I am working on a lighting design involving a relay panel.  The fixtures are located 150'-300' away from the relay panel and the source panel.  The relay panel is fed from three (3) 20A,1P breakers from a 120/208V,3ph,4w panelboard.  There are 6 relays coming out of the relay panel.  I am having trouble figuring out how to size the wires from the source panel to the relay panel and then from the relay panel to the lighting fixtures including the voltage drop.  I understand that only the hot wire goes to the relay panel.  I've added up the loads for each relay and also designated relays to one of the three input circuits.  Do I size each individual relay for the load on the relay (taking into account voltage drop)?  Do I size each circuit from the source panel for the entire load (all associated relays) and take into account voltage drop?  Since the neutral and ground do not go through the relay panel and sized of the entire circuit load, won't these be too large for the associated relay?

Example:  circuit number 1 consists of relay r1 r2 and r3.  r1=500W, r2=250W, r3=350W.  Total load =1100W.  So the conductors (H,N &G) from the source panel to the relay panel will be sized for 1100W?  On the other side of the relay panel, the hot wire for r1 is sized off of 500W, r2 for 250W and r3 for 350W?  Or should the conductors be sized off of 1100W for everything?

I am sorry if my post is confusing, but any help would be much appreciated.

Thanks!

Example:

### RE: Relay Panels

Um.. Yeah.

You need to add up the current needed for one relay circuit.

The chosen relay must be rated for that current or more and for that type of load.  Tungsten requires larger relays.

The breaker feeding that relay must be rated to handle that load.

The wire from the breaker to the relay must be sized according to the breaker protecting it.

Then the wire from the relay to the lights must be at least the size protected by the breaker. In reality it must be up-sized because you need to reduce the voltage drop on that long line.

The neutrals need to sized to handle the return current and also be over-sized to reduce the drop.

Of course the grounds SHALL NOT BE BROKEN.

I suggest you find a local electrician to help you out.  There are lots of little disasters waiting for you on this project.  It would be good to have two sets of eyes on the job.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: Relay Panels

Unless you provide individual overcurrent protection for each of the relay controlled legs on the secondary side of the relay output, you cannot downsize the conductors.  The only overcurrent protection  for them is the breaker at the branch circuit panel so if the breaker is a 20 amp circuit, the conductors will be # 12 AWG minimum ,(or the equivalent for wherever you are), for the length of the circuit.

You can calculate the voltage drop for each relay controlled branch of the circuit and calculate the voltage drop from the source panel to the relay panel for the total load on the circuit, which will give you a close approximation of the total voltage drop for each leg of the control circuit.

### RE: Relay Panels

#12 wire for everything, unless you want to use #10 to reduce voltage drop.  Neutral must remain with phase conductors; into and out of relay panel. (Or the phase conductors have to enter and leave the relay panel in the same conduit.)

### RE: Relay Panels

David is right of course. A circuit up to 300' in length will need to use a larger conductor.  My spec's upsize the conductor at 100' in length as that gives you almost a 3% Voltage drop for a 10 amp load at 120V.

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!