Controller, SDS, Sign Disco?

[Jerramundi]

Thinking about a 240V DPST Mech. Timer near the panel, feeding a 240V/120V 3kVA Xfrmr near the end of the run for VD purposes, which will feed an electric sign.

Question is... would the timer/controller be considered:
(1) Timer/Controller for the SDS/Xfrmr?
(2) Timer/Controller for the Sign?

The more I think about and learn about SDS', I feel like this controller would be considered (1) a controller for the SDS/Xfrmr.

The result being, that a disconnect near the timer, such as the OCPD, could be not be counted as the sign disconnect near the controller, even if it were lockable.

The isolation of the SDS would dictate that I need a disconnect for the sign on the secondary side of the Xfrmr... and with this controller being on the primary side of the SDS, said disconnect would need to be either (1) at the point of termination to the sign or (2) within sign of the sign...

Unless I moved the controller, but a 120V version, to the secondary...

Thoughts?

 

[winnie]

I don't believe that the SDS aspect makes a difference; there is no way that the secondary of the transformer to be energized if the primary is not.

However signs have local disconnect requirements anyway, so you would need to have a disconnect at the sign SDS or no.

-Jon

 

[Jerramundi]

However signs have local disconnect requirements anyway, so you would need to have a disconnect at the sign SDS or no.

Not entirely true. You're allowed to have a disconnect within sight of a remote location controller, but it's conditional upon the presence of said controller.

You have (3) options for signage disconnects.
(1) At the point of termination to the sign
(2) Within sight of the sign
(3) Within sight of the controller

 

[Jerramundi]

I don't believe that the SDS aspect makes a difference; there is no way that the secondary of the transformer to be energized if the primary is not.

I agree that there's no way for the secondary to be energized if the primary is not, but I feel like the requirements may "start over" once on the secondary side of an SDS, which is technically the beginning of a "new system." A perfect example of this would be the requirements for a new grounding/bonding system.

Relatively speaking, the secondary side of an SDS is actually the primary side of system #2. I feel like the primary/secondary protection of an SDS is relative ONLY to the transformer.

Looking at it another way... say I have something on the secondary side of the SDS that requires over-current protection, does the OCPD on the primary side of the SDS suffice if rated properly? I would think not (unless the SDS was a 1:1 ratio, then maybe).

 

[Jerramundi]

My problem is that I'm thinking about placing the controller on the primary side of the SDS near the main panel.

If that controller is still considered a controller for the sign, as opposed to a controller for the transformer itself, then the panel OCPD or transformer primary OCPD should suffice as a disconnect for the sign per the allowable condition of having a disconnect within sight of a remote location controller.

Conversely, if this controller is considered a controller for the transformer and NOT the sign, then I need an additional/new disconnect on the secondary side of the SDS... which ultimately may not be a bad idea, code required or not.

 

[winnie]

Relatively speaking, the secondary side of an SDS is actually the primary side of system #2. I feel like the primary/secondary protection of an SDS is relative ONLY to the transformer.

I think there is an important nugget of truth here, but I still think that the sign control/protection aspects have to be considered separately from the transformer protection requirements.

Transformer primary protection carries over to the secondary side only in some circumstances. For example if you have a common delta/wye 3 phase transformer, load on one of the secondary phases is carried by all of the primary phases, so you could have a secondary overload without a primary overload. Such a transformer requires primary and secondary protection.

On the other hand, with a single phase transformer there is a direct correspondence between primary and secondary current, so with proper selection of components primary OCPD can protect the secondary.

Looking at it another way... say I have something on the secondary side of the SDS that requires over-current protection, does the OCPD on the primary side of the SDS suffice if rated properly? I would think not (unless the SDS was a 1:1 ratio, then maybe).

IMHO if you have one of the suitable transformer types where primary current is strictly related to secondary current, then primary OCPD can protect the secondary, as long as you take into account turns ratio.

Say you have a 240:120V single phase transformer, and your secondary must be protected at 30A. IMHO 15A primary protection would provide this.

-Jon

 

[Jerramundi]

IMHO if you have one of the suitable transformer types where primary current is strictly related to secondary current, then primary OCPD can protect the secondary, as long as you take into account turns ratio.

Say you have a 240:120V single phase transformer, and your secondary must be protected at 30A. IMHO 15A primary protection would provide this.

-Jon

My situation is a single phase 240v/120v @ 3kVA. Primary protection only, per 450.3, that's a max OCPD of 20A (3000VA/240V = 12.5A; 12.5A * 125% = 15.625A; Note #1 allows rounding up to the next standard size).

Standard sizes given, the lowest I could do would be a 15A OCPD on the primary (unless they make a 10A breaker). This 15A OCPD provides 3600W of protection on the primary (15A * 240V), which is 30A on the secondary (3600W/120V).

Problem is a sign cannot be fed by a circuit greater than 20A. Even though the 15A OCPD ultimately determines the circuit rating at 15A, there is not 15A of over current protection on the secondary. There is 30A.

 

[Jerramundi]

I'm tempted to argue that if there exists anything on the secondary side of an SDS that requires things like (1) Overcurrent Protection or (2) a Disconnect , that the Primary OCPD of the SDS does NOT suffice (unless perhaps the SDS is a 1:1 ratio).

The exception to this being certain Class II and Class III Power Supplies that do NOT require Overcurrent Protection.

 

[winnie]

I'm tempted to argue that if there exists anything on the secondary side of an SDS that requires things like (1) Overcurrent Protection or (2) a Disconnect , that the Primary OCPD of the SDS does NOT suffice (unless perhaps the SDS is a 1:1 ratio).

Again, if (and only if) the SDS is one of the known types where a primary OCPD can protect the secondary, and if suitable breakers are available, then IMHO primary ocpd can suffice.

In your particular example, you _must_ have 15A OCPD on the secondary. The transformer has a 1:2 current ratio, so to get your 15A protection on the secondary side you would need 7.5A on the primary side; which I believe would be permitted but is likely impractical.

This might be possible with a fuse, but you might then have problems with transformer inrush tripping the primary OCPD. So I think as a practical matter you would need secondary OCPD.

-Jon

 

[Jerramundi]

In your particular example, you _must_ have 15A OCPD on the secondary. The transformer has a 1:2 current ratio, so to get your 15A protection on the secondary side you would need 7.5A on the primary side; which I believe would be permitted but is likely impractical.

Where do you get that I MUST have a 15A OCPD on the secondary?

If I introduced secondary overcurrent protection, per 450.3, the maximum OCPD I could use would be 125% of the secondary current as a function of the transformer VA rating (i.e. 3000VA/120V = 25A; 25A * 125% = 31.25A; No rounding up allowed, so 30A OCPD max).

I was thinking about putting in a 20A OCPD on the secondary to fulfill the requirement of a sign being by no more than a 20A rated circuit.

But if it were primary OCPD only, I would need a 10A breaker on the primary, which I believe would be allowable, but as you said, impractical.

 

[Jerramundi]

This particular question is less about the actual math, which I understand is important, and more so about if whether or not, for example, a timer on the primary side of the SDS would be considered a controller for either (1) the transformer, (2) the sign, or (3) both.

The OCPD example was just another iteration of the same question, but perhaps I muddied the discussion up by bringing up OCPD's.

I'm leaning toward (1) a controller for the transformer, but you seem to be of the opinion that it could be considered both?

 

[winnie]

Where do you get that I MUST have a 15A OCPD on the secondary?

I misread that the _sign_ required 15A OCPD.

If I introduced secondary overcurrent protection, per 450.3, the maximum OCPD I could use would be 125% of the secondary current as a function of the transformer VA rating (i.e. 3000VA/120V = 25A; 25A * 125% = 31.25A; No rounding up allowed, so 30A OCPD max).

I believe that is correct for transformer protection.

I was thinking about putting in a 20A OCPD on the secondary to fulfill the requirement of a sign being by no more than a 20A rated circuit.

But if it were primary OCPD only, I would need a 10A breaker on the primary, which I believe would be allowable, but as you said, impractical.

Exactly. 10A on the primary would provide the necessary protection for the sign. Additionally it is lower than what is permitted for protecting the transformer and would be allowed there. However getting the breaker might be difficult, and might be subject to inrush current tripping.

-Jon