OCP Requirements for ATS

[gepman]

Situation
Generator with 125/2 CB feeding through 100A unfused disconnect to "emergency" side of 100A ATS. 100/2 CB feeding through 100A unfused disconnect to "normal" side of 100A ATS. "Common" side of ATS connected to panelboard with 100/2 main CB.

Question
What is the required OCP for the ATS (i.e. what size CB is required at the generator)?

From my reading of Article 240, it applies only to conductors. Table 240.3 describes which articles apply to OCP for equipment which I assume an ATS is categorized as. I looked in article 445 (generators), articles 700, 701, and 702 (emergency and standby systems), and even article 404 (switches). None had any OCP requirements.

Is there any article which gives requirements for OCP for ATS's, switches, or other equipment not listed in Table 240.3? Is there any article which gives sizing requirements (i.e. equipment rating must be greater than the calculated load) for ATS's, switches, or other equipment not listed in Table 240.3. The closest that I could come is 110.3(A)(7) and perhaps 110.3(B).

I am not that interested in opinions on the safety of the above scenario. I am interested in the exact article number that gives the requirements. I have referenced the 2008 NEC. If replying with any other edition of the NEC, please state the year.

 

[360Youth]

Situation
Generator with 125/2 CB feeding through 100A unfused disconnect to "emergency" side of 100A ATS. 100/2 CB feeding through 100A unfused disconnect to "normal" side of 100A ATS. "Common" side of ATS connected to panelboard with 100/2 main CB.

Question
What is the required OCP for the ATS (i.e. what size CB is required at the generator)?

From my reading of Article 240, it applies only to conductors. Table 240.3 describes which articles apply to OCP for equipment which I assume an ATS is categorized as. I looked in article 445 (generators), articles 700, 701, and 702 (emergency and standby systems), and even article 404 (switches). None had any OCP requirements.

Is there any article which gives requirements for OCP for ATS's, switches, or other equipment not listed in Table 240.3? Is there any article which gives sizing requirements (i.e. equipment rating must be greater than the calculated load) for ATS's, switches, or other equipment not listed in Table 240.3. The closest that I could come is 110.3(A)(7) and perhaps 110.3(B).

I am not that interested in opinions on the safety of the above scenario. I am interested in the exact article number that gives the requirements. I have referenced the 2008 NEC. If replying with any other edition of the NEC, please state the year.

Your question is a bit confusing, or maybe I am in a hurry. I am not exactly sure what a 125/2 is. My guess is 125 amp, 2-pole (240v) circuit breaker. If so, your transfer switch is too small unless you reduce the OCP of the generator . 125 amp breaker sounds like 25kw genset which is rated 104 amps. Transfer switch must be rated for the size of the service or generator, whichever is greater. A 200 amp service with 5kw generator still needs 200 TS. IMO, a generator within site and with proper OCP does not need additional OCP at the service, but many ATS have it built-in now. It depends on the equipment you are using.

Question
What is the required OCP for the ATS (i.e. what size CB is required at the generator)?

OCP is going to be based on KW rating.

 

[ron]

I'm sure there is a section somewhere, that I cannot locate, that generically requires equipment to be protected at its ratings.
The best I can do without finding that section mentioned above is 445.12(A) which requires the protection to be suitable for the conditions of use.

 

[jwelectric]

Your question is a bit confusing, or maybe I am in a hurry. I am not exactly sure what a 125/2 is. My guess is 125 amp, 2-pole (240v) circuit breaker.

I am also not sure of what his is trying to accomplish here but do agree that it is a referral to a two pole breaker.

If so, your transfer switch is too small unless you reduce the OCP of the generator . 125 amp breaker sounds like 25kw genset which is rated 104 amps. Transfer switch must be rated for the size of the service or generator, whichever is greater. A 200 amp service with 5kw generator still needs 200 TS.

This way of thinking was shot down with the 2008 code cycle. A lot of electricians think that the transfer is to be sized according to the service.
Notice the language found in the 2008 code cycle for an optional standby system;
702.5(B) (2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.
(b) Load Management. Where a system is employed that will automatically manage the connected load, the standby source shall have a capacity sufficient to supply the maximum load that will be connected by the load management system.

IMO, a generator within site and with proper OCP does not need additional OCP at the service, but many ATS have it built-in now. It depends on the equipment you are using.

This is not true nor has it ever been true. It is not the generator that has to be within sight but the disconnect on the generator that is required to be within sight. The relief of the within sight and readily assessable relates to the disconnect and not to the generator.

We also need to remember that if this generator is located outside then the feeders from the generator must also comply with the article concerning outside feeders. The relief of the requirements for outside feeders is in no way relieved by 702 although a lot of electricians believe this to be true.
If the generator is located on the inside to the same building that it serves then the requirements of 225 would not apply.

Language was also added to 702 to clear this up;
702.11 Outdoor Generator Sets.
Where an outdoor housed generator set is equipped with a readily accessible disconnecting means located within sight of the building or structure supplied, an additional disconnecting means shall not be required where ungrounded conductors serve or pass through the building or structure. The disconnecting means shall meet the requirements of 225.36.

As far as protecting the transfer switch with overcurrent, I just can?t see why a device would need overcurrent protection.
Is overcurrent protection required for any switch that is installed?

 

[360Youth]

I am waiting at supply house and posting mobile style, so bear with me. I agree that '08 sizing rrules have changed things but it still remains that a 200a service will require a 200a TS with a 25kw generator or a 100a service will need a 200a TS if someone decides to put a 45kw generator in place. More later.

 

[gepman]

Thank you for all of the replies.


360Youth

I am not exactly sure what a 125/2 is. My guess is 125 amp, 2-pole (240v) circuit breaker.

125/2 does indeed indicate a 125A, two pole circuit breaker. The generator is actually a 26kW/26kVA unit which would give it a full load amps of 108A.

Transfer switch must be rated for the size of the service or generator, whichever is greater.

I see no code requirement for that. If you know of a code section please quote the exact section. As jwelectric said in NEC 2008 702.5(B)(2)

702.5(B) (2) Automatic Transfer Equipment. Where automatic transfer equipment is used, an optional standby system shall comply with (2)(a) or (2)(b).
(a) Full Load. The standby source shall be capable of supplying the full load that is transferred by the automatic transfer equipment.

IN fact 702.5(b)(2) only states that the generator must be capable of supplying the load but does not mention the required rating of the transfer switch or the OCP required

ron

I'm sure there is a section somewhere, that I cannot locate, that generically requires equipment to be protected at its ratings.
The best I can do without finding that section mentioned above is 445.12(A) which requires the protection to be suitable for the conditions of use.

This is what I thought also. But as I said in my original post, Table 240.3 lists the articles which give the required OCP for the type of equipment covered. If it is not in Table 240.3 I guess it does not require OCP.

jwelectric

I think the you are right when you say

As far as protecting the transfer switch with overcurrent, I just can?t see why a device would need overcurrent protection.
Is overcurrent protection required for any switch that is installed?

As I stated I can't find any requirement for overcurrent protection for a switch. I can't even find a rquirement that a switch be rated for the load on it unless you consider the switch a conductor. Some devices (or equipment) such as receptacles and panelboards require OCP as listed in Table 240.3 which refers you to specific articles.

I have always been in the habit of using equipment that is rated at or above the OCP ahead of it since almost all of my work is industrial (this generator is for a small communications building at a large facility) and I never know what load will be added in the future (almost everything gets maxed out).

In this situation the 100/2 CB in the panelboard prevents continuous loading above 100A. However I have a nearly identical situation where instead of a single panelboard the ATS will feed two panelboards, each with a 100/2 CB. Therefore there is nothing (except the connected load which is below 100A) to prevent the generator from delivering its full load capability of 108A. In the latter case, even if I can't find a code requirement, I will use a 150A ATS and a 200A unfused switch on the generator side.

 

[jwelectric]

Gepman,

Is this generator install outdoors?

If the generator is installed outdoors then the requirements for outside feeders found in Article 225 will apply to those conductors coming from the generator to the building they serve.
Pay close attention to Sections 225.30 through 225.39 and also 702.11 unless this standby system is legally required.

 

[RB1]

Our favorite Section

Our favorite Section

Gepman,

From the UL White Book: Transfer switches without integral overcurrent protection, are marked to indicate the maximum rating of overcurrent protection to be provided ahead of the transfer switch.

So I am going with Section 110.3(B).

 

[360Youth]

Quote:
Transfer switch must be rated for the size of the service or generator, whichever is greater.


I see no code requirement for that. If you know of a code section please quote the exact section.

I cannot give you a reference right now, but let me phrase it this way (although I think all this is insignificant to you original question :smile: ): If you have a 200 amp service on a house and you do a load calc. in order to properly size your generator and you come up with 102 amps, allowing a 26kw generator to be installed, your service still runs through the TS at all times so the TS must therefore be sized for the existing or installed service. That being said, if the standby system is designed to run only a portion of the home, than the TS can be sized accordingly.

It is stated very well here...

Gepman,

From the UL White Book: Transfer switches without integral overcurrent protection, are marked to indicate the maximum rating of overcurrent protection to be provided ahead of the transfer switch.

So I am going with Section 110.3(B).

That would go for either the main service or the generator OCP. Sorry if I sidetracked the post.

PS Maybe I missed that the 100a TS had intergral protection. Oops.

 

[gepman]

jwelectric
Thanks for the reminders of the code sections. My design will meet those.

RB1
I believe that you have the best answer.

I mentioned that code section, 110.3(B), previously because I have been burned by it before. You can design everything to code and then come to some installation requirement that applies only to this particular piece of equipment. I didn't look in the UL White Book to see that statement. I believe that the UL listings and the UL listing requirements (UL Code for that particular equipment) should be available for free to the end user since the manufacturer conveniently makes the restrictions of use unavailable until it shows up inside the box that the equipment is delivered in. Many times equipment manufacturer's literature leaves out the information that is included with the equipment such as the installation requirements. I had some motor disconnect switches that came with requirements to use 75 deg. wire rated at 60 deg. ampacity. Industrially no one uses wire with the ampacity rated at 60 deg. Yes I know the code section that requires that design for circuits less than 100A, 110.14(C)(1), but all the equipment that I normally specify is listed for 75 deg. use.

I will check with the manufacturer on this.

Thanks

 

[gepman]

I just looked up the information in the UL White Book. To excerpts are below:

Transfer switches without integral overcurrent protective devices are
suitable for continuous use at 100% of rated current.

This is sort of a "duh" statement but if you considered a transfer switch a conductor then a 100A transfer switch should be protected with a 100A CB. A 101A transfer switch could be protected with a 125A CB.

Transfer switches not provided with integral overcurrent protection are
marked in accordance with a), b) or c) below.
a) ??When protected by ____ ampere maximum Class ____ fuse or Type
____ circuit breaker rated no more than ____ amperes, this transfer
switch is rated for use on a circuit capable of delivering not more
than ____ rms symmetrical amperes, ____ volts maximum.?? The first
two blanks in this marking are filled with the maximum ampere rating
and Class of fuse to be used. The third blank is filled with the
specific circuit breaker to be used, including the manufacturer and
type designation of the circuit breaker. The fourth blank is filled with
the maximum current rating of the circuit breaker. Transfer switches
may be marked with only the fuse information or the circuit breaker
information, when investigated for use only with fuses or circuit
breakers, respectively.
b) ??When protected by a circuit breaker rated no more than ____
amperes, this transfer switch is rated for use on a circuit capable of
delivering not more than 10 kA rms symmetrical amperes, ____ volts
maximum.?? The first blank is filled with the maximum current rating
of circuit breaker, and the second blank is filled with the maximum
circuit voltage. Transfer switches that bear this marking are intended
only for use with a molded-case circuit breaker as the overcurrent
protection, and may not be rated more than 400 A.
c) ??When protected by a circuit breaker without an adjustable shorttime
response only or by fuses, this transfer switch is rated for use
on a circuit capable of delivering no more than ____ rms symmetrical
amperes, ____ volts maximum.??

This requirement doesn't quite give the maximum overcurrent protection rating, just the maximum overcurrent protection allowable to have the labeled withstand or AIC rating.

 

[jwelectric]

IThis is sort of a "duh" statement but if you considered a transfer switch a conductor then a 100A transfer switch should be protected with a 100A CB. A 101A transfer switch could be protected with a 125A CB.

I don't think I fully understand just what you are saying with, "100A transfer switch should be protected with a 100A CB."

I don't think that there is a requirement to protect the transfer switch.

 

[gepman]

jwelectric

if you considered a transfer switch a conductor

If a transfer switch was considered a conductor it would be covered by 240.4(B).

Unfortunately the NEC does NOT define "conductor".